This is gambit-c.info, produced by makeinfo version 4.7 from
gambit-c.txi.

START-INFO-DIR-ENTRY
* Gambit-C: (gambit-c).		A portable implementation of Scheme.
* gsi: (gambit-c) interpreter.	Gambit interpreter.
* gsc: (gambit-c) compiler.	Gambit compiler.
END-INFO-DIR-ENTRY

   This file documents Gambit-C, a portable implementation of Scheme.

   Copyright (C) 1994-2006 Marc Feeley.

   Permission is granted to make and distribute verbatim copies of this
manual provided the copyright notice and this permission notice are
preserved on all copies.

   Permission is granted to copy and distribute modified versions of
this manual under the conditions for verbatim copying, provided that
the entire resulting derived work is distributed under the terms of a
permission notice identical to this one.

   Permission is granted to copy and distribute translations of this
manual into another language, under the above conditions for modified
versions, except that this permission notice may be stated in a
translation approved by the copyright holder.


File: gambit-c.info,  Node: Measuring time,  Next: File information,  Prev: Environment variables,  Up: Host environment

16.7 Measuring time
===================

Procedures are available for measuring real time (aka "wall" time) and
cpu time (the amount of time the cpu has been executing the process).
The resolution of the real time and cpu time clock is operating system
dependent.  Typically the resolution of the cpu time clock is rather
coarse (measured in "ticks" of 1/60th or 1/100th of a second).  Real
time is internally computed relative to some arbitrary point in time
using floating point numbers, which means that there is a gradual loss
of resolution as time elapses.  Moreover, some operating systems report
time in number of ticks using a 32 bit integer so the value returned by
the time related procedures may wraparound much before any significant
loss of resolution occurs (for example 2.7 years if ticks are 1/50th of
a second).

 -- procedure: current-time
 -- procedure: time? OBJ
 -- procedure: time->seconds TIME
 -- procedure: seconds->time X
     The procedure `current-time' returns a "time object" representing
     the current point in real time.

     The procedure `time?' returns `#t' when OBJ is a time object and
     `#f' otherwise.

     The procedure `time->seconds' converts the time object TIME into
     an inexact real number representing the number of seconds elapsed
     since the "epoch" (which is 00:00:00 Coordinated Universal Time
     01-01-1970).

     The procedure `seconds->time' converts the real number X
     representing the number of seconds elapsed since the "epoch" into a
     time object.

     For example:

          > (current-time)
          #<time #2>
          > (time? (current-time))
          #t
          > (time? 123)
          #f
          > (time->seconds (current-time))
          1083118758.63973
          > (time->seconds (current-time))
          1083118759.909163
          > (seconds->time (+ 10 (time->seconds (current-time))
          #<time #3>  ; a time object representing 10 seconds in the future


 -- procedure: process-times
 -- procedure: cpu-time
 -- procedure: real-time
     The procedure `process-times' returns a three element f64vector
     containing the cpu time that has been used by the program and the
     real time that has elapsed since it was started.  The first element
     corresponds to "user" time in seconds, the second element
     corresponds to "system" time in seconds and the third element is
     the elapsed real time in seconds.  On operating systems that can't
     differentiate user and system time, the system time is zero.  On
     operating systems that can't measure cpu time, the user time is
     equal to the elapsed real time and the system time is zero.

     The procedure `cpu-time' returns the cpu time in seconds that has
     been used by the program (user time plus system time).

     The procedure `real-time' returns the real time that has elapsed
     since the program was started.

     For example:

          > (process-times)
          #f64(.02794 .021754 .159926176071167)
          > (cpu-time)
          .051223
          > (real-time)
          .40660619735717773


 -- special form: time expr
     The `time' special form evaluates expr and returns the result.  As
     a side effect it displays a message on the interaction channel
     which indicates how long the evaluation took (in real time and cpu
     time), how much time was spent in the garbage collector, how much
     memory was allocated during the evaluation and how many minor and
     major page faults occured (0 is reported if not running under
     UNIX).

     For example:

          > (define (f x)
              (let loop ((x x) (lst '()))
                (if (= x 0)
                    lst
                    (loop (- x 1) (cons x lst)))))
          > (length (time (f 100000)))
          (time (f 100000))
              683 ms real time
              558 ms cpu time (535 user, 23 system)
              8 collections accounting for 102 ms real time (70 user, 5 system)
              6400160 bytes allocated
              no minor faults
              no major faults
          100000



File: gambit-c.info,  Node: File information,  Next: Group information,  Prev: Measuring time,  Up: Host environment

16.8 File information
=====================

 -- procedure: file-exists? PATH [CHASE?]
     The PATH argument must be a string.  This procedure returns `#t'
     when a file by that name exists, and returns `#f' otherwise.

     When CHASE? is present and `#f', symbolic links will not be
     chased, in other words if PATH refers to a symbolic link,
     `file-exists?' will return `#t' whether or not it points to an
     existing file.

     For example:

          > (file-exists? "nofile")
          #f


 -- procedure: file-info PATH [CHASE?]
     This procedure accesses the filesystem to get information about the
     file whose location is given by the string PATH.  A
     file-information record is returned that contains the file's type,
     the device number, the inode number, the mode (permission bits),
     the number of links, the file's user id, the file's group id, the
     file's size in bytes, the times of last-access, last-modification
     and last-change, the attributes, and the creation time.

     When CHASE? is present and `#f', symbolic links will not be
     chased, in other words if PATH refers to a symbolic link the
     `file-info' procedure will return information about the link
     rather than the file it links to.

     For example:

          > (file-info "/dev/tty")
          #<file-info #2
             type: character-special
             device: 19513156
             inode: 20728196
             mode: 438
             number-of-links: 1
             owner: 0
             group: 0
             size: 0
             last-access-time: #<time #3>
             last-modification-time: #<time #4>
             last-change-time: #<time #5>
             attributes: 128
             creation-time: #<time #6>>


 -- procedure: file-info? OBJ
     This procedure returns `#t' when OBJ is a file-information record
     and `#f' otherwise.

     For example:

          > (file-info? (file-info "/dev/tty"))
          #t
          > (file-info? 123)
          #f


 -- procedure: file-info-type FILE-INFO
     Returns the type field of the file-information record FILE-INFO.
     The type is denoted by a symbol.  The following types are possible:

    `regular'
          Regular file

    `directory'
          Directory

    `character-special'
          Character special device

    `block-special'
          Block special device

    `fifo'
          FIFO

    `symbolic-link'
          Symbolic link

    `socket'
          Socket

    `unknown'
          File is of an unknown type

     For example:

          > (file-info-type (file-info "/dev/tty"))
          character-special
          > (file-info-type (file-info "/dev"))
          directory


 -- procedure: file-info-device FILE-INFO
     Returns the device field of the file-information record FILE-INFO.

     For example:

          > (file-info-device (file-info "/dev/tty"))
          19513156


 -- procedure: file-info-inode FILE-INFO
     Returns the inode field of the file-information record FILE-INFO.

     For example:

          > (file-info-inode (file-info "/dev/tty"))
          20728196


 -- procedure: file-info-mode FILE-INFO
     Returns the mode field of the file-information record FILE-INFO.

     For example:

          > (file-info-mode (file-info "/dev/tty"))
          438


 -- procedure: file-info-number-of-links FILE-INFO
     Returns the number-of-links field of the file-information record
     FILE-INFO.

     For example:

          > (file-info-number-of-links (file-info "/dev/tty"))
          1


 -- procedure: file-info-owner FILE-INFO
     Returns the owner field of the file-information record FILE-INFO.

     For example:

          > (file-info-owner (file-info "/dev/tty"))
          0


 -- procedure: file-info-group FILE-INFO
     Returns the group field of the file-information record FILE-INFO.

     For example:

          > (file-info-group (file-info "/dev/tty"))
          0


 -- procedure: file-info-size FILE-INFO
     Returns the size field of the file-information record FILE-INFO.

     For example:

          > (file-info-size (file-info "/dev/tty"))
          0


 -- procedure: file-info-last-access-time FILE-INFO
     Returns the last-access-time field of the file-information record
     FILE-INFO.

     For example:

          > (file-info-last-access-time (file-info "/dev/tty"))
          #<time #2>


 -- procedure: file-info-last-modification-time FILE-INFO
     Returns the last-modification-time field of the file-information
     record FILE-INFO.

     For example:

          > (file-info-last-modification-time (file-info "/dev/tty"))
          #<time #2>


 -- procedure: file-info-last-change-time FILE-INFO
     Returns the last-change-time field of the file-information record
     FILE-INFO.

     For example:

          > (file-info-last-change-time (file-info "/dev/tty"))
          #<time #2>


 -- procedure: file-info-attributes FILE-INFO
     Returns the attributes field of the file-information record
     FILE-INFO.

     For example:

          > (file-info-attributes (file-info "/dev/tty"))
          128


 -- procedure: file-info-creation-time FILE-INFO
     Returns the creation-time field of the file-information record
     FILE-INFO.

     For example:

          > (file-info-creation-time (file-info "/dev/tty"))
          #<time #2>


 -- procedure: file-type PATH
 -- procedure: file-device PATH
 -- procedure: file-inode PATH
 -- procedure: file-mode PATH
 -- procedure: file-number-of-links PATH
 -- procedure: file-owner PATH
 -- procedure: file-group PATH
 -- procedure: file-size PATH
 -- procedure: file-last-access-time PATH
 -- procedure: file-last-modification-time PATH
 -- procedure: file-last-change-time PATH
 -- procedure: file-attributes PATH
 -- procedure: file-creation-time PATH
     These procedures combine a call to the `file-info' procedure and a
     call to a file-information record field accessor.  For instance
     `(file-type PATH)' is equivalent to `(file-info-type (file-info
     PATH))'.



File: gambit-c.info,  Node: Group information,  Next: User information,  Prev: File information,  Up: Host environment

16.9 Group information
======================

 -- procedure: group-info GROUP-NAME-OR-ID
     This procedure accesses the group database to get information
     about the group identified by GROUP-NAME-OR-ID, which is the
     group's symbolic name (string) or the group's GID (exact integer).
     A group-information record is returned that contains the group's
     symbolic name, the group's id (GID), and the group's members (list
     of symbolic user names).

     For example:

          > (group-info "staff")
          #<group-info #2 name: "staff" gid: 20 members: ("root")>
          > (group-info 29)
          #<group-info #3
             name: "certusers"
             gid: 29
             members: ("root" "jabber" "postfix" "cyrusimap")>
          > (group-info 5000)
          *** ERROR IN (console)@3.1 -- Resource temporarily unavailable
          (group-info 5000)


 -- procedure: group-info? OBJ
     This procedure returns `#t' when OBJ is a group-information record
     and `#f' otherwise.

     For example:

          > (group-info? (group-info "daemon"))
          #t
          > (group-info? 123)
          #f


 -- procedure: group-info-name GROUP-INFO
     Returns the symbolic name field of the group-information record
     GROUP-INFO.

     For example:

          > (group-info-name (group-info 29))
          "certusers"


 -- procedure: group-info-gid GROUP-INFO
     Returns the group id field of the group-information record
     GROUP-INFO.

     For example:

          > (group-info-gid (group-info "staff"))
          20


 -- procedure: group-info-members GROUP-INFO
     Returns the members field of the group-information record
     GROUP-INFO.

     For example:

          > (group-info-members (group-info "staff"))
          ("root")



File: gambit-c.info,  Node: User information,  Next: Host information,  Prev: Group information,  Up: Host environment

16.10 User information
======================

 -- procedure: user-name
     This procedure returns the user's name as a string.

     For example:

          > (user-name)
          "feeley"


 -- procedure: user-info USER-NAME-OR-ID
     This procedure accesses the user database to get information about
     the user identified by USER-NAME-OR-ID, which is the user's
     symbolic name (string) or the user's UID (exact integer).  A
     user-information record is returned that contains the user's
     symbolic name, the user's id (UID), the user's group id (GID), the
     path to the user's home directory, and the user's login shell.

     For example:

          > (user-info "feeley")
          #<user-info #2
             name: "feeley"
             uid: 506
             gid: 506
             home: "/Users/feeley"
             shell: "/bin/bash">
          > (user-info 0)
          #<user-info #3 name: "root" uid: 0 gid: 0 home: "/var/root" shell: "/bin/sh">
          > (user-info 5000)
          *** ERROR IN (console)@3.1 -- Resource temporarily unavailable
          (user-info 5000)


 -- procedure: user-info? OBJ
     This procedure returns `#t' when OBJ is a user-information record
     and `#f' otherwise.

     For example:

          > (user-info? (user-info "feeley"))
          #t
          > (user-info? 123)
          #f


 -- procedure: user-info-name USER-INFO
     Returns the symbolic name field of the user-information record
     USER-INFO.

     For example:

          > (user-info-name (user-info 0))
          "root"


 -- procedure: user-info-uid USER-INFO
     Returns the user id field of the user-information record USER-INFO.

     For example:

          > (user-info-uid (user-info "feeley"))
          506


 -- procedure: user-info-gid USER-INFO
     Returns the group id field of the user-information record
     USER-INFO.

     For example:

          > (user-info-gid (user-info "feeley"))
          506


 -- procedure: user-info-home USER-INFO
     Returns the home directory field of the user-information record
     USER-INFO.

     For example:

          > (user-info-home (user-info 0))
          "/var/root"


 -- procedure: user-info-shell USER-INFO
     Returns the shell field of the user-information record USER-INFO.

     For example:

          > (user-info-shell (user-info 0))
          "/bin/sh"



File: gambit-c.info,  Node: Host information,  Next: Service information,  Prev: User information,  Up: Host environment

16.11 Host information
======================

 -- procedure: host-name
     This procedure returns the machine's host name as a string.

     For example:

          > (host-name)
          "mega.iro.umontreal.ca"


 -- procedure: host-info HOST-NAME
     This procedure accesses the internet host database to get
     information about the machine whose name is denoted by the string
     HOST-NAME.  A host-information record is returned that contains
     the official name of the machine, a list of aliases (alternative
     names), and a non-empty list of IP addresses for this machine.  An
     exception is raised when HOST-NAME does not appear in the database.

     For example:

          > (host-info "www.google.com")
          #<host-info #2
             name: "www.l.google.com"
             aliases: ("www.google.com")
             addresses: (#u8(66 249 85 99) #u8(66 249 85 104))>
          > (host-info "unknown.domain")
          *** ERROR IN (console)@2.1 -- Unknown host
          (host-info "unknown.domain")


 -- procedure: host-info? OBJ
     This procedure returns `#t' when OBJ is a host-information record
     and `#f' otherwise.

     For example:

          > (host-info? (host-info "www.google.com"))
          #t
          > (host-info? 123)
          #f


 -- procedure: host-info-name HOST-INFO
     Returns the official name field of the host-information record
     HOST-INFO.

     For example:

          > (host-info-name (host-info "www.google.com"))
          "www.l.google.com"


 -- procedure: host-info-aliases HOST-INFO
     Returns the aliases field of the host-information record
     HOST-INFO.  This field is a possibly empty list of strings.

     For example:

          > (host-info-aliases (host-info "www.google.com"))
          ("www.google.com")


 -- procedure: host-info-addresses HOST-INFO
     Returns the addresses field of the host-information record
     HOST-INFO.  This field is a non-empty list of u8vectors denoting
     IP addresses.

     For example:

          > (host-info-addresses (host-info "www.google.com"))
          (#u8(66 249 85 99) #u8(66 249 85 104))



File: gambit-c.info,  Node: Service information,  Next: Protocol information,  Prev: Host information,  Up: Host environment

16.12 Service information
=========================

 -- procedure: service-info SERVICE-NAME-OR-ID
     This procedure accesses the service database to get information
     about the service identified by SERVICE-NAME-OR-ID, which is the
     service's symbolic name (string) or the service's port number
     (exact integer).  A service-information record is returned that
     contains the service's symbolic name, a list of aliases
     (alternative names), the port number (exact integer), and the
     protocol name (string).  An exception is raised when
     SERVICE-NAME-OR-ID does not appear in the database.

     For example:

          > (service-info "time")
          #<service-info #2
             name: "loopback"
             aliases: ("loopback-net")
             number: 2130706432>
          > (service-info 127)
          #<service-info #3
             name: "loopback"
             aliases: ("loopback-net")
             number: 2130706432>
          > (service-info 0)
          *** ERROR IN (console)@3.1 -- Resource temporarily unavailable
          (service-info 0)


 -- procedure: service-info? OBJ
     This procedure returns `#t' when OBJ is a service-information
     record and `#f' otherwise.

     For example:

          > (service-info? (service-info "loopback"))
          #t
          > (service-info? 123)
          #f


 -- procedure: service-info-name SERVICE-INFO
     Returns the symbolic name field of the service-information record
     SERVICE-INFO.

     For example:

          > (service-info-name (service-info 127))
          "loopback"


 -- procedure: service-info-aliases SERVICE-INFO
     Returns the aliases field of the service-information record
     SERVICE-INFO.  This field is a possibly empty list of strings.

     For example:

          > (service-info-aliases (service-info "loopback"))
          ("loopback-net")


 -- procedure: service-info-number SERVICE-INFO
     Returns the service number field of the service-information record
     SERVICE-INFO.

     For example:

          > (service-info-number (service-info "loopback"))
          2130706432



File: gambit-c.info,  Node: Protocol information,  Next: Network information,  Prev: Service information,  Up: Host environment

16.13 Protocol information
==========================

 -- procedure: protocol-info PROTOCOL-NAME-OR-ID
     This procedure accesses the protocol database to get information
     about the protocol identified by PROTOCOL-NAME-OR-ID, which is the
     protocol's symbolic name (string) or the protocol's number (exact
     integer).  A protocol-information record is returned that contains
     the protocol's symbolic name, a list of aliases (alternative
     names), and the protocol number (32 bit unsigned exact integer).
     An exception is raised when PROTOCOL-NAME-OR-ID does not appear in
     the database.

     For example:

          > (protocol-info "loopback")
          #<protocol-info #2
             name: "loopback"
             aliases: ("loopback-net")
             number: 2130706432>
          > (protocol-info 127)
          #<protocol-info #3
             name: "loopback"
             aliases: ("loopback-net")
             number: 2130706432>
          > (protocol-info 0)
          *** ERROR IN (console)@3.1 -- Resource temporarily unavailable
          (protocol-info 0)


 -- procedure: protocol-info? OBJ
     This procedure returns `#t' when OBJ is a protocol-information
     record and `#f' otherwise.

     For example:

          > (protocol-info? (protocol-info "loopback"))
          #t
          > (protocol-info? 123)
          #f


 -- procedure: protocol-info-name PROTOCOL-INFO
     Returns the symbolic name field of the protocol-information record
     PROTOCOL-INFO.

     For example:

          > (protocol-info-name (protocol-info 127))
          "loopback"


 -- procedure: protocol-info-aliases PROTOCOL-INFO
     Returns the aliases field of the protocol-information record
     PROTOCOL-INFO.  This field is a possibly empty list of strings.

     For example:

          > (protocol-info-aliases (protocol-info "loopback"))
          ("loopback-net")


 -- procedure: protocol-info-number PROTOCOL-INFO
     Returns the protocol number field of the protocol-information
     record PROTOCOL-INFO.

     For example:

          > (protocol-info-number (protocol-info "loopback"))
          2130706432



File: gambit-c.info,  Node: Network information,  Prev: Protocol information,  Up: Host environment

16.14 Network information
=========================

 -- procedure: network-info NETWORK-NAME-OR-ID
     This procedure accesses the network database to get information
     about the network identified by NETWORK-NAME-OR-ID, which is the
     network's symbolic name (string) or the network's number (exact
     integer).  A network-information record is returned that contains
     the network's symbolic name, a list of aliases (alternative
     names), and the network number (32 bit unsigned exact integer).
     An exception is raised when NETWORK-NAME-OR-ID does not appear in
     the database.

     For example:

          > (network-info "loopback")
          #<network-info #2
             name: "loopback"
             aliases: ("loopback-net")
             number: 2130706432>
          > (network-info 127)
          #<network-info #3
             name: "loopback"
             aliases: ("loopback-net")
             number: 2130706432>
          > (network-info 0)
          *** ERROR IN (console)@3.1 -- Resource temporarily unavailable
          (network-info 0)


 -- procedure: network-info? OBJ
     This procedure returns `#t' when OBJ is a network-information
     record and `#f' otherwise.

     For example:

          > (network-info? (network-info "loopback"))
          #t
          > (network-info? 123)
          #f


 -- procedure: network-info-name NETWORK-INFO
     Returns the symbolic name field of the network-information record
     NETWORK-INFO.

     For example:

          > (network-info-name (network-info 127))
          "loopback"


 -- procedure: network-info-aliases NETWORK-INFO
     Returns the aliases field of the network-information record
     NETWORK-INFO.  This field is a possibly empty list of strings.

     For example:

          > (network-info-aliases (network-info "loopback"))
          ("loopback-net")


 -- procedure: network-info-number NETWORK-INFO
     Returns the network number field of the network-information record
     NETWORK-INFO.

     For example:

          > (network-info-number (network-info "loopback"))
          2130706432



File: gambit-c.info,  Node: I/O and ports,  Next: Lexical syntax and readtables,  Prev: Host environment,  Up: Top

17 I/O and ports
****************

* Menu:

* Unidirectional and bidirectional ports::  Unidirectional and bidirectional ports
* Port classes::                            Port classes
* Port settings::                           Port settings
* Object-ports::                            Object-ports
* Character-ports::                         Character-ports
* Byte-ports::                              Byte-ports
* Device-ports::                            Device-ports
* Directory-ports::                         Directory-ports
* Vector-ports::                            Vector-ports
* String-ports::                            String-ports
* U8vector-ports::                          U8vector-ports
* Parameter objects related to I/O::        Parameter objects related to I/O


File: gambit-c.info,  Node: Unidirectional and bidirectional ports,  Next: Port classes,  Prev: I/O and ports,  Up: I/O and ports

17.1 Unidirectional and bidirectional ports
===========================================

Unidirectional ports allow communication between a producer of
information and a consumer.  An input-port's producer is typically a
resource managed by the operating system (such as a file, a process or
a network connection) and the consumer is the Scheme program.  The
roles are reversed for an output-port.

   Associated with each port are settings that affect I/O operations on
that port (encoding of characters to bytes, end-of-line encoding, type
of buffering, etc).  Port settings are specified when the port is
created.  Some port settings can be changed after a port is created.

   Bidirectional ports, also called input-output-ports, allow
communication in both directions.  They are best viewed as an object
that groups two separate unidirectional ports (one in each direction).
Each direction has its own port settings and can be closed
independently from the other direction.


File: gambit-c.info,  Node: Port classes,  Next: Port settings,  Prev: Unidirectional and bidirectional ports,  Up: I/O and ports

17.2 Port classes
=================

The four classes of ports listed below form an inheritance hierarchy.
Operations possible for a certain class of port are also possible for
the subclasses.  Only device-ports are connected to a device managed by
the operating system.  For instance it is possible to create ports that
behave as a FIFO where the Scheme program is both the producer and
consumer of information (possibly one thread is the producer and
another thread is the consumer).

  1. An "object-port" (or simply a port) provides operations to read
     and write Scheme data (i.e. any Scheme object) to/from the port.
     It also provides operations to force output to occur, to change
     the way threads block on the port, and to close the port.  Note
     that the class of objects for which write/read invariance is
     guaranteed depends on the particular class of port.

  2. A "character-port" provides all the operations of an object-port,
     and also operations to read and write individual characters to/from
     the port.  When a Scheme object is written to a character-port, it
     is converted into the sequence of characters that corresponds to
     its external-representation.  When reading a Scheme object, an
     inverse conversion occurs.  Note that some Scheme objects do not
     have an external textual representation that can be read back.

  3. A "byte-port" provides all the operations of a character-port, and
     also operations to read and write individual bytes to/from the
     port.  When a character is written to a byte-port, some encoding
     of that character into a sequence of bytes will occur (for example,
     `#\newline' will be encoded as the 2 bytes CR-LF when using
     ISO-8859-1 character encoding and `cr-lf' end-of-line encoding, and
     a non-ASCII character will generate more than 1 byte when using
     UTF-8 character encoding).  When reading a character, a similar
     decoding occurs.

  4. A "device-port" provides all the operations of a byte-port, and
     also operations to control the operating system managed device
     (file, network connection, terminal, etc) that is connected to the
     port.



File: gambit-c.info,  Node: Port settings,  Next: Object-ports,  Prev: Port classes,  Up: I/O and ports

17.3 Port settings
==================

Some port settings are only valid for specific port classes whereas
some others are valid for all ports.  Port settings are specified when
a port is created.  The settings that are not specified will default to
some reasonable values.  Keyword objects are used to name the settings
to be set.  As a simple example, a device-port connected to the file
`"foo"' can be created using the call

     (open-input-file "foo")

   This will use default settings for the character encoding, buffering,
etc.  If the UTF-8 character encoding is desired, then the port could
be opened using the call

     (open-input-file (list path: "foo" char-encoding: 'UTF-8))

   Here the argument of the procedure `open-input-file' has been
replaced by a "port settings list" which specifies the value of each
port setting that should not be set to the default value.  Note that
some port settings have no useful default and it is therefore required
to specify a value for them, such as the `path:' in the case of the
file opening procedures.  All port creation procedures (i.e. named
`open-...') take a single argument that can either be a port settings
list or a value of a type that depends on the kind of port being
created (a path string for files, an IP port number for socket servers,
etc).


File: gambit-c.info,  Node: Object-ports,  Next: Character-ports,  Prev: Port settings,  Up: I/O and ports

17.4 Object-ports
=================

* Menu:

* Object-port settings::             Object-port settings
* Object-port operations::           Object-port operations


File: gambit-c.info,  Node: Object-port settings,  Next: Object-port operations,  Prev: Object-ports,  Up: Object-ports

17.4.1 Object-port settings
---------------------------

The following is a list of port settings that are valid for all types
of ports.

   * `direction:' ( `input' | `output' | `input-output' )

     This setting controls the direction of the port.  The symbol
     `input' indicates a unidirectional input-port, the symbol `output'
     indicates a unidirectional output-port, and the symbol
     `input-output' indicates a bidirectional port.  The default value
     of this setting depends on the port creation procedure.

   * `buffering:' ( `#f' | `#t' | `line' )

     This setting controls the buffering of the port.  To set each
     direction separately the keywords `input-buffering:' and
     `output-buffering:' must be used instead of `buffering:'.  The
     value `#f' selects unbuffered I/O, the value `#t' selects fully
     buffered I/O, and the symbol `line' selects line buffered I/O (the
     output buffer is drained when a `#\newline' character is written).
     Line buffered I/O only applies to character-ports.  The default
     value of this setting is operating system dependent except
     consoles which are unbuffered.



File: gambit-c.info,  Node: Object-port operations,  Prev: Object-port settings,  Up: Object-ports

17.4.2 Object-port operations
-----------------------------

 -- procedure: input-port? OBJ
 -- procedure: output-port? OBJ
 -- procedure: port? OBJ
     The procedure `input-port?' returns `#t' when OBJ is a
     unidirectional input-port or a bidirectional port and `#f'
     otherwise.

     The procedure `output-port?' returns `#t' when OBJ is a
     unidirectional output-port or a bidirectional port and `#f'
     otherwise.

     The procedure `port?' returns `#t' when OBJ is a port (either
     unidirectional or bidirectional) and `#f' otherwise.

     For example:

          > (input-port? (current-input-port))
          #t
          > (call-with-input-string "some text" output-port?)
          #f
          > (port? (current-output-port))
          #t


 -- procedure: read [PORT]
     This procedure reads and returns the next Scheme datum from the
     input-port PORT.  The end-of-file object is returned when the end
     of the stream is reached.  If it is not specified, PORT defaults
     to the current input-port.

     For example:

          > (call-with-input-string "some text" read)
          some
          > (call-with-input-string "" read)
          #!eof


 -- procedure: read-all [PORT [READER]]
     This procedure repeatedly calls the procedure READER with PORT as
     the sole argument and accumulates a list of each value returned up
     to the end-of-file object.  The procedure `read-all' returns the
     accumulated list without the end-of-file object.  If it is not
     specified, PORT defaults to the current input-port.  If it is not
     specified, READER defaults to the procedure `read'.

     For example:

          > (call-with-input-string "3,2,1\ngo!" read-all)
          (3 ,2 ,1 go!)
          > (call-with-input-string "3,2,1\ngo!"
                                    (lambda (p) (read-all p read-char)))
          (#\3 #\, #\2 #\, #\1 #\newline #\g #\o #\!)
          > (call-with-input-string "3,2,1\ngo!"
                                    (lambda (p) (read-all p read-line)))
          ("3,2,1" "go!")


 -- procedure: write OBJ [PORT]
     This procedure writes the Scheme datum OBJ to the output-port PORT
     and the value returned is unspecified.  If it is not specified,
     PORT defaults to the current output-port.

     For example:

          > (write (list 'compare (list 'quote '@x) 'and (list 'unquote '@x)))
          (compare '@x and , @x)>


 -- procedure: newline [PORT]
     This procedure writes an "object separator" to the output-port
     PORT and the value returned is unspecified.  The separator ensures
     that the next Scheme datum written with the `write' procedure will
     not be confused with the latest datum that was written.  On
     character-ports this is done by writing the character `#\newline'.
     On ports where successive objects are implicitly distinct (such
     as "vector ports") this procedure does nothing.

     Regardless of the class of a port P and assuming that the external
     textual representation of the object X is readable, the expression
     `(begin (write X P) (newline P))' will write to P a representation
     of X that can be read back with the procedure `read'.  If it is
     not specified, PORT defaults to the current output-port.

     For example:

          > (begin (write 123) (newline) (write 456) (newline))
          123
          456


 -- procedure: force-output [PORT]
     The procedure `force-output' causes the output buffers of the
     output-port PORT to be drained (i.e. the data is sent to its
     destination).  If PORT is not specified, the current output-port
     is used.

     For example:

          > (define p (open-tcp-client
                        (list server-address: "www.iro.umontreal.ca"
                              port-number: 80)))
          > (display "GET /\n" p)
          > (force-output p)
          > (read-line p)
          "<!DOCTYPE HTML PUBLIC \"-//W3C//DTD HTML 4.01 Transitional//EN\""


 -- procedure: close-input-port PORT
 -- procedure: close-output-port PORT
 -- procedure: close-port PORT
     The PORT argument of these procedures must be a unidirectional or
     a bidirectional port.  For all three procedures the value returned
     is unspecified.

     The procedure `close-input-port' closes the input-port side of
     PORT, which must not be a unidirectional output-port.

     The procedure `close-output-port' closes the output-port side of
     PORT, which must not be a unidirectional input-port.  The ouput
     buffers are drained before PORT is closed.

     The procedure `close-port' closes all sides of the PORT.  Unless
     PORT is a unidirectional input-port, the output buffers are
     drained before PORT is closed.

     For example:

          > (define p (open-tcp-client
                        (list server-address: "www.iro.umontreal.ca"
                              port-number: 80)))
          > (display "GET /\n" p)
          > (close-output-port p)
          > (read-line p)
          "<!DOCTYPE HTML PUBLIC \"-//W3C//DTD HTML 4.01 Transitional//EN\""


 -- procedure: input-port-timeout-set! PORT TIMEOUT [THUNK]
 -- procedure: output-port-timeout-set! PORT TIMEOUT [THUNK]
     When a thread tries to perform an I/O operation on a port, the
     requested operation may not be immediately possible and the thread
     must wait.  For example, the thread may be trying to read a line of
     text from the console and the user has not typed anything yet, or
     the thread may be trying to write to a network connection faster
     than the network can handle.  In such situations the thread
     normally blocks until the operation becomes possible.

     It is sometimes necessary to guarantee that the thread will not
     block too long.  For this purpose, to each input-port and
     output-port is attached a "timeout" and "timeout-thunk".  The
     timeout indicates the point in time beyond which the thread should
     stop waiting on an input and output operation respectively.  When
     the timeout is reached, the thread calls the port's timeout-thunk.
     If the timeout-thunk returns `#f' the thread abandons trying to
     perform the operation (in the case of an input operation an
     end-of-file is read and in the case of an output operation an
     exception is raised).  Otherwise, the thread will block again
     waiting for the operation to become possible (note that if the
     port's timeout has not changed the thread will immediately call
     the timeout-thunk again).

     The procedure `input-port-timeout-set!' sets the timeout of the
     input-port PORT to TIMEOUT and the timeout-thunk to THUNK.  The
     procedure `output-port-timeout-set!' sets the timeout of the
     output-port PORT to TIMEOUT and the timeout-thunk to THUNK.  If it
     is not specified, the THUNK defaults to a thunk that returns `#f'.
     The TIMEOUT is either a time object indicating an absolute point
     in time, or it is a real number which indicates the number of
     seconds relative to the moment the procedure is called.  For both
     procedures the value returned is unspecified.

     When a port is created the timeout is set to infinity (`+inf.0').
     This causes the thread to wait as long as needed for the operation
     to become possible.  Setting the timeout to a point in the past
     (`-inf.0') will cause the thread to attempt the I/O operation and
     never block (i.e. the timeout-thunk is called if the operation is
     not immediately possible).

     The following example shows how to cause the REPL to terminate
     when the user does not enter an expression within the next 60
     seconds.

          > (input-port-timeout-set! (repl-input-port) 60)
          >
          *** EOF again to exit



File: gambit-c.info,  Node: Character-ports,  Next: Byte-ports,  Prev: Object-ports,  Up: I/O and ports

17.5 Character-ports
====================

* Menu:

* Character-port settings::          Character-port settings
* Character-port operations::        Character-port operations


File: gambit-c.info,  Node: Character-port settings,  Next: Character-port operations,  Prev: Character-ports,  Up: Character-ports

17.5.1 Character-port settings
------------------------------

The following is a list of port settings that are valid for
character-ports.

   * `readtable:' READTABLE

     This setting determines the readtable attached to the
     character-port.  To set each direction separately the keywords
     `input-readtable:' and `output-readtable:' must be used instead of
     `readtable:'.  Readtables control the external textual
     representation of Scheme objects, that is the encoding of Scheme
     objects using characters.  The behavior of the `read' procedure
     depends on the port's input-readtable and the behavior of the
     procedures `write', `pretty-print', and related procedures is
     affected by the port's output-readtable.  The default value of this
     setting is the value bound to the parameter object
     `current-readtable'.

   * `output-width:' POSITIVE-INTEGER

     This setting indicates the width of the character output-port in
     number of characters.  This information is used by the
     pretty-printer.  The default value of this setting is 80.



File: gambit-c.info,  Node: Character-port operations,  Prev: Character-port settings,  Up: Character-ports

17.5.2 Character-port operations
--------------------------------

 -- procedure: input-port-line PORT
 -- procedure: input-port-column PORT
 -- procedure: output-port-line PORT
 -- procedure: output-port-column PORT
     The current character location of a character input-port is the
     location of the next character to read.  The current character
     location of a character output-port is the location of the next
     character to write.  Location is denoted by a line number (the
     first line is line 1) and a column number, that is the location on
     the current line (the first column is column 1).  The procedures
     `input-port-line' and `input-port-column' return the line location
     and the column location respectively of the character input-port
     PORT.  The procedures `output-port-line' and `output-port-column'
     return the line location and the column location respectively of
     the character output-port PORT.

     For example:

          > (call-with-output-string
              '()
              (lambda (p)
                (display "abc\n123def" p)
                (write (list (output-port-line p) (output-port-column p))
                       p)))
          "abc\n123def(2 7)"


 -- procedure: output-port-width PORT
     This procedure returns the width, in characters, of the character
     output-port PORT.  The value returned is the port's output-width
     setting.

     For example:

          > (output-port-width (repl-output-port))
          80


 -- procedure: read-char [PORT]
     This procedure reads the character input-port PORT and returns the
     character at the current character location and advances the
     current character location to the next character, unless the PORT
     is already at end-of-file in which case `read-char' returns the
     end-of-file object.  If it is not specified, PORT defaults to the
     current input-port.

     For example:

          > (call-with-input-string
              "some text"
              (lambda (p)
                (let ((a (read-char p))) (list a (read-char p)))))
          (#\s #\o)
          > (call-with-input-string "" read-char)
          #!eof


 -- procedure: peek-char [PORT]
     This procedure returns the same result as `read-char' but it does
     not advance the current character location of the input-port PORT.
     If it is not specified, PORT defaults to the current input-port.

     For example:

          > (call-with-input-string
              "some text"
              (lambda (p)
                (let ((a (peek-char p))) (list a (read-char p)))))
          (#\s #\s)
          > (call-with-input-string "" peek-char)
          #!eof


 -- procedure: write-char CHAR [PORT]
     This procedure writes the character CHAR to the character
     output-port PORT and advances the current character location of
     that output-port.  The value returned is unspecified.  If it is not
     specified, PORT defaults to the current output-port.

     For example:

          > (write-char #\=)
          =>


 -- procedure: read-line [PORT [SEPARATOR [INCLUDE-SEPARATOR?]]]
     This procedure reads characters from the character input-port PORT
     until a specific SEPARATOR or the end-of-file is encountered and
     returns a string containing the sequence of characters read.  The
     SEPARATOR is included at the end of the string only if it was the
     last character read and INCLUDE-SEPARATOR? is not `#f'.  The
     SEPARATOR must be a character or `#f' (in which case all the
     characters until the end-of-file are read).  If it is not
     specified, PORT defaults to the current input-port.  If it is not
     specified, SEPARATOR defaults to `#\newline'.  If it is not
     specified, INCLUDE-SEPARATOR? defaults to `#f'.

     For example:

          > (define (split sep)
              (lambda (str)
                (call-with-input-string
                  str
                  (lambda (p)
                    (read-all p (lambda (p) (read-line p sep)))))))
          > ((split #\,) "a,b,c")
          ("a" "b" "c")
          > (map (split #\,)
                 (call-with-input-string "1,2,3\n4,5"
                                         (lambda (p) (read-all p read-line))))
          (("1" "2" "3") ("4" "5"))


 -- procedure: read-substring STRING START END [PORT]
 -- procedure: write-substring STRING START END [PORT]
     These procedures support bulk character I/O.  The part of the
     string STRING starting at index START and ending just before index
     END is used as a character buffer that will be the target of
     `read-substring' or the source of the `write-substring'.  Up to
     END-START characters will be transferred.  The number of
     characters transferred, possibly zero, is returned by these
     procedures.  Fewer characters will be read by `read-substring' if
     an end-of-file is read, or a timeout occurs before all the
     requested characters are transferred and the timeout thunk returns
     `#f' (see the procedure `input-port-timeout-set!').  Fewer
     characters will be written by `write-substring' if a timeout
     occurs before all the requested characters are transferred and the
     timeout thunk returns `#f' (see the procedure
     `output-port-timeout-set!').  If it is not specified, PORT
     defaults to the current input-port and current output-port
     respectively.

     For example:

          > (define s (make-string 10 #\x))
          > (read-substring s 2 5)123456789
          3
          > 456789
          > s
          "xx123xxxxx"


 -- procedure: input-port-readtable PORT
 -- procedure: output-port-readtable PORT
     These procedures return the readtable attached to the
     character-port PORT.  The PORT parameter of `input-port-readtable'
     must be an input-port.  The PORT parameter of
     `output-port-readtable' must be an output-port.


 -- procedure: input-port-readtable-set! PORT READTABLE
 -- procedure: output-port-readtable-set! PORT READTABLE
     These procedures change the readtable attached to the
     character-port PORT to the readtable READTABLE.  The PORT parameter
     of `input-port-readtable-set!'  must be an input-port.  The PORT
     parameter of `output-port-readtable-set!' must be an output-port.
     The value returned is unspecified.



File: gambit-c.info,  Node: Byte-ports,  Next: Device-ports,  Prev: Character-ports,  Up: I/O and ports

17.6 Byte-ports
===============

* Menu:

* Byte-port settings::               Byte-port settings
* Byte-port operations::             Byte-port operations


File: gambit-c.info,  Node: Byte-port settings,  Next: Byte-port operations,  Prev: Byte-ports,  Up: Byte-ports

17.6.1 Byte-port settings
-------------------------

The following is a list of port settings that are valid for byte-ports.

   * `char-encoding:' ENCODING

     This setting controls the character encoding of the byte-port.  For
     bidirectional byte-ports, the character encoding for input and
     output is set.  To set each direction separately the keywords
     `input-char-encoding:' and `output-char-encoding:' must be used
     instead of `char-encoding:'.  The default value of this setting is
     operating system dependent, but this can be overriden through the
     runtime options (*note Runtime options::).  The following
     encodings are supported:

    `ISO-8859-1'
          ISO-8859-1 character encoding.  Each character is encoded by
          a single byte.  Only Unicode characters with a code in the
          range 0 to 255 are allowed.

    `ASCII'
          ASCII character encoding.  Each character is encoded by a
          single byte.  In principle only Unicode characters with a
          code in the range 0 to 127 are allowed but most types of
          ports treat this exactly like `ISO-8859-1'.

    `UTF-8'
          UTF-8 character encoding.  Each character is encoded by a
          sequence of one to four bytes.  The minimum length UTF-8
          encoding is used.  If a BOM is needed at the beginning of the
          stream then it must be explicitly written.

    `UTF-16'
          UTF-16 character encoding.  Each character is encoded by one
          or two 16 bit integers (2 or 4 bytes).  The 16 bit integers
          may be encoded using little-endian encoding or big-endian
          encoding.  If the port is an input-port and the first two
          bytes read are a BOM ("Byte Order Mark" character with
          hexadecimal code FEFF) then the BOM will be discarded and the
          endianness will be set accordingly, otherwise the endianness
          depends on the operating system and how the Gambit runtime was
          compiled.  If the port is an output-port then a BOM will be
          output at the beginning of the stream and the endianness
          depends on the operating system and how the Gambit runtime
          was compiled.

    `UTF-16LE'
          UTF-16 character encoding with little-endian endianness.  It
          is like `UTF-16' except the endianness is set to
          little-endian and there is no BOM processing.  If a BOM is
          needed at the beginning of the stream then it must be
          explicitly written.

    `UTF-16BE'
          UTF-16 character encoding with big-endian endianness.  It is
          like `UTF-16LE' except the endianness is set to big-endian.

    `UCS-2'
          UCS-2 character encoding.  Each character is encoded by a 16
          bit integer (2 bytes).  The 16 bit integers may be encoded
          using little-endian encoding or big-endian encoding.  If the
          port is an input-port and the first two bytes read are a BOM
          ("Byte Order Mark" character with hexadecimal code FEFF) then
          the BOM will be discarded and the endianness will be set
          accordingly, otherwise the endianness depends on the
          operating system and how the Gambit runtime was compiled.  If
          the port is an output-port then a BOM will be output at the
          beginning of the stream and the endianness depends on the
          operating system and how the Gambit runtime was compiled.

    `UCS-2LE'
          UCS-2 character encoding with little-endian endianness.  It
          is like `UCS-2' except the endianness is set to little-endian
          and there is no BOM processing.  If a BOM is needed at the
          beginning of the stream then it must be explicitly written.

    `UCS-2BE'
          UCS-2 character encoding with big-endian endianness.  It is
          like `UCS-2LE' except the endianness is set to big-endian.

    `UCS-4'
          UCS-4 character encoding.  Each character is encoded by a 32
          integer (4 bytes).  The 32 bit integers may be encoded using
          little-endian encoding or big-endian encoding.  If the port
          is an input-port and the first four bytes read are a BOM
          ("Byte Order Mark" character with hexadecimal code 0000FEFF)
          then the BOM will be discarded and the endianness will be set
          accordingly, otherwise the endianness depends on the
          operating system and how the Gambit runtime was compiled.  If
          the port is an output-port then a BOM will be output at the
          beginning of the stream and the endianness depends on the
          operating system and how the Gambit runtime was compiled.

    `UCS-4LE'
          UCS-4 character encoding with little-endian endianness.  It
          is like `UCS-4' except the endianness is set to little-endian
          and there is no BOM processing.  If a BOM is needed at the
          beginning of the stream then it must be explicitly written.

    `UCS-4BE'
          UCS-4 character encoding with big-endian endianness.  It is
          like `UCS-4LE' except the endianness is set to big-endian.


   * `eol-encoding:' ENCODING

     This setting controls the end-of-line encoding of the byte-port.
     To set each direction separately the keywords `input-eol-encoding:'
     and `output-eol-encoding:' must be used instead of
     `eol-encoding:'.  The default value of this setting is operating
     system dependent, but this can be overriden through the runtime
     options (*note Runtime options::).  Note that for output-ports the
     end-of-line encoding is applied before the character encoding, and
     for input-ports it is applied after.  The following encodings are
     supported:

    `lf'
          For an output-port, writing a `#\newline' character outputs a
          `#\linefeed' character to the stream (Unicode character code
          10).  For an input-port, a `#\newline' character is read when
          a `#\linefeed' character is encountered on the stream.  Note
          that `#\linefeed' and `#\newline' are two names for the same
          character, so this end-of-line encoding is actually the
          identity function.  Text files created by UNIX applications
          typically use this end-of-line encoding.

    `cr'
          For an output-port, writing a `#\newline' character outputs a
          `#\return' character to the stream (Unicode character code
          10).  For an input-port, a `#\newline' character is read when
          a `#\linefeed' character or a `#\return' character is
          encountered on the stream.  Text files created by Classic Mac
          OS applications typically use this end-of-line encoding.

    `cr-lf'
          For an output-port, writing a `#\newline' character outputs to
          the stream a `#\return' character followed by a `#\linefeed'
          character.  For an input-port, a `#\newline' character is read
          when a `#\linefeed' character or a `#\return' character is
          encountered on the stream.  Moreover, if this character is
          immediately followed by the opposite character (`#\linefeed'
          followed by `#\return' or `#\return' followed by
          `#\linefeed') then the second character is ignored.  In other
          words, all four possible end-of-line encodings are read as a
          single `#\newline' character.  Text files created by DOS and
          Microsoft Windows applications typically use this end-of-line
          encoding.




File: gambit-c.info,  Node: Byte-port operations,  Prev: Byte-port settings,  Up: Byte-ports

17.6.2 Byte-port operations
---------------------------

 -- procedure: read-u8 [PORT]
     This procedure reads the byte input-port PORT and returns the byte
     at the current byte location and advances the current byte
     location to the next byte, unless the PORT is already at
     end-of-file in which case `read-u8' returns the end-of-file
     object.  If it is not specified, PORT defaults to the current
     input-port.

     This procedure must be called when the port's input character
     buffer is empty otherwise the character-stream and byte-stream may
     be out of sync due to buffering.  The input character buffer is
     used for bulk decoding of encoded characters (i.e. to translate
     the byte-stream into a character-stream).  The input character
     buffer is initially empty.  It is only when characters are read
     that it is filled with characters obtained by decoding the
     byte-stream.

     One way to ensure that the port's input character buffer is empty
     is to call `read-u8' strictly before any use of the port in a
     character input operation (i.e. a call to the procedures `read',
     `read-char', `peek-char', etc).  Alternatively
     `input-port-characters-buffered' can be used to get the number of
     characters in the port's input character buffer, and to empty the
     buffer with calls to `read-char' or `read-substring'.

     For example:

          > (call-with-input-u8vector
              '#u8(11 22 33 44)
              (lambda (p)
                (let ((a (read-u8 p))) (list a (read-u8 p)))))
          (11 22)
          > (call-with-input-u8vector '#u8() read-u8)
          #!eof


 -- procedure: write-u8 N [PORT]
     This procedure writes the byte N to the byte output-port PORT and
     advances the current byte location of that output-port.  The value
     returned is unspecified.  If it is not specified, PORT defaults to
     the current output-port.

     For example:

          > (call-with-output-u8vector '() (lambda (p) (write-u8 33 p)))
          #u8(33)


 -- procedure: read-subu8vector U8VECTOR START END [PORT]
 -- procedure: write-subu8vector U8VECTOR START END [PORT]
     These procedures support bulk byte I/O.  The part of the u8vector
     U8VECTOR starting at index START and ending just before index END
     is used as a byte buffer that will be the target of
     `read-subu8vector' or the source of the `write-subu8vector'.  Up
     to END-START bytes will be transferred.  The number of bytes
     transferred, possibly zero, is returned by these procedures.
     Fewer bytes will be read by `read-subu8vector' if an end-of-file
     is read, or a timeout occurs before all the requested bytes are
     transferred and the timeout thunk returns `#f' (see the procedure
     `input-port-timeout-set!').  Fewer bytes will be written by
     `write-subu8vector' if a timeout occurs before all the requested
     bytes are transferred and the timeout thunk returns `#f' (see the
     procedure `output-port-timeout-set!').  If it is not specified,
     PORT defaults to the current input-port and current output-port
     respectively.

     The procedure `read-subu8vector' must be called before any use of
     the port in a character input operation (i.e. a call to the
     procedures `read', `read-char', `peek-char', etc) because
     otherwise the character-stream and byte-stream may be out of sync
     due to the port buffering.

     For example:

          > (define v (make-u8vector 10))
          > (read-subu8vector v 2 5)123456789
          3
          > 456789
          > v
          #u8(0 0 49 50 51 0 0 0 0 0)



File: gambit-c.info,  Node: Device-ports,  Next: Directory-ports,  Prev: Byte-ports,  Up: I/O and ports

17.7 Device-ports
=================

* Menu:

* Filesystem devices::            Filesystem devices
* Process devices::               Process devices
* Network devices::               Network devices


File: gambit-c.info,  Node: Filesystem devices,  Next: Process devices,  Prev: Device-ports,  Up: Device-ports

17.7.1 Filesystem devices
-------------------------

 -- procedure: open-file PATH-OR-SETTINGS
 -- procedure: open-input-file PATH-OR-SETTINGS
 -- procedure: open-output-file PATH-OR-SETTINGS
 -- procedure: call-with-input-file PATH-OR-SETTINGS PROC
 -- procedure: call-with-output-file PATH-OR-SETTINGS PROC
 -- procedure: with-input-from-file PATH-OR-SETTINGS THUNK
 -- procedure: with-output-to-file PATH-OR-SETTINGS THUNK
     All of these procedures create a port to interface to a byte-stream
     device (such as a file, console, serial port, named pipe, etc)
     whose name is given by a path of the filesystem.  The `direction:'
     setting will default to the value `input' for the procedures
     `open-input-file', `call-with-input-file' and
     `with-input-from-file', to the value `output' for the procedures
     `open-output-file', `call-with-output-file' and
     `with-output-to-file', and to the value `input-output' for the
     procedure `open-file'.  The procedures `open-file',
     `open-input-file' and `open-output-file' return the port that is
     created.  The procedures `call-with-input-file' and
     `call-with-output-file' call the procedure PROC with the port as
     single argument, and then return the value(s) of this call after
     closing the port.  The procedures `with-input-from-file' and
     `with-output-to-file' dynamically bind the current input-port and
     current output-port respectively to the port created for the
     duration of a call to the procedure THUNK with no argument.  The
     value(s) of the call to THUNK are returned after closing the port.

     The first argument of these procedures is either a string denoting
     a filesystem path or a list of port settings which must contain a
     `path:' setting.  Here are the settings allowed in addition to the
     generic settings of byte-ports:

        * `path:' STRING

          This setting indicates the location of the file in the
          filesystem.  There is no default value for this setting.

        * `append:' ( `#f' | `#t' )

          This setting controls whether output will be added to the end
          of the file.  This is useful for writing to log files that
          might be open by more than one process.  The default value of
          this setting is `#f'.

        * `create:' ( `#f' | `#t' | `maybe' )

          This setting controls whether the file will be created when
          it is opened.  A setting of `#f' requires that the file exist
          (otherwise an exception is raised).  A setting of `#t'
          requires that the file does not exist (otherwise an exception
          is raised).  A setting of `maybe' will create the file if it
          does not exist.  The default value of this setting is `maybe'
          for output-ports and `#f' for input-ports and bidirectional
          ports.

        * `permissions:' 12-BIT-EXACT-INTEGER

          This setting controls the UNIX permissions that will be
          attached to the file if it is created.  The default value of
          this setting is `#o666'.

        * `truncate:' ( `#f' | `#t' )

          This setting controls whether the file will be truncated when
          it is opened.  For input-ports, the default value of this
          setting is `#f'.  For output-ports, the default value of this
          setting is `#t' when the `append:' setting is `#f', and `#f'
          otherwise.


     For example:

          > (with-output-to-file
              (list path: "nofile"
                    create: #f)
              (lambda ()
                (display "hello world!\n")))
          *** ERROR IN (console)@1.1 -- No such file or directory
          (with-output-to-file '(path: "nofile" create: #f) '#<procedure #2>)


 -- procedure: input-port-byte-position PORT [POSITION [WHENCE]]
 -- procedure: output-port-byte-position PORT [POSITION [WHENCE]]
     When called with a single argument these procedures return the byte
     position where the next I/O operation would take place in the file
     attached to the given PORT (relative to the beginning of the
     file).  When called with two or three arguments, the byte position
     for subsequent I/O operations on the given PORT is changed to
     POSITION, which must be an exact integer.  When WHENCE is omitted
     or is 0, the POSITION is relative to the beginning of the file.
     When WHENCE is 1, the POSITION is relative to the current byte
     position of the file.  When WHENCE is 2, the POSITION is relative
     to the end of the file.  The return value is the new byte
     position.  On most operating systems the byte position for reading
     and writing of a given bidirectional port are the same.

     When `input-port-byte-position' is called to change the byte
     position of an input-port, all input buffers will be flushed so
     that the next byte read will be the one at the given position.

     When `output-port-byte-position' is called to change the byte
     position of an output-port, there is an implicit call to
     `force-output' before the position is changed.

     For example:

          > (define p  ; p is an input-output-port
              (open-file '(path: "test" char-encoding: ISO-8859-1 create: maybe)))
          > (list (input-port-byte-position p) (output-port-byte-position p))
          (0 0)
          > (display "abcdefghij\n" p)
          > (list (input-port-byte-position p) (output-port-byte-position p))
          (0 0)
          > (force-output p)
          > (list (input-port-byte-position p) (output-port-byte-position p))
          (11 11)
          > (input-port-byte-position p 2)
          2
          > (list (input-port-byte-position p) (output-port-byte-position p))
          (2 2)
          > (peek-char p)
          #\c
          > (list (input-port-byte-position p) (output-port-byte-position p))
          (11 11)
          > (output-port-byte-position p -7 2)
          4
          > (list (input-port-byte-position p) (output-port-byte-position p))
          (4 4)
          > (write-char #\! p)
          > (list (input-port-byte-position p) (output-port-byte-position p))
          (4 4)
          > (force-output p)
          > (list (input-port-byte-position p) (output-port-byte-position p))
          (5 5)
          > (input-port-byte-position p 1)
          1
          > (read p)
          bcd!fghij



File: gambit-c.info,  Node: Process devices,  Next: Network devices,  Prev: Filesystem devices,  Up: Device-ports

17.7.2 Process devices
----------------------

 -- procedure: open-process PATH-OR-SETTINGS
     This procedure starts a new process and returns a port that allows
     communication with that process on its standard input and standard
     output.  The default value of the `direction:' setting is
     `input-output', i.e. the Scheme program can write to the process'
     standard input and can read from the process' standard output.

     The first argument of this procedure is either a string denoting a
     filesystem path of an executable program or a list of port settings
     which must contain a `path:' setting.  Here are the settings
     allowed in addition to the generic settings of byte-ports:

        * `path:' STRING

          This setting indicates the location of the executable program
          in the filesystem.  There is no default value for this
          setting.

        * `arguments:' LIST-OF-STRINGS

          This setting indicates the string arguments that are passed
          to the program.  The default value of this setting is the
          empty list (i.e. no arguments).

        * `environment:' LIST-OF-STRINGS

          This setting indicates the set of environment variable
          bindings that the process receives.  Each element of the list
          is a string of the form "`VAR=VALUE'", where `VAR' is the
          name of the variable and `VALUE' is its binding.  When
          LIST-OF-STRINGS is `#f', the process inherits the environment
          variable bindings of the Scheme program.  The default value
          of this setting is `#f'.

        * `directory:' DIR

          This setting indicates the current working directory of the
          process.  When DIR is `#f', the process uses the value of
          `(current-directory)'.  The default value of this setting is
          `#f'.

        * `stderr-redirection:' ( `#f' | `#t' )

          This setting indicates how the standard error of the process
          is redirected.  A setting of `#t' will redirect the standard
          error to the standard output (i.e. all output to standard
          error can be read from the process-port).  A setting of `#f'
          will leave the standard error as-is, which typically results
          in error messages being output to the console.  The default
          value of this setting is `#f'.

        * `pseudo-terminal:' ( `#f' | `#t' )

          This setting indicates what type of device will be bound to
          the process' standard input and standard output.  A setting
          of `#t' will use a pseudo-terminal device (this is a device
          that behaves like a tty device even though there is no real
          terminal or user directly involved).  A setting of `#f' will
          use a pair of pipes.  The difference is important for
          programs which behave differently when they are used
          interactively, for example shells.  The default value of this
          setting is `#f'.


     For example:

          > (define p (open-process (list path: "ls"
                                          arguments: '("../examples"))))
          > (read-line p)
          "README"
          > (read-line p)
          "Xlib-simple"
          > (close-port p)
          > (define p (open-process "/usr/bin/dc"))
          > (display "2 100 ^ p\n" p)
          > (force-output p)
          > (read-line p)
          "1267650600228229401496703205376"



File: gambit-c.info,  Node: Network devices,  Prev: Process devices,  Up: Device-ports

17.7.3 Network devices
----------------------

 -- procedure: open-tcp-client SETTINGS
     This procedure opens a network connection to a socket server and
     returns a tcp-client-port (a subtype of device-port) that
     represents this connection and allows communication with that
     server.  The default value of the `direction:' setting is
     `input-output', i.e. the Scheme program can send information to
     the server and receive information from the server.  The sending
     direction can be "shutdown" using the `close-output-port'
     procedure and the receiving direction can be "shutdown" using the
     `close-input-port' procedure.  The `close-port' procedure closes
     both directions of the connection.

     The first argument of this procedure is a list of port settings
     which must contain a `server-address:' setting and a
     `port-number:' setting.  Here are the settings allowed in addition
     to the generic settings of byte-ports:

        * `server-address:' STRING-OR-IP-ADDRESS

          This setting indicates the internet address of the server.
          It can be a string denoting a host name, which will be
          translated to an IP address by the `host-info' procedure, or
          a 4 element u8vector which contains the 32-bit IPv4 address
          or an 8 element u16vector which contains the 128-bit IPv6
          address.  There is no default value for this setting.

        * `port-number:' 16-BIT-EXACT-INTEGER

          This setting indicates the IP port-number of the server to
          connect to (e.g. 80 for the standard HTTP server, 23 for the
          standard telnet server).  There is no default value for this
          setting.

        * `keep-alive:' ( `#f' | `#t' )

          This setting controls the use of the "keep alive" option on
          the connection.  The "keep alive" option will periodically
          send control packets on otherwise idle network connections to
          ensure that the server host is active and reachable.  The
          default value of this setting is `#f'.

        * `coalesce:' ( `#f' | `#t' )

          This setting controls the use of TCP's "Nagle algorithm" which
          reduces the number of small packets by delaying their
          transmission and coalescing them into larger packets.  A
          setting of `#t' will coalesce small packets into larger ones.
          A setting of `#f' will transmit packets as soon as possible.
          The default value of this setting is `#t'.  Note that this
          setting does not affect the buffering of the port.


     Here is an example of the client-side code that opens a connection
     to an HTTP server on port 8080 on the same computer (for the
     server-side code see the example for the procedure
     `open-tcp-server'):

          > (define p (open-tcp-client (list server-address: '#u8(127 0 0 1)
                                             port-number: 8080
                                             eol-encoding: 'cr-lf)))
          > p
          #<input-output-port #2 (tcp-client #u8(127 0 0 1) 8080)>
          > (display "GET / HTTP/1.1\n" p)
          > (force-output p)
          > (read-line p)
          "<HTML>"


 -- procedure: open-tcp-server PORT-NUMBER-OR-SETTINGS
     This procedure sets up a socket to accept network connection
     requests from clients and returns a tcp-server-port from which
     network connections to clients are obtained.  Tcp-server-ports are
     a direct subtype of object-ports (i.e. they are not
     character-ports) and are input-ports.  Reading from a
     tcp-server-port with the `read' procedure will block until a
     network connection request is received from a client.  The `read'
     procedure will then return a tcp-client-port (a subtype of
     device-port) that represents this connection and allows
     communication with that client.  Closing a tcp-server-port with
     either the `close-input-port' or `close-port' procedures will
     cause the network subsystem to stop accepting connections on that
     socket.

     The first argument of this procedure is an IP port-number (16-bit
     nonnegative exact integer) or a list of port settings which must
     contain a `port-number:' setting.  Below is a list of the settings
     allowed in addition to the settings `keep-alive:' and `coalesce:'
     allowed by the `open-tcp-client' procedure and the generic
     settings of byte-ports.  The settings which are not listed below
     apply to the tcp-client-port that is returned by `read' when a
     connection is accepted and have the same meaning as if they were
     used in a call to the `open-tcp-client' procedure.

        * `server-address:' STRING-OR-IP-ADDRESS

          This setting indicates the internet address of the network
          interface on which connections requests are accepted.  When
          this parameter is not specified or is `#f', the connection
          requests are accepted on all network interfaces (i.e. address
          INADDR_ANY).  The parameter can be a string denoting a host
          name, which will be translated to an IP address by the
          `host-info' procedure, or a 4 element u8vector which contains
          the 32-bit IPv4 address or an 8 element u16vector which
          contains the 128-bit IPv6 address.

        * `port-number:' 16-BIT-EXACT-INTEGER

          This setting indicates the IP port-number assigned to the
          socket which accepts connection requests from clients.  So
          called "well-known ports", which are reserved for standard
          services, have a port-number below 1024 and can only be
          assigned to a socket by a process with superuser priviledges
          (e.g. 80 for the HTTP service, 23 for the telnet service).
          No special priviledges are needed to assign higher
          port-numbers to a socket.  There is no default value for this
          setting.

        * `backlog:' POSITIVE-EXACT-INTEGER

          This setting indicates the maximum number of connection
          requests that can be waiting to be accepted by a call to
          `read' (technically it is the value passed as the second
          argument of the UNIX `listen()' function).  The default value
          of this setting is 128.

        * `reuse-address:' ( `#f' | `#t' )

          This setting controls whether it is possible to assign a
          port-number that is currently active.  Note that when a
          server process terminates, the socket it was using to accept
          connection requests does not become inactive immediately.
          Instead it remains active for a few minutes to ensure clean
          termination of the connections.  A setting of `#f' will cause
          an exception to be raised in that case.  A setting of `#t'
          will allow a port-number to be used even if it is active.
          The default value of this setting is `#t'.


     Here is an example of the server-side code that accepts
     connections on port 8080 (for the client-side code see the example
     for the procedure `open-tcp-client'):

          > (define s (open-tcp-server (list port-number: 8080
                                             eol-encoding: 'cr-lf)))
          > (define p (read s))  ; blocks until client connects
          > p
          #<input-output-port #2 (tcp-client 8080)>
          > (read-line p)
          "GET / HTTP/1.1"
          > (display "<HTML>\n" p)
          > (force-output p)



File: gambit-c.info,  Node: Directory-ports,  Next: Vector-ports,  Prev: Device-ports,  Up: I/O and ports

17.8 Directory-ports
====================

 -- procedure: open-directory PATH-OR-SETTINGS
     This procedure opens a directory of the filesystem for reading its
     entries and returns a directory-port from which the entries can be
     enumerated.  Directory-ports are a direct subtype of object-ports
     (i.e. they are not character-ports) and are input-ports.  Reading
     from a directory-port with the `read' procedure returns the next
     file name in the directory as a string.  The end-of-file object is
     returned when all the file names have been enumerated.  Another
     way to get the list of all files in a directory is the
     `directory-files' procedure which returns a list of the files in
     the directory.  The advantage of using directory-ports is that it
     allows iterating over the files in a directory in constant space,
     which is interesting when the number of files in the directory is
     not known in advance and may be large.  Note that the order in
     which the names are returned is operating-system dependent.

     The first argument of this procedure is either a string denoting a
     filesystem path to a directory or a list of port settings which
     must contain a `path:' setting.  Here are the settings allowed in
     addition to the generic settings of object-ports:

        * `path:' STRING

          This setting indicates the location of the directory in the
          filesystem.  There is no default value for this setting.

        * `ignore-hidden:' ( `#f' | `#t' | `dot-and-dot-dot' )

          This setting controls whether hidden-files will be returned.
          Under UNIX and Mac OS X hidden-files are those that start
          with a period (such as `.', `..', and `.profile').  Under
          Microsoft Windows hidden files are the `.' and `..' entries
          and the files whose "hidden file" attribute is set.  A
          setting of `#f' will enumerate all the files.  A setting of
          `#t' will only enumerate the files that are not hidden.  A
          setting of `dot-and-dot-dot' will enumerate all the files
          except for the `.' and `..' hidden files.  The default value
          of this setting is `#t'.


     For example:

          > (let ((p (open-directory (list path: "../examples"
                                           ignore-hidden: #f))))
              (let loop ()
                (let ((fn (read p)))
                  (if (string? fn)
                      (begin
                        (pp (path-expand fn))
                        (loop)))))
              (close-input-port p))
          "/u/feeley/examples/."
          "/u/feeley/examples/.."
          "/u/feeley/examples/complex"
          "/u/feeley/examples/README"
          "/u/feeley/examples/simple"
          > (define x (open-directory "../examples"))
          > (read-all x)
          ("complex" "README" "simple")



File: gambit-c.info,  Node: Vector-ports,  Next: String-ports,  Prev: Directory-ports,  Up: I/O and ports

17.9 Vector-ports
=================

 -- procedure: open-vector [VECTOR-OR-SETTINGS]
 -- procedure: open-input-vector [VECTOR-OR-SETTINGS]
 -- procedure: open-output-vector [VECTOR-OR-SETTINGS]
 -- procedure: call-with-input-vector VECTOR-OR-SETTINGS PROC
 -- procedure: call-with-output-vector VECTOR-OR-SETTINGS PROC
 -- procedure: with-input-from-vector VECTOR-OR-SETTINGS THUNK
 -- procedure: with-output-to-vector VECTOR-OR-SETTINGS THUNK
     Vector-ports represent streams of Scheme objects.  They are a
     direct subtype of object-ports (i.e. they are not
     character-ports).  All of these procedures create vector-ports
     that are either unidirectional or bidirectional.  The `direction:'
     setting will default to the value `input' for the procedures
     `open-input-vector', `call-with-input-vector' and
     `with-input-from-vector', to the value `output' for the procedures
     `open-output-vector', `call-with-output-vector' and
     `with-output-to-vector', and to the value `input-output' for the
     procedure `open-vector'.  Bidirectional vector-ports behave like
     FIFOs: data written to the port is added to the end of the stream
     that is read.  It is only when a bidirectional vector-port's
     output-side is closed with a call to the `close-output-port'
     procedure that the stream's end is known (when the stream's end is
     reached, reading the port returns the end-of-file object).

     The procedures `open-vector', `open-input-vector' and
     `open-output-vector' return the port that is created.  The
     procedures `call-with-input-vector' and `call-with-output-vector'
     call the procedure PROC with the port as single argument, and then
     return the value(s) of this call after closing the port.  The
     procedures `with-input-from-vector' and `with-output-to-vector'
     dynamically bind the current input-port and current output-port
     respectively to the port created for the duration of a call to the
     procedure THUNK with no argument.  The value(s) of the call to
     THUNK are returned after closing the port.

     The first argument of these procedures is either a vector of the
     elements used to initialize the stream or a list of port settings.
     If it is not specified, the argument of the `open-vector',
     `open-input-vector', and `open-output-vector' procedures defaults
     to an empty list of port settings.  Here are the settings allowed
     in addition to the generic settings of object-ports:

        * `init:' VECTOR

          This setting indicates the initial content of the stream.
          The default value of this setting is an empty vector.

        * `permanent-close:' ( `#f' | `#t' )

          This setting controls whether a call to the procedures
          `close-output-port' will close the output-side of a
          bidirectional vector-port permanently or not.  A permanently
          closed bidirectional vector-port whose end-of-file has been
          reached on the input-side will return the end-of-file object
          for all subsequent calls to the `read' procedure.  A
          non-permanently closed bidirectional vector-port will return
          to its opened state when its end-of-file is read.  The
          default value of this setting is `#t'.


     For example:

          > (define p (open-vector))
          > (write 1 p)
          > (write 2 p)
          > (write 3 p)
          > (read p)
          1
          > (read p)
          2
          > (close-output-port p)
          > (read p)
          3
          > (read p)
          #!eof


 -- procedure: open-vector-pipe [VECTOR-OR-SETTINGS1
          [VECTOR-OR-SETTINGS2]]
     The procedure `open-vector-pipe' creates two vector-ports and
     returns these two ports.  The two ports are interrelated as
     follows: the first port's output-side is connected to the second
     port's input-side and the first port's input-side is connected to
     the second port's output-side.  The value VECTOR-OR-SETTINGS1 is
     used to setup the first vector-port and VECTOR-OR-SETTINGS2 is
     used to setup the second vector-port.  The same settings as for
     `open-vector' are allowed.  The default `direction:' setting is
     `input-output' (i.e. a bidirectional port is created).  If it is
     not specified VECTOR-OR-SETTINGS1 defaults to the empty list.  If
     it is not specified VECTOR-OR-SETTINGS2 defaults to
     VECTOR-OR-SETTINGS1 but with the `init:' setting set to the empty
     vector and with the input and output settings exchanged (e.g. if
     the first port is an input-port then the second port is an
     output-port, if the first port's input-side is non-buffered then
     the second port's output-side is non-buffered).

     For example:

          > (define (server op)
              (receive (c s) (open-vector-pipe)  ; client-side and server-side ports
                (thread-start!
                  (make-thread
                    (lambda ()
                      (let loop ()
                        (let ((request (read s)))
                          (if (not (eof-object? request))
                              (begin
                                (write (op request) s)
                                (newline s)
                                (force-output s)
                                (loop))))))))
                c))
          > (define a (server (lambda (x) (expt 2 x))))
          > (define b (server (lambda (x) (expt 10 x))))
          > (write 100 a)
          > (write 30 b)
          > (read a)
          1267650600228229401496703205376
          > (read b)
          1000000000000000000000000000000


 -- procedure: get-output-vector VECTOR-PORT
     The procedure `get-output-vector' takes an output vector-port or a
     bidirectional vector-port as argument and removes all the objects
     currently on the output-side, returning them in a vector.  The port
     remains open and subsequent output to the port and calls to the
     procedure `get-output-vector' are possible.

     For example:

          > (define p (open-vector '#(1 2 3)))
          > (write 4 p)
          > (get-output-vector p)
          #(1 2 3 4)
          > (write 5 p)
          > (write 6 p)
          > (get-output-vector p)
          #(5 6)



File: gambit-c.info,  Node: String-ports,  Next: U8vector-ports,  Prev: Vector-ports,  Up: I/O and ports

17.10 String-ports
==================

 -- procedure: open-string [STRING-OR-SETTINGS]
 -- procedure: open-input-string [STRING-OR-SETTINGS]
 -- procedure: open-output-string [STRING-OR-SETTINGS]
 -- procedure: call-with-input-string STRING-OR-SETTINGS PROC
 -- procedure: call-with-output-string STRING-OR-SETTINGS PROC
 -- procedure: with-input-from-string STRING-OR-SETTINGS THUNK
 -- procedure: with-output-to-string STRING-OR-SETTINGS THUNK
 -- procedure: open-string-pipe [STRING-OR-SETTINGS1
          [STRING-OR-SETTINGS2]]
 -- procedure: get-output-string STRING-PORT
     String-ports represent streams of characters.  They are a direct
     subtype of character-ports.  These procedures are the string-port
     analog of the procedures specified in the vector-ports section.
     Note that these procedures are a superset of the procedures
     specified in the "Basic String Ports SRFI" (SRFI 6).


 -- procedure: object->string OBJ [N]
     This procedure converts the object OBJ to its external
     representation and returns it in a string.  The parameter N
     specifies the maximal width of the resulting string.  If the
     external representation is wider than N, the resulting string will
     be truncated to N characters and the last 3 characters will be set
     to periods.  Note that the current readtable is used.



File: gambit-c.info,  Node: U8vector-ports,  Next: Parameter objects related to I/O,  Prev: String-ports,  Up: I/O and ports

17.11 U8vector-ports
====================

 -- procedure: open-u8vector [U8VECTOR-OR-SETTINGS]
 -- procedure: open-input-u8vector [U8VECTOR-OR-SETTINGS]
 -- procedure: open-output-u8vector [U8VECTOR-OR-SETTINGS]
 -- procedure: call-with-input-u8vector U8VECTOR-OR-SETTINGS PROC
 -- procedure: call-with-output-u8vector U8VECTOR-OR-SETTINGS PROC
 -- procedure: with-input-from-u8vector U8VECTOR-OR-SETTINGS THUNK
 -- procedure: with-output-to-u8vector U8VECTOR-OR-SETTINGS THUNK
 -- procedure: open-u8vector-pipe [U8VECTOR-OR-SETTINGS1
          [U8VECTOR-OR-SETTINGS2]]
 -- procedure: get-output-u8vector U8VECTOR-PORT
     U8vector-ports represent streams of bytes.  They are a direct
     subtype of byte-ports.  These procedures are the u8vector-port
     analog of the procedures specified in the vector-ports section.



File: gambit-c.info,  Node: Parameter objects related to I/O,  Prev: U8vector-ports,  Up: I/O and ports

17.12 Parameter objects related to I/O
======================================

 -- procedure: current-input-port [NEW-VALUE]
 -- procedure: current-output-port [NEW-VALUE]
 -- procedure: current-error-port [NEW-VALUE]
 -- procedure: current-readtable [NEW-VALUE]
     These procedures are parameter objects which represent
     respectively: the current input-port, the current output-port, the
     current error-port, and the current readtable.



File: gambit-c.info,  Node: Lexical syntax and readtables,  Next: C-interface,  Prev: I/O and ports,  Up: Top

18 Lexical syntax and readtables
********************************

* Menu:

* Readtables::                        Readtables
* Boolean syntax::                    Boolean syntax
* Character syntax::                  Character syntax
* String syntax::                     String syntax
* Symbol syntax::                     Symbol syntax
* Keyword syntax::                    Keyword syntax
* Box syntax::                        Box syntax
* Number syntax::                     Number syntax
* Homogeneous vector syntax::         Homogeneous vector syntax
* Special #! syntax::                 Special #! syntax
* Multiline comment syntax::          Multiline comment syntax
* Scheme infix syntax extension::     Scheme infix syntax extension


File: gambit-c.info,  Node: Readtables,  Next: Boolean syntax,  Prev: Lexical syntax and readtables,  Up: Lexical syntax and readtables

18.1 Readtables
===============

Readtables control the external textual representation of Scheme
objects, that is the encoding of Scheme objects using characters.
Readtables affect the behavior of the reader (i.e. the `read' procedure
and the parser used by the `load' procedure and the interpreter and
compiler) and the printer (i.e. the procedures `write', `display',
`pretty-print', and `pp', and the procedure used by the REPL to print
results).  To preserve write/read invariance the printer and reader
must be using compatible readtables.  For example a symbol which
contains upper case letters will be printed with special escapes if the
readtable indicates that the reader is case-insensitive.

   Readtables are immutable records whose fields specify various textual
representation aspects.  There are accessor procedures to retrieve the
content of specific fields.  There are also functional update
procedures that create a copy of a readtable, with a specific field set
to a new value.

 -- procedure: readtable? OBJ
     This procedure returns `#t' when OBJ is a readtable and `#f'
     otherwise.

     For example:

          > (readtable? (current-readtable))
          #t
          > (readtable? 123)
          #f


 -- procedure: readtable-case-conversion? READTABLE
 -- procedure: readtable-case-conversion?-set READTABLE NEW-VALUE
     The procedure `readtable-case-conversion?' returns the content of
     the `case-conversion?' field of READTABLE.  When the content of
     this field is `#f', the reader preserves the case of symbols and
     keyword objects that are read (i.e. `Ice' and `ice' are distinct
     symbols).  When the content of this field is the symbol `upcase',
     the reader converts lowercase letters to uppercase when reading
     symbols and keywords (i.e. `Ice' is read as the symbol
     `(string->symbol "ICE")').  Otherwise the reader converts
     uppercase letters to lowercase when reading symbols and keywords
     (i.e. `Ice' is read as the symbol `(string->symbol "ice")').

     The procedure `readtable-case-conversion?-set' returns a copy of
     READTABLE where only the `case-conversion?' field has been changed
     to NEW-VALUE.

     For example:

          > (output-port-readtable-set!
              (repl-output-port)
              (readtable-case-conversion?-set
                (output-port-readtable (repl-output-port))
                #f))
          > (input-port-readtable-set!
              (repl-input-port)
              (readtable-case-conversion?-set
                (input-port-readtable (repl-input-port))
                #f))
          > 'Ice
          Ice
          > (input-port-readtable-set!
              (repl-input-port)
              (readtable-case-conversion?-set
                (input-port-readtable (repl-input-port))
                #t))
          > 'Ice
          ice
          > (input-port-readtable-set!
              (repl-input-port)
              (readtable-case-conversion?-set
                (input-port-readtable (repl-input-port))
                'upcase))
          > 'Ice
          ICE


 -- procedure: readtable-keywords-allowed? READTABLE
 -- procedure: readtable-keywords-allowed?-set READTABLE NEW-VALUE
     The procedure `readtable-keywords-allowed?' returns the content of
     the `keywords-allowed?' field of READTABLE.  When the content of
     this field is `#f', the reader does not recognize keyword objects
     (i.e. `:foo' and `foo:' are read as the symbols `(string->symbol
     ":foo")' and `(string->symbol "foo:")' respectively).  When the
     content of this field is the symbol `prefix', the reader
     recognizes keyword objects that start with a colon, as in Common
     Lisp (i.e. `:foo' is read as the keyword `(string->keyword
     "foo")').  Otherwise the reader recognizes keyword objects that
     end with a colon, as in DSSSL (i.e. `foo:' is read as the symbol
     `(string->symbol "foo")').

     The procedure `readtable-keywords-allowed?-set' returns a copy of
     READTABLE where only the `keywords-allowed?' field has been
     changed to NEW-VALUE.

     For example:

          > (input-port-readtable-set!
              (repl-input-port)
              (readtable-keywords-allowed?-set
                (input-port-readtable (repl-input-port))
                #f))
          > (map keyword? '(foo :foo foo:))
          (#f #f #f)
          > (input-port-readtable-set!
              (repl-input-port)
              (readtable-keywords-allowed?-set
                (input-port-readtable (repl-input-port))
                #t))
          > (map keyword? '(foo :foo foo:))
          (#f #f #t)
          > (input-port-readtable-set!
              (repl-input-port)
              (readtable-keywords-allowed?-set
                (input-port-readtable (repl-input-port))
                'prefix))
          > (map keyword? '(foo :foo foo:))
          (#f #t #f)


 -- procedure: readtable-sharing-allowed? READTABLE
 -- procedure: readtable-sharing-allowed?-set READTABLE NEW-VALUE
     The procedure `readtable-sharing-allowed?' returns the content of
     the `sharing-allowed?' field of READTABLE.  The reader recognizes
     the `#N#' and `#N=DATUM' notation for circular structures and the
     printer uses this notation if and only if the content of the
     `sharing-allowed?' field is not `#f'.  Moreover when the content
     of the `sharing-allowed?' field is the symbol `serialize', the
     printer uses a special external representation that the reader
     understands and that extends write/read invariance to the
     following types: records, procedures and continuations.  Note that
     an object can be serialized and deserialized if and only if all of
     its components are serializable.

     The procedure `readtable-sharing-allowed?-set' returns a copy of
     READTABLE where only the `sharing-allowed?' field has been changed
     to NEW-VALUE.

     Here is a simple example:

          > (define (wr obj allow?)
              (call-with-output-string
                '()
                (lambda (p)
                  (output-port-readtable-set!
                    p
                    (readtable-sharing-allowed?-set
                      (output-port-readtable p)
                      allow?))
                  (write obj p))))
          > (define (rd str allow?)
              (call-with-input-string
                str
                (lambda (p)
                  (input-port-readtable-set!
                    p
                    (readtable-sharing-allowed?-set
                      (input-port-readtable p)
                      allow?))
                  (read p))))
          > (define x (list 1 2 3))
          > (set-car! (cdr x) (cddr x))
          > (wr x #f)
          "(1 (3) 3)"
          > (wr x #t)
          "(1 #0=(3) . #0#)"
          > (define y (rd (wr x #t) #t))
          > y
          (1 (3) 3)
          > (eq? (cadr y) (cddr y))
          #t
          > (define f #f)
          > (let ((free (expt 2 10)))
            (set! f (lambda (x) (+ x free))))
          > (define s (wr f 'serialize))
          > (string-length s)
          4196
          > (define g (rd s 'serialize))
          > (eq? f g)
          #f
          > (g 4)
          1028

     Continuations are tricky to serialize because they contain a
     dynamic environment and this dynamic environment may contain
     non-serializable objects, in particular ports attached to
     operating-system streams such as files, the console or standard
     input/output.  Indeed, all dynamic environments contain a binding
     for the `current-input-port' and `current-output-port'.  Moreover,
     any thread that has started a REPL has a continuation which refers
     to the "repl-context" object in its dynamic environment.  A
     repl-context object contains the interaction channel, which is
     typically connected to a non-serializable port, such as the
     console.  Another problem is that the `parameterize' form saves
     the old binding of the parameter in the continuation, so it is not
     possible to eliminate the references to these ports in the
     continuation by using the `parameterize' form alone.

     Serialization of continuations can be achieved dependably by taking
     advantage of string-ports, which are serializable objects (unless
     there is a blocked thread), and the following features of threads:
     they inherit the dynamic environment of the parent thread and they
     start with an initial continuation that contains only serializable
     objects.  So a thread created in a dynamic environment where
     `current-input-port' and `current-output-port' are bound to a
     dummy string-port has a serializable continuation.

     Here is an example where continuations are serialized:

          > (define (wr obj)
              (call-with-output-string
               '()
               (lambda (p)
                 (output-port-readtable-set!
                  p
                  (readtable-sharing-allowed?-set
                   (output-port-readtable p)
                   'serialize))
                 (write obj p))))
          > (define (rd str)
              (call-with-input-string
               str
               (lambda (p)
                 (input-port-readtable-set!
                  p
                  (readtable-sharing-allowed?-set
                   (input-port-readtable p)
                   'serialize))
                 (read p))))
          > (define fifo (open-vector))
          > (define (suspend-and-die!)
              (call-with-current-continuation
               (lambda (k)
                 (write (wr k) fifo)
                 (newline fifo)
                 (force-output fifo)
                 (thread-terminate! (current-thread)))))
          > (let ((dummy-port (open-string)))
              (parameterize ((current-input-port dummy-port)
                             (current-output-port dummy-port))
                (thread-start!
                 (make-thread
                  (lambda ()
                    (* 100
                       (suspend-and-die!)))))))
          #<thread #2>
          > (define s (read fifo))
          > (thread-join!
              (thread-start!
                (make-thread
                  (lambda ()
                    ((rd s) 111)))))
          11100
          > (thread-join!
              (thread-start!
                (make-thread
                  (lambda ()
                    ((rd s) 222)))))
          22200
          > (string-length s)
          13114


 -- procedure: readtable-eval-allowed? READTABLE
 -- procedure: readtable-eval-allowed?-set READTABLE NEW-VALUE
     The procedure `readtable-eval-allowed?' returns the content of the
     `eval-allowed?' field of READTABLE.  The reader recognizes the
     `#.EXPRESSION' notation for read-time evaluation if and only if
     the content of the `eval-allowed?' field is not `#f'.

     The procedure `readtable-eval-allowed?-set' returns a copy of
     READTABLE where only the `eval-allowed?' field has been changed to
     NEW-VALUE.

     For example:

          > (input-port-readtable-set!
              (repl-input-port)
              (readtable-eval-allowed?-set
                (input-port-readtable (repl-input-port))
                #t))
          > '(5 plus 7 is #.(+ 5 7))
          (5 plus 7 is 12)
          > '(buf = #.(make-u8vector 25))
          (buf = #u8(0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0))


 -- procedure: readtable-max-write-level READTABLE
 -- procedure: readtable-max-write-level-set READTABLE NEW-VALUE
     The procedure `readtable-max-write-level' returns the content of
     the `max-write-level' field of READTABLE.  The printer will
     display an ellipsis for the elements of lists and vectors that are
     nested deeper than this level.

     The procedure `readtable-max-write-level-set' returns a copy of
     READTABLE where only the `max-write-level' field has been changed
     to NEW-VALUE, which must be an nonnegative fixnum.

     For example:

          > (define (wr obj n)
              (call-with-output-string
                '()
                (lambda (p)
                  (output-port-readtable-set!
                    p
                    (readtable-max-write-level-set
                      (output-port-readtable p)
                      n))
                  (write obj p))))
          > (wr '(a #(b (c c) #u8(9 9 9) b) a) 3)
          "(a #(b (c c) #u8(9 9 9) b) a)"
          > (wr '(a #(b (c c) #u8(9 9 9) b) a) 2)
          "(a #(b (...) #u8(...) b) a)"
          > (wr '(a #(b (c c) #u8(9 9 9) b) a) 1)
          "(a #(...) a)"
          > (wr '(a #(b (c c) #u8(9 9 9) b) a) 0)
          "(...)"
          > (wr 'hello 0)
          "hello"


 -- procedure: readtable-max-write-length READTABLE
 -- procedure: readtable-max-write-length-set READTABLE NEW-VALUE
     The procedure `readtable-max-write-length' returns the content of
     the `max-write-length' field of READTABLE.  The printer will
     display an ellipsis for the elements of lists and vectors that are
     at an index beyond that length.

     The procedure `readtable-max-write-length-set' returns a copy of
     READTABLE where only the `max-write-length' field has been changed
     to NEW-VALUE, which must be an nonnegative fixnum.

     For example:

          > (define (wr obj n)
              (call-with-output-string
                '()
                (lambda (p)
                  (output-port-readtable-set!
                    p
                    (readtable-max-write-length-set
                      (output-port-readtable p)
                      n))
                  (write obj p))))
          > (wr '(a #(b (c c) #u8(9 9 9) b) . a) 4)
          "(a #(b (c c) #u8(9 9 9) b) . a)"
          > (wr '(a #(b (c c) #u8(9 9 9) b) . a) 3)
          "(a #(b (c c) #u8(9 9 9) ...) . a)"
          > (wr '(a #(b (c c) #u8(9 9 9) b) . a) 2)
          "(a #(b (c c) ...) . a)"
          > (wr '(a #(b (c c) #u8(9 9 9) b) . a) 1)
          "(a ...)"
          > (wr '(a #(b (c c) #u8(9 9 9) b) . a) 0)
          "(...)"


 -- procedure: readtable-start-syntax READTABLE
 -- procedure: readtable-start-syntax-set READTABLE NEW-VALUE
     The procedure `readtable-start-syntax' returns the content of the
     `start-syntax' field of READTABLE.  The reader uses this field to
     determine in which syntax to start parsing the input.  When the
     content of this field is the symbol `six', the reader starts in
     the infix syntax.  Otherwise the reader starts in the prefix
     syntax.

     The procedure `readtable-start-syntax-set' returns a copy of
     READTABLE where only the `start-syntax' field has been changed to
     NEW-VALUE.

     For example:

          > (+ 2 3)
          5
          > (input-port-readtable-set!
             (repl-input-port)
             (readtable-start-syntax-set
               (input-port-readtable (repl-input-port))
               'six))
          > 2+3;
          5
          > exit();



File: gambit-c.info,  Node: Boolean syntax,  Next: Character syntax,  Prev: Readtables,  Up: Lexical syntax and readtables

18.2 Boolean syntax
===================

Booleans are required to be followed by a delimiter (i.e. `#f64()' is
not the boolean `#f' followed by the number `64' and the empty list).


File: gambit-c.info,  Node: Character syntax,  Next: String syntax,  Prev: Boolean syntax,  Up: Lexical syntax and readtables

18.3 Character syntax
=====================

Characters are required to be followed by a delimiter (i.e.
`#\spaceballs' is not the character `#\space' followed by the symbol
`balls').  The lexical syntax of characters is extended to allow the
following:

`#\nul'
     Unicode character 0

`#\alarm'
     Unicode character 7

`#\backspace'
     Unicode character 8

`#\tab'
     Unicode character 9

`#\newline'
     Unicode character 10 (newline character)

`#\linefeed'
     Unicode character 10

`#\vtab'
     Unicode character 11

`#\page'
     Unicode character 12

`#\return'
     Unicode character 13

`#\space'
     Unicode character 32 (space character)

`#\rubout'
     Unicode character 127

`#\xHH'
     character encoded in hexadecimal (>= 1 hexadecimal digit)

`#\uHHHH'
     character encoded in hexadecimal (exactly 4 hexadecimal digits)

`#\UHHHHHHHH'
     character encoded in hexadecimal (exactly 8 hexadecimal digits)


File: gambit-c.info,  Node: String syntax,  Next: Symbol syntax,  Prev: Character syntax,  Up: Lexical syntax and readtables

18.4 String syntax
==================

The lexical syntax of quoted strings is extended to allow the following
escape codes:

`\a'
     Unicode character 7

`\b'
     Unicode character 8

`\t'
     Unicode character 9

`\n'
     Unicode character 10 (newline character)

`\v'
     Unicode character 11

`\f'
     Unicode character 12

`\r'
     Unicode character 13

`\"'
     `"'

`\\'
     `\'

`\|'
     `|'

`\?'
     `?'

`\OOO'
     character encoded in octal (1 to 3 octal digits, first digit must
     be less than 4 when there are 3 octal digits)

`\xHH'
     character encoded in hexadecimal (>= 1 hexadecimal digit)

`\uHHHH'
     character encoded in hexadecimal (exactly 4 hexadecimal digits)

`\UHHHHHHHH'
     character encoded in hexadecimal (exactly 8 hexadecimal digits)

`\<space>'
     Unicode character 32 (space character)

`\<newline><whitespace-except-newline>*'
     This sequence expands to nothing (it is useful for splitting a long
     string literal on multiple lines while respecting proper
     indentation of the source code)

   Gambit also supports a "here string" syntax that is similar to shell
"here documents".  For example:

     > (pretty-print #<<THE-END
     hello
     world
     THE-END
     )
     "hello\nworld"

   The here string starts with the sequence `#<<'.  The part of the
line after the `#<<' up to and including the newline character is the
key. The first line afterward that matches the key marks the end of the
here string.  The string contains all the characters between the start
key and the end key, with the exception of the newline character before
the end key.


File: gambit-c.info,  Node: Symbol syntax,  Next: Keyword syntax,  Prev: String syntax,  Up: Lexical syntax and readtables

18.5 Symbol syntax
==================

The lexical syntax of symbols is extended to allow a leading and
trailing vertical bar (e.g. `|a\|b"c:|').  The symbol's name
corresponds verbatim to the characters between the vertical bars except
for escaped characters.  The same escape sequences as for strings are
permitted except that `"' does not need to be escaped and `|' needs to
be escaped.

   For example:

     > (symbol->string '|a\|b"c:|)
     "a|b\"c:"


File: gambit-c.info,  Node: Keyword syntax,  Next: Box syntax,  Prev: Symbol syntax,  Up: Lexical syntax and readtables

18.6 Keyword syntax
===================

The lexical syntax of keywords is like symbols, but with a colon at the
end (note that this can be changed to a leading colon by setting the
`keywords-allowed?' field of the readtable to the symbol `prefix').  A
colon by itself is not a keyword, it is a symbol.  Vertical bars can be
used like symbols but the colon must be outside the vertical bars.
Note that the string returned by the `keyword->string' procedure does
not include the colon.

   For example:

     > (keyword->string 'foo:)
     "foo"
     > (map keyword? '(|ab()cd:| |ab()cd|: : ||:))
     (#f #t #f #t)


File: gambit-c.info,  Node: Box syntax,  Next: Number syntax,  Prev: Keyword syntax,  Up: Lexical syntax and readtables

18.7 Box syntax
===============

The lexical syntax of boxes is `#&OBJ' where OBJ is the content of the
box.

   For example:

     > (list '#&"hello" '#&123)
     (#&"hello" #&123)
     > (box (box (+ 10 20)))
     #&#&30


File: gambit-c.info,  Node: Number syntax,  Next: Homogeneous vector syntax,  Prev: Box syntax,  Up: Lexical syntax and readtables

18.8 Number syntax
==================

The lexical syntax of the special inexact real numbers is as follows:

`+inf.0'
     positive infinity

`-inf.0'
     negative infinity

`+nan.0'
     "not a number"

`-0.'
     negative zero (`0.' is the positive zero)


File: gambit-c.info,  Node: Homogeneous vector syntax,  Next: Special #! syntax,  Prev: Number syntax,  Up: Lexical syntax and readtables

18.9 Homogeneous vector syntax
==============================

Homogeneous vectors are vectors containing raw numbers of the same type
(signed or unsigned exact integers or inexact reals).  There are 10
types of homogeneous vectors: `s8vector' (vector of 8 bit signed
integers), `u8vector' (vector of 8 bit unsigned integers), `s16vector'
(vector of 16 bit signed integers), `u16vector' (vector of 16 bit
unsigned integers), `s32vector' (vector of 32 bit signed integers),
`u32vector' (vector of 32 bit unsigned integers), `s64vector' (vector
of 64 bit signed integers), `u64vector' (vector of 64 bit unsigned
integers), `f32vector' (vector of 32 bit floating point numbers), and
`f64vector' (vector of 64 bit floating point numbers).

   The external representation of homogeneous vectors is similar to
normal vectors but with the `#(' prefix replaced respectively with
`#s8(', `#u8(', `#s16(', `#u16(', `#s32(', `#u32(', `#s64(', `#u64(',
`#f32(', and `#f64('.

   The elements of the integer homogeneous vectors must be exact
integers fitting in the given precision.  The elements of the floating
point homogeneous vectors must be inexact reals.


File: gambit-c.info,  Node: Special #! syntax,  Next: Multiline comment syntax,  Prev: Homogeneous vector syntax,  Up: Lexical syntax and readtables

18.10 Special `#!' syntax
=========================

The lexical syntax of the special `#!' objects is as follows:

`#!eof'
     end-of-file object

`#!void'
     void object

`#!optional'
     optional object

`#!rest'
     rest object

`#!key'
     key object


File: gambit-c.info,  Node: Multiline comment syntax,  Next: Scheme infix syntax extension,  Prev: Special #! syntax,  Up: Lexical syntax and readtables

18.11 Multiline comment syntax
==============================

Multiline comments are delimited by the tokens `#|' and `|#'.  These
comments can be nested.


File: gambit-c.info,  Node: Scheme infix syntax extension,  Prev: Multiline comment syntax,  Up: Lexical syntax and readtables

18.12 Scheme infix syntax extension
===================================

The reader supports an infix syntax extension which is called SIX
(Scheme Infix eXtension).  This extension is both supported by the
`read' procedure and in program source code.

   The backslash character is a delimiter that marks the beginning of a
single datum expressed in the infix syntax (the details are given
below).  One way to think about it is that the backslash character
escapes the prefix syntax temporarily to use the infix syntax.  For
example a three element list could be written as `(X \Y Z)', the
elements X and Z are expressed using the normal prefix syntax and Y is
expressed using the infix syntax.

   When the reader encounters an infix datum, it constructs a syntax
tree for that particular datum.  Each node of this tree is represented
with a list whose first element is a symbol indicating the type of node.
For example, `(six.identifier abc)' is the representation of the infix
identifier `abc' and `(six.index (six.identifier abc) (six.identifier
i))' is the representation of the infix datum `abc[i];'.

18.12.1 SIX grammar
-------------------

The SIX grammar is given below.  On the left hand side are the
production rules.  On the right hand side is the datum that is
constructed by the reader.  The notation $I denotes the datum that is
constructed by the reader for the Ith part of the production rule.

<infix datum> ::=                           
<stat>                                      $1
<stat> ::=                                  
<if stat>                                   $1
| <for stat>                                $1
| <while stat>                              $1
| <do stat>                                 $1
| <switch stat>                             $1
| <case stat>                               $1
| <break stat>                              $1
| <continue stat>                           $1
| <label stat>                              $1
| <goto stat>                               $1
| <return stat>                             $1
| <expression stat>                         $1
| <procedure definition>                    $1
| <variable definition> `;'                 $1
| <clause stat>                             $1
| <compound stat>                           $1
| `;'                                       `(six.compound)'
<if stat> ::=                               
`if' `(' <pexpr> `)' <stat>                 `(six.if $3 $5)'
| `if' `(' <pexpr> `)' <stat> `else'        `(six.if $3 $5 $7)'
<stat>                                      
<for stat> ::=                              
`for' `(' <stat> `;' <oexpr> `;' <oexpr>    `(six.for $3 $5 $7 $9)'
`)' <stat>                                  
<while stat> ::=                            
`while' `(' <pexpr> `)' <stat>              `(six.while $3 $5)'
<do stat> ::=                               
`do' <stat> `while' `(' <pexpr> `)' `;'     `(six.do-while $2 $5)'
<switch stat> ::=                           
`switch' `(' <pexpr> `)' <stat>             `(six.switch $3 $5)'
<case stat> ::=                             
`case' <expr> `:' <stat>                    `(six.case $2 $4)'
<break stat> ::=                            
`break' `;'                                 `(six.break)'
<continue stat> ::=                         
`continue' `;'                              `(six.continue)'
<label stat> ::=                            
<identifier> `:' <stat>                     `(six.label $1 $3)'
<goto stat> ::=                             
`goto' <expr> `;'                           `(six.goto $2)'
<return stat> ::=                           
`return' `;'                                `(six.return)'
| `return' <expr> `;'                       `(six.return $2)'
<expression stat> ::=                       
<expr> `;'                                  $1
<clause stat> ::=                           
<expr> `.'                                  `(six.clause $1)'
<pexpr> ::=                                 
<procedure definition>                      $1
| <variable definition>                     $1
| <expr>                                    $1
<procedure definition> ::=                  
<type> <id-or-prefix> `(' <parameters> `)'  `(six.define-procedure $2
<body>                                      (six.procedure $1 $4 $6))'
<variable definition> ::=                   
<type> <id-or-prefix> <dimensions> <iexpr>  `(six.define-variable $2 $1
                                            $3 $4)'
<iexpr> ::=                                 
`=' <expr>                                  $2
|                                           `#f'
<dimensions> ::=                            
| `[' <expr> `]' <dimensions>               `($2 . $4)'
|                                           `()'
<oexpr> ::=                                 
<expr>                                      $1
|                                           `#f'
<expr> ::=                                  
<expr18>                                    $1
<expr18> ::=                                
<expr17> `:-' <expr18>                      `(six.x:-y $1 $3)'
| <expr17>                                  $1
<expr17> ::=                                
<expr17> `,' <expr16>                       `(|six.x,y| $1 $3)'
| <expr16>                                  $1
<expr16> ::=                                
<expr15> `:=' <expr16>                      `(six.x:=y $1 $3)'
| <expr15>                                  $1
<expr15> ::=                                
<expr14> `%=' <expr15>                      `(six.x%=y $1 $3)'
| <expr14> `&=' <expr15>                    `(six.x&=y $1 $3)'
| <expr14> `*=' <expr15>                    `(six.x*=y $1 $3)'
| <expr14> `+=' <expr15>                    `(six.x+=y $1 $3)'
| <expr14> `-=' <expr15>                    `(six.x-=y $1 $3)'
| <expr14> `/=' <expr15>                    `(six.x/=y $1 $3)'
| <expr14> `<<=' <expr15>                   `(six.x<<=y $1 $3)'
| <expr14> `=' <expr15>                     `(six.x=y $1 $3)'
| <expr14> `>>=' <expr15>                   `(six.x>>=y $1 $3)'
| <expr14> `^=' <expr15>                    `(six.x^=y $1 $3)'
| <expr14> `|=' <expr15>                    `(|six.x\|=y| $1 $3)'
| <expr14>                                  $1
<expr14> ::=                                
<expr13> `:' <expr14>                       `(six.x:y $1 $3)'
| <expr13>                                  $1
<expr13> ::=                                
<expr12> `?' <expr> `:' <expr13>            `(six.x?y:z $1 $3 $5)'
| <expr12>                                  $1
<expr12> ::=                                
<expr12> `||' <expr11>                      `(|six.x\|\|y| $1 $3)'
| <expr11>                                  $1
<expr11> ::=                                
<expr11> `&&' <expr10>                      `(six.x&&y $1 $3)'
| <expr10>                                  $1
<expr10> ::=                                
<expr10> `|' <expr9>                        `(|six.x\|y| $1 $3)'
| <expr9>                                   $1
<expr9> ::=                                 
<expr9> `^' <expr8>                         `(six.x^y $1 $3)'
| <expr8>                                   $1
<expr8> ::=                                 
<expr8> `&' <expr7>                         `(six.x&y $1 $3)'
| <expr7>                                   $1
<expr7> ::=                                 
<expr7> `!=' <expr6>                        `(six.x!=y $1 $3)'
| <expr7> `==' <expr6>                      `(six.x==y $1 $3)'
| <expr6>                                   $1
<expr6> ::=                                 
<expr6> `<' <expr5>                         `(six.x<y $1 $3)'
| <expr6> `<=' <expr5>                      `(six.x<=y $1 $3)'
| <expr6> `>' <expr5>                       `(six.x>y $1 $3)'
| <expr6> `>=' <expr5>                      `(six.x>=y $1 $3)'
| <expr5>                                   $1
<expr5> ::=                                 
<expr5> `<<' <expr4>                        `(six.x<<y $1 $3)'
| <expr5> `>>' <expr4>                      `(six.x>>y $1 $3)'
| <expr4>                                   $1
<expr4> ::=                                 
<expr4> `+' <expr3>                         `(six.x+y $1 $3)'
| <expr4> `-' <expr3>                       `(six.x-y $1 $3)'
| <expr3>                                   $1
<expr3> ::=                                 
<expr3> `%' <expr2>                         `(six.x%y $1 $3)'
| <expr3> `*' <expr2>                       `(six.x*y $1 $3)'
| <expr3> `/' <expr2>                       `(six.x/y $1 $3)'
| <expr2>                                   $1
<expr2> ::=                                 
`&' <expr2>                                 `(six.&x $2)'
| `+' <expr2>                               `(six.+x $2)'
| `-' <expr2>                               `(six.-x $2)'
| `*' <expr2>                               `(six.*x $2)'
| `!' <expr2>                               `(six.!x $2)'
| `!'                                       `(six.!)'
| `++' <expr2>                              `(six.++x $2)'
| `--' <expr2>                              `(six.--x $2)'
| `~' <expr2>                               `(six.~x $2)'
| `new' <id-or-prefix> `(' <arguments> `)'  `(six.new $2 . $4)'
| <expr1>                                   $1
<expr1> ::=                                 
<expr1> `++'                                `(six.x++ $1)'
| <expr1> `--'                              `(six.x-- $1)'
| <expr1> `(' <arguments> `)'               `(six.call $1 . $3)'
| <expr1> `[' <expr> `]'                    `(six.index $1 $3)'
| <expr1> `->' <id-or-prefix>               `(six.arrow $1 $3)'
| <expr1> `.' <id-or-prefix>                `(six.dot $1 $3)'
| <expr0>                                   $1
<expr0> ::=                                 
<id-or-prefix>                              $1
| <string>                                  `(six.literal $1)'
| <char>                                    `(six.literal $1)'
| <number>                                  `(six.literal $1)'
| `(' <expr> `)'                            $2
| `(' <block stat> `)'                      $2
| <datum-starting-with-#-or-backquote>      `(six.prefix $1)'
| `[' <elements> `]'                        $2
| <type> `(' <parameters> `)' <body>        `(six.procedure $1 $3 $5)'
<block stat> ::=                            
`{' <stat list> `}'                         `(six.compound . $2)'
<body> ::=                                  
`{' <stat list> `}'                         `(six.procedure-body . $2)'
<stat list> ::=                             
<stat> <stat list>                          `($1 . $2)'
|                                           `()'
<parameters> ::=                            
<nonempty parameters>                       $1
|                                           `()'
<nonempty parameters> ::=                   
<parameter> `,' <nonempty parameters>       `($1 . $3)'
| <parameter>                               `($1)'
<parameter> ::=                             
<type> <id-or-prefix>                       `($2 $1)'
<arguments> ::=                             
<nonempty arguments>                        $1
|                                           `()'
<nonempty arguments> ::=                    
<expr> `,' <nonempty arguments>             `($1 . $3)'
| <expr>                                    `($1)'
<elements> ::=                              
<nonempty elements>                         $1
|                                           `(six.null)'
<nonempty elements> ::=                     
<expr>                                      `(six.list $1 (six.null))'
| <expr> `,' <nonempty elements>            `(six.list $1 $3)'
| <expr> `|' <expr>                         `(six.cons $1 $3)'
<id-or-prefix> ::=                          
<identifier>                                `(six.identifier $1)'
| `\' <datum>                               `(six.prefix $2)'
<type> ::=                                  
`int'                                       `int'
| `char'                                    `char'
| `bool'                                    `bool'
| `void'                                    `void'
| `float'                                   `float'
| `double'                                  `double'
| `obj'                                     `obj'

18.12.2 SIX semantics
---------------------

The semantics of SIX depends on the definition of the `six.XXX'
identifiers (as functions and macros).  Many of these identifiers are
predefined macros which give SIX a semantics that is close to C's.  The
user may override these definitions to change the semantics either
globally or locally.  For example, `six.x^y' is a predefined macro that
expands `(six.x^y x y)' into `(bitwise-xor x y)'.  If the user prefers
the `^' operator to express exponentiation in a specific function, then
in that function `six.x^y' can be redefined as a macro that expands
`(six.x^y x y)' into `(expt x y)'.  Note that the associativity and
precedence of operators cannot be changed as that is a syntactic issue.

   Note that the following identifiers are not predefined, and
consequently they do not have a predefined semantics: `six.label',
`six.goto', `six.switch', `six.case', `six.break', `six.continue',
`six.return', `six.clause', `six.x:-y', and `six.!'.

   The following is an example showing some of the predefined semantics
of SIX:

     > (list (+ 1 2) \3+4; (+ 5 6))
     (3 7 11)
     > \[ 1+2, \(+ 3 4), 5+6 ];
     (3 7 11)
     > (map (lambda (x) \(x*x-1)/log(x+1);) '(1 2 3 4))
     (0 2.730717679880512 5.7707801635558535 9.320024018394177)
     > \obj n = expt(10,5);
     > n
     100000
     > \obj t[3][10] = 88;
     > \t[0][9] = t[2][1] = 11;
     11
     > t
     #(#(88 88 88 88 88 88 88 88 88 11)
       #(88 88 88 88 88 88 88 88 88 88)
       #(88 11 88 88 88 88 88 88 88 88))
     > \obj radix = new parameter (10);
     > \radix(2);
     > \radix();
     2
     > \for (int i=0; i<5; i++) pp(1<<i*8);
     1
     256
     65536
     16777216
     4294967296
     > \obj \make-adder (obj x) { obj (obj y) { x+y; }; }
     > \map (new adder (100), [1,2,3,4]);
     (101 102 103 104)
     > (map (make-adder 100) (list 1 2 3 4))
     (101 102 103 104)


File: gambit-c.info,  Node: C-interface,  Next: System limitations,  Prev: Lexical syntax and readtables,  Up: Top

19 C-interface
**************

The Gambit Scheme system offers a mechanism for interfacing Scheme code
and C code called the "C-interface".  A Scheme program indicates which
C functions it needs to have access to and which Scheme procedures can
be called from C, and the C interface automatically constructs the
corresponding Scheme procedures and C functions.  The conversions needed
to transform data from the Scheme representation to the C representation
(and back), are generated automatically in accordance with the argument
and result types of the C function or Scheme procedure.

   The C-interface places some restrictions on the types of data that
can be exchanged between C and Scheme.  The mapping of data types
between C and Scheme is discussed in the next section.  The remaining
sections of this chapter describe each special form of the C-interface.

* Menu:

* mapping of types::                The mapping of types between C and Scheme
* c-declare::                       The `c-declare' special form
* c-initialize::                    The `c-initialize' special form
* c-lambda::                        The `c-lambda' special form
* c-define::                        The `c-define' special form
* c-define-type::                   The `c-define-type' special form
* continuations::                   Continuations and the C-interface


File: gambit-c.info,  Node: mapping of types,  Next: c-declare,  Prev: C-interface,  Up: C-interface

19.1 The mapping of types between C and Scheme
==============================================

Scheme and C do not provide the same set of built-in data types so it is
important to understand which Scheme type is compatible with which C
type and how values get mapped from one environment to the other.  To
improve compatibility a new type is added to Scheme, the `foreign'
object type, and the following data types are added to C:

`scheme-object'
     denotes the universal type of Scheme objects (type `___SCMOBJ'
     defined in `gambit.h')

`bool'
     denotes the C++ `bool' type or the C `int' type (type `___BOOL'
     defined in `gambit.h')

`int8'
     8 bit signed integer (type `___S8' defined in `gambit.h')

`unsigned-int8'
     8 bit unsigned integer (type `___U8' defined in `gambit.h')

`int16'
     16 bit signed integer (type `___S16' defined in `gambit.h')

`unsigned-int16'
     16 bit unsigned integer (type `___U16' defined in `gambit.h')

`int32'
     32 bit signed integer (type `___S32' defined in `gambit.h')

`unsigned-int32'
     32 bit unsigned integer (type `___U32' defined in `gambit.h')

`int64'
     64 bit signed integer (type `___S64' defined in `gambit.h')

`unsigned-int64'
     64 bit unsigned integer (type `___U64' defined in `gambit.h')

`float32'
     32 bit floating point number (type `___F32' defined in `gambit.h')

`float64'
     64 bit floating point number (type `___F64' defined in `gambit.h')

`ISO-8859-1'
     denotes ISO-8859-1 encoded characters (8 bit unsigned integer,
     type `___ISO_8859_1' defined in `gambit.h')

`UCS-2'
     denotes UCS-2 encoded characters (16 bit unsigned integer, type
     `___UCS_2' defined in `gambit.h')

`UCS-4'
     denotes UCS-4 encoded characters (32 bit unsigned integer, type
     `___UCS_4' defined in `gambit.h')

`char-string'
     denotes the C `char*' type when used as a null terminated string

`nonnull-char-string'
     denotes the nonnull C `char*' type when used as a null terminated
     string

`nonnull-char-string-list'
     denotes an array of nonnull C `char*' terminated with a null
     pointer

`ISO-8859-1-string'
     denotes ISO-8859-1 encoded strings (null terminated string of 8
     bit unsigned integers, i.e. `___ISO_8859_1*')

`nonnull-ISO-8859-1-string'
     denotes nonnull ISO-8859-1 encoded strings (null terminated string
     of 8 bit unsigned integers, i.e. `___ISO_8859_1*')

`nonnull-ISO-8859-1-stringlist'
     denotes an array of nonnull ISO-8859-1 encoded strings terminated
     with a null pointer

`UTF-8-string'
     denotes UTF-8 encoded strings (null terminated string of `char',
     i.e. `char*')

`nonnull-UTF-8-string'
     denotes nonnull UTF-8 encoded strings (null terminated string of
     `char', i.e. `char*')

`nonnull-UTF-8-string-list'
     denotes an array of nonnull UTF-8 encoded strings terminated with
     a null pointer

`UCS-2-string'
     denotes UCS-2 encoded strings (null terminated string of 16 bit
     unsigned integers, i.e. `___UCS_2*')

`nonnull-UCS-2-string'
     denotes nonnull UCS-2 encoded strings (null terminated string of
     16 bit unsigned integers, i.e. `___UCS_2*')

`nonnull-UCS-2-string-list'
     denotes an array of nonnull UCS-2 encoded strings terminated with
     a null pointer

`UCS-4-string'
     denotes UCS-4 encoded strings (null terminated string of 32 bit
     unsigned integers, i.e. `___UCS_4*')

`nonnull-UCS-4-string'
     denotes nonnull UCS-4 encoded strings (null terminated string of
     32 bit unsigned integers, i.e. `___UCS_4*')

`nonnull-UCS-4-string-list'
     denotes an array of nonnull UCS-4 encoded strings terminated with
     a null pointer

`wchar_t-string'
     denotes `wchar_t' encoded strings (null terminated string of
     `wchar_t', i.e. `wchar_t*')

`nonnull-wchar_t-string'
     denotes nonnull `wchar_t' encoded strings (null terminated string
     of `wchar_t', i.e. `wchar_t*')

`nonnull-wchar_t-string-list'
     denotes an array of nonnull `wchar_t' encoded strings terminated
     with a null pointer

   To specify a particular C type inside the `c-lambda', `c-define' and
`c-define-type' forms, the following "Scheme notation" is used:

`Scheme notation'
     C type

`void'
     `void'

`bool'
     `bool'

`char'
     `char'  (may be signed or unsigned depending on the C compiler)

`signed-char'
     `signed char'

`unsigned-char'
     `unsigned char'

`ISO-8859-1'
     `ISO-8859-1'

`UCS-2'
     `UCS-2'

`UCS-4'
     `UCS-4'

`wchar_t'
     `wchar_t'

`short'
     `short'

`unsigned-short'
     `unsigned short'

`int'
     `int'

`unsigned-int'
     `unsigned int'

`long'
     `long'

`unsigned-long'
     `unsigned long'

`long-long'
     `long long'

`unsigned-long-long'
     `unsigned long long'

`float'
     `float'

`double'
     `double'

`int8'
     `int8'

`unsigned-int8'
     `unsigned-int8'

`int16'
     `int16'

`unsigned-int16'
     `unsigned-int16'

`int32'
     `int32'

`unsigned-int32'
     `unsigned-int32'

`int64'
     `int64'

`unsigned-int64'
     `unsigned-int64'

`float32'
     `float32'

`float64'
     `float64'

`(struct "C-STRUCT-ID" [TAG [RELEASE-FUNCTION]])'
     `struct C-STRUCT-ID'  (where C-STRUCT-ID is the name of a C
     structure; see below for the meaning of TAG and RELEASE-FUNCTION)

`(union "C-UNION-ID" [TAG [RELEASE-FUNCTION]])'
     `union C-UNION-ID'  (where C-UNION-ID is the name of a C union;
     see below for the meaning of TAG and RELEASE-FUNCTION)

`(type "C-TYPE-ID" [TAG [RELEASE-FUNCTION]])'
     `C-TYPE-ID'  (where C-TYPE-ID is an identifier naming a C type;
     see below for the meaning of TAG and RELEASE-FUNCTION)

`(pointer TYPE [TAG [RELEASE-FUNCTION]])'
     `T*'  (where T is the C equivalent of TYPE which must be the
     Scheme notation of a C type; see below for the meaning of TAG and
     RELEASE-FUNCTION)

`(nonnull-pointer TYPE [TAG [RELEASE-FUNCTION]])'
     same as `(pointer TYPE [TAG [RELEASE-FUNCTION]])' except the
     `NULL' pointer is not allowed

`(function (TYPE1...) RESULT-TYPE)'
     function with the given argument types and result type

`(nonnull-function (TYPE1...) RESULT-TYPE)'
     same as `(function (TYPE1...) RESULT-TYPE)' except the `NULL'
     pointer is not allowed

`char-string'
     `char-string'

`nonnull-char-string'
     `nonnull-char-string'

`nonnull-char-string-list'
     `nonnull-char-string-list'

`ISO-8859-1-string'
     `ISO-8859-1-string'

`nonnull-ISO-8859-1-string'
     `nonnull-ISO-8859-1-string'

`nonnull-ISO-8859-1-string-list'
     `nonnull-ISO-8859-1-string-list'

`UTF-8-string'
     `UTF-8-string'

`nonnull-UTF-8-string'
     `nonnull-UTF-8-string'

`nonnull-UTF-8-string-list'
     `nonnull-UTF-8-string-list'

`UCS-2-string'
     `UCS-2-string'

`nonnull-UCS-2-string'
     `nonnull-UCS-2-string'

`nonnull-UCS-2-string-list'
     `nonnull-UCS-2-string-list'

`UCS-4-string'
     `UCS-4-string'

`nonnull-UCS-4-string'
     `nonnull-UCS-4-string'

`nonnull-UCS-4-string-list'
     `nonnull-UCS-4-string-list'

`wchar_t-string'
     `wchar_t-string'

`nonnull-wchar_t-string'
     `nonnull-wchar_t-string'

`nonnull-wchar_t-string-list'
     `nonnull-wchar_t-string-list'

`scheme-object'
     `scheme-object'

`NAME'
     appropriate translation of NAME (where NAME is a C type defined
     with `c-define-type')

`"C-TYPE-ID"'
     C-TYPE-ID (this form is equivalent to `(type "C-TYPE-ID")')

   The `struct', `union', `type', `pointer' and `nonnull-pointer' types
are "foreign types" and they are represented on the Scheme side as
"foreign objects".  A foreign object is internally represented as a
pointer.  This internal pointer is identical to the C pointer being
represented in the case of the `pointer' and `nonnull-pointer' types.

   In the case of the `struct', `union' and `type' types, the internal
pointer points to a copy of the C data type being represented.  When an
instance of one of these types is converted from C to Scheme, a block
of memory is allocated from the C heap and initialized with the
instance and then a foreign object is allocated from the Scheme heap
and initialized with the pointer to this copy.  This approach may
appear overly complex, but it allows the conversion of C++ classes that
do not have a zero parameter constructor or an assignment method (i.e.
when compiling with a C++ compiler an instance is copied using `new
TYPE (INSTANCE)', which calls the copy-constructor of TYPE if it is a
class; TYPE's assignment operator is never used).  Conversion from
Scheme to C simply dereferences the internal pointer (no allocation
from the C heap is performed).  Deallocation of the copy on the C heap
is under the control of the release function attached to the foreign
object (see below).

   For type checking on the Scheme side, a TAG can be specified within
a foreign type specification.  The TAG must be `#f' or a symbol.  When
it is not specified the TAG defaults to a symbol whose name, as
returned by `symbol->string', is the C type declaration for that type.
For example the default tag for the type `(pointer (pointer char))' is
the symbol `char**'.  Two foreign types are compatible (i.e. can be
converted from one to the other) if they have identical tags or if at
least one of the tags is `#f'.  For the safest code the `#f' tag should
be used sparingly, as it completely bypasses type checking.  The
external representation of Scheme foreign objects (used by the `write'
procedure) contains the tag if it is not `#f', and the hexadecimal
address denoted by the internal pointer, for example `#<char** #2
0x2AAC535C>'.  Note that the hexadecimal address is in C notation, which
can be easily transferred to a C debugger with a "cut-and-paste".

   A RELEASE-FUNCTION can also be specified within a foreign type
specification.  The RELEASE-FUNCTION must be `#f' or a string naming a
C function with a single parameter of type `void*' (in which the
internal pointer is passed) and with a result of type `___SCMOBJ' (for
returning an error code).  When the RELEASE-FUNCTION is not specified
or is `#f' a default function is constructed by the C-interface.  This
default function does nothing in the case of the `pointer' and
`nonnull-pointer' types (deallocation is not the responsibility of the
C-interface) and returns the fixnum `___FIX(___NO_ERR)' to indicate no
error.  In the case of the `struct', `union' and `type' types, the
default function reclaims the copy on the C heap referenced by the
internal pointer (when using a C++ compiler this is done using `delete
(TYPE*)INTERNAL-POINTER', which calls the destructor of TYPE if it is a
class) and returns `___FIX(___NO_ERR)'.  In many situations the default
RELEASE-FUNCTION will perform the appropriate cleanup for the foreign
type.  However, in certain cases special operations (such as
decrementing a reference count, removing the object from a table, etc)
must be performed.  For such cases a user supplied RELEASE-FUNCTION is
needed.

   The RELEASE-FUNCTION is invoked at most once for any foreign object.
After the RELEASE-FUNCTION is invoked, the foreign object is
considered "released" and can no longer be used in a foreign type
conversion.  When the garbage collector detects that a foreign object
is no longer reachable by the program, it will invoke the
RELEASE-FUNCTION if the foreign object is not yet released.  When there
is a need to release the foreign object promptly, the program can
explicitly call `(foreign-release! OBJ)' which invokes the
RELEASE-FUNCTION if the foreign object is not yet released, and does
nothing otherwise.  The call `(foreign-released? OBJ)' returns a
boolean indicating whether the foreign object OBJ has been released yet
or not.  Finally, the call `(foreign-address OBJ)' returns the address
denoted by the internal pointer of foreign object OBJ or 0 if it has
been released.

   The following table gives the C types to which each Scheme type can
be converted:

Scheme type
     Allowed target C types

boolean `#f'
     `scheme-object'; `bool'; `pointer'; `function'; `char-string';
     `ISO-8859-1-string'; `UTF-8-string'; `UCS-2-string';
     `UCS-4-string'; `wchar_t-string'

boolean `#t'
     `scheme-object'; `bool'

character
     `scheme-object'; `bool'; [`[un]signed'] `char'; `ISO-8859-1';
     `UCS-2'; `UCS-4'; `wchar_t'

exact integer
     `scheme-object'; `bool'; [`unsigned-']
     `int8'/`int16'/`int32'/`int64'; [`unsigned'] `short'/`int'/`long'

inexact real
     `scheme-object'; `bool'; `float'; `double'; `float32'; `float64'

string
     `scheme-object'; `bool'; `char-string'; `nonnull-char-string';
     `ISO-8859-1-string'; `nonnull-ISO-8859-1-string'; `UTF-8-string';
     `nonnull-UTF-8-string'; `UCS-2-string'; `nonnull-UCS-2-string';
     `UCS-4-string'; `nonnull-UCS-4-string'; `wchar_t-string';
     `nonnull-wchar_t-string'

foreign object
     `scheme-object'; `bool';
     `struct'/`union'/`type'/`pointer'/`nonnull-pointer' with the
     appropriate tag

vector
     `scheme-object'; `bool'

symbol
     `scheme-object'; `bool'

procedure
     `scheme-object'; `bool'; `function'; `nonnull-function'

other objects
     `scheme-object'; `bool'

   The following table gives the Scheme types to which each C type will
be converted:

C type
     Resulting Scheme type

scheme-object
     the Scheme object encoded

bool
     boolean

[`[un]signed'] `char'; `ISO-8859-1'; `UCS-2'; `UCS-4'; `wchar_t'
     character

[`unsigned-'] `int8'/`int16'/`int32'/`int64'; [`unsigned'] `short'/`int'/`long'
     exact integer

`float'; `double'; `float32'; `float64'
     inexact real

`char-string'; `ISO-8859-1-string'; `UTF-8-string'; `UCS-2-string'; `UCS-4-string'; `wchar_t-string'
     string or `#f' if it is equal to `NULL'

`nonnull-char-string'; `nonnull-ISO-8859-1-string'; `nonnull-UTF-8-string'; `nonnull-UCS-2-string'; `nonnull-UCS-4-string'; `nonnull-wchar_t-string'
     string

`struct'/`union'/`type'/`pointer'/`nonnull-pointer'
     foreign object with the appropriate tag or `#f' in the case of a
     `pointer' equal to `NULL'

`function'
     procedure or `#f' if it is equal to `NULL'

`nonnull-function'
     procedure

`void'
     void object

   All Scheme types are compatible with the C types `scheme-object' and
`bool'.  Conversion to and from the C type `scheme-object' is the
identity function on the object encoding.  This provides a low-level
mechanism for accessing Scheme's object representation from C (with the
help of the macros in the `gambit.h' header file).  When a C `bool'
type is expected, an extended Scheme boolean can be passed (`#f' is
converted to 0 and all other values are converted to 1).

   The Scheme boolean `#f' can be passed to the C environment where a
`char-string', `ISO-8859-1-string', `UTF-8-string', `UCS-2-string',
`UCS-4-string', `wchar_t-string', `pointer' or `function' type is
expected.  In this case, `#f' is converted to the `NULL' pointer.  C
`bool's are extended booleans so any value different from 0 represents
true.  Thus, a C `bool' passed to the Scheme environment is mapped as
follows: 0 to `#f' and all other values to `#t'.

   A Scheme character passed to the C environment where any C character
type is expected is converted to the corresponding character in the C
environment.  An error is signaled if the Scheme character does not fit
in the C character.  Any C character type passed to Scheme is converted
to the corresponding Scheme character.  An error is signaled if the C
character does not fit in the Scheme character.

   A Scheme exact integer passed to the C environment where a C integer
type (other than `char') is expected is converted to the corresponding
integral value.  An error is signaled if the value falls outside of the
range representable by that integral type.  C integer values passed to
the Scheme environment are mapped to the same Scheme exact integer.  If
the value is outside the fixnum range, a bignum is created.

   A Scheme inexact real passed to the C environment is converted to the
corresponding `float', `double', `float32' or `float64' value.  C
`float', `double', `float32' and `float64' values passed to the Scheme
environment are mapped to the closest Scheme inexact real.

   Scheme's rational numbers and complex numbers are not compatible with
any C numeric type.

   A Scheme string passed to the C environment where any C string type
is expected is converted to a null terminated string using the
appropriate encoding.  The C string is a fresh copy of the Scheme
string.  If the C string was created for an argument of a `c-lambda',
the C string will be reclaimed when the `c-lambda' returns.  If the C
string was created for returning the result of a `c-define' to C, the
caller is responsible for reclaiming the C string with a call to the
`___release_string' function (see below for an example).  Any C string
type passed to the Scheme environment causes the creation of a fresh
Scheme string containing a copy of the C string (unless the C string is
equal to `NULL', in which case it is converted to `#f').

   A foreign type passed to the Scheme environment causes the creation
and initialization of a Scheme foreign object with the appropriate tag
(except for the case of a `pointer' equal to `NULL' which is converted
to `#f').  A Scheme foreign object can be passed where a foreign type
is expected, on the condition that the tags are appropriate (identical
or one is `#f') and the Scheme foreign object is not yet released.  The
value `#f' is also acceptable for a `pointer' type, and is converted to
`NULL'.

   Scheme procedures defined with the `c-define' special form can be
passed where the `function' and `nonnull-function' types are expected.
The value `#f' is also acceptable for a `function' type, and is
converted to `NULL'.  No other Scheme procedures are acceptable.
Conversion from the `function' and `nonnull-function' types to Scheme
procedures is not currently implemented.


File: gambit-c.info,  Node: c-declare,  Next: c-initialize,  Prev: mapping of types,  Up: C-interface

19.2 The `c-declare' special form
=================================

Synopsis:

     (c-declare c-declaration)

   Initially, the C file produced by `gsc' contains only an `#include'
of `gambit.h'.  This header file provides a number of macro and
procedure declarations to access the Scheme object representation.  The
special form `c-declare' adds c-declaration (which must be a string
containing the C declarations) to the C file.  This string is copied to
the C file on a new line so it can start with preprocessor directives.
All types of C declarations are allowed (including type declarations,
variable declarations, function declarations, `#include' directives,
`#define's, and so on).  These declarations are visible to subsequent
`c-declare's, `c-initialize's, and `c-lambda's, and `c-define's in the
same module.  The most common use of this special form is to declare
the external functions that are referenced in `c-lambda' special forms.
Such functions must either be declared explicitly or by including a
header file which contains the appropriate C declarations.

   The `c-declare' special form does not return a value.  It can only
appear at top level.

   For example:

     (c-declare #<<c-declare-end

     #include <stdio.h>

     extern char *getlogin ();

     #ifdef sparc
     char *host = "sparc";
     #else
     char *host = "unknown";
     #endif

     FILE *tfile;

     c-declare-end
     )


File: gambit-c.info,  Node: c-initialize,  Next: c-lambda,  Prev: c-declare,  Up: C-interface

19.3 The `c-initialize' special form
====================================

Synopsis:

     (c-initialize c-code)

   Just after the program is loaded and before control is passed to the
Scheme code, each C file is initialized by calling its associated
initialization function.  The body of this function is normally empty
but it can be extended by using the `c-initialize' form.  Each
occurence of the `c-initialize' form adds code to the body of the
initialization function in the order of appearance in the source file.
c-code must be a string containing the C code to execute.  This string
is copied to the C file on a new line so it can start with preprocessor
directives.

   The `c-initialize' special form does not return a value.  It can
only appear at top level.

   For example:

     (c-initialize "tfile = tmpfile ();")


File: gambit-c.info,  Node: c-lambda,  Next: c-define,  Prev: c-initialize,  Up: C-interface

19.4 The `c-lambda' special form
================================

Synopsis:

     (c-lambda (type1...) result-type c-name-or-code)

   The `c-lambda' special form makes it possible to create a Scheme
procedure that will act as a representative of some C function or C code
sequence.  The first subform is a list containing the type of each
argument.  The type of the function's result is given next.  Finally,
the last subform is a string that either contains the name of the C
function to call or some sequence of C code to execute.  Variadic C
functions are not supported.  The resulting Scheme procedure takes
exactly the number of arguments specified and delivers them in the same
order to the C function.  When the Scheme procedure is called, the
arguments will be converted to their C representation and then the C
function will be called.  The result returned by the C function will be
converted to its Scheme representation and this value will be returned
from the Scheme procedure call.  An error will be signaled if some
conversion is not possible.  The temporary memory allocated from the C
heap for the conversion of the arguments and result will be reclaimed
whether there is an error or not.

   When c-name-or-code is not a valid C identifier, it is treated as an
arbitrary piece of C code.  Within the C code the variables `___arg1',
`___arg2', etc. can be referenced to access the converted arguments.
Similarly, the result to be returned from the call should be assigned
to the variable `___result' except when the result is of type `struct',
`union', `type', `pointer', `nonnull-pointer', `function' or
`nonnull-function' in which case a pointer must be assigned to the
variable `___result_voidstar' which is of type `void*'.  For results of
type `pointer', `nonnull-pointer', `function' and `nonnull-function',
the value assigned to the variable `___result_voidstar' must be the
pointer or function cast to `void*'.  For results of type `struct',
`union', and `type', the value assigned to the variable
`___result_voidstar' must be a pointer to a memory allocated block
containing a copy of the result.  Note that this block will be
reclaimed by the RELEASE-FUNCTION associated with the type.  If no
result needs to be returned, the result-type should be `void' and no
assignment to the variable `___result' or `___result_voidstar' should
take place.  Note that the C code should not contain `return'
statements as this is meaningless.  Control must always fall off the
end of the C code.  The C code is copied to the C file on a new line so
it can start with preprocessor directives.  Moreover the C code is
always placed at the head of a compound statement whose lifetime
encloses the C to Scheme conversion of the result.  Consequently,
temporary storage (strings in particular) declared at the head of the C
code can be returned by assigning them to `___result' or
`___result_voidstar'.  In the c-name-or-code, the macro `___AT_END' may
be defined as the piece of C code to execute before control is returned
to Scheme but after the result is converted to its Scheme
representation.  This is mainly useful to deallocate temporary storage
contained in the result.

   When passed to the Scheme environment, the C `void' type is
converted to the void object.

   For example:

     (define fopen
       (c-lambda (nonnull-char-string nonnull-char-string)
                 (pointer "FILE")
        "fopen"))

     (define fgetc
       (c-lambda ((pointer "FILE"))
                 int
        "fgetc"))

     (let ((f (fopen "datafile" "r")))
       (if f (write (fgetc f))))

     (define char-code (c-lambda (char) int "___result = ___arg1;"))

     (define host ((c-lambda () nonnull-char-string "___result = host;")))

     (define stdin ((c-lambda () (pointer "FILE") "___result = stdin;")))

     ((c-lambda () void
     #<<c-lambda-end
       printf( "hello\n" );
       printf( "world\n" );
     c-lambda-end
     ))

     (define pack-1-char
       (c-lambda (char)
                 nonnull-char-string
     #<<c-lambda-end
        ___result = malloc (2);
        if (___result != NULL) { ___result[0] = ___arg1; ___result[1] = 0; }
        #define ___AT_END if (___result != NULL) free (___result);
     c-lambda-end
     ))

     (define pack-2-chars
       (c-lambda (char char)
                 nonnull-char-string
     #<<c-lambda-end
        char s[3];
        s[0] = ___arg1;
        s[1] = ___arg2;
        s[2] = 0;
        ___result = s;
     c-lambda-end
     ))


File: gambit-c.info,  Node: c-define,  Next: c-define-type,  Prev: c-lambda,  Up: C-interface

19.5 The `c-define' special form
================================

Synopsis:

     (c-define (variable define-formals) (type1...) result-type c-name scope
       body)

   The `c-define' special form makes it possible to create a C function
that will act as a representative of some Scheme procedure.  A C
function named c-name as well as a Scheme procedure bound to the
variable variable are defined.  The parameters of the Scheme procedure
are define-formals and its body is at the end of the form.  The type of
each argument of the C function, its result type and c-name (which must
be a string) are specified after the parameter specification of the
Scheme procedure.  When the C function c-name is called from C, its
arguments are converted to their Scheme representation and passed to
the Scheme procedure.  The result of the Scheme procedure is then
converted to its C representation and the C function c-name returns it
to its caller.

   The scope of the C function can be changed with the scope parameter,
which must be a string.  This string is placed immediately before the
declaration of the C function.  So if scope is the string `"static"',
the scope of c-name is local to the module it is in, whereas if scope
is the empty string, c-name is visible from other modules.

   The `c-define' special form does not return a value.  It can only
appear at top level.

   For example:

     (c-define (proc x #!optional (y x) #!rest z) (int int char float) int "f" ""
       (write (cons x (cons y z)))
       (newline)
       (+ x y))

     (proc 1 2 #\x 1.5) => 3 and prints (1 2 #\x 1.5)
     (proc 1)           => 2 and prints (1 1)

     ; if f is called from C with the call  f (1, 2, 'x', 1.5)
     ; the value 3 is returned and (1 2 #\x 1.5) is printed.
     ; f has to be called with 4 arguments.

   The `c-define' special form is particularly useful when the driving
part of an application is written in C and Scheme procedures are called
directly from C.  The Scheme part of the application is in a sense a
"server" that is providing services to the C part.  The Scheme
procedures that are to be called from C need to be defined using the
`c-define' special form.  Before it can be used, the Scheme part must
be initialized with a call to the function `___setup'.  Before the
program terminates, it must call the function `___cleanup' so that the
Scheme part may do final cleanup.  A sample application is given in the
file `tests/server.scm'.


File: gambit-c.info,  Node: c-define-type,  Next: continuations,  Prev: c-define,  Up: C-interface

19.6 The `c-define-type' special form
=====================================

Synopsis:

     (c-define-type name type [c-to-scheme scheme-to-c [cleanup]])

   This form associates the type identifier name to the C type type.
The name must not clash with predefined types (e.g. `char-string',
`ISO-8859-1', etc.) or with types previously defined with
`c-define-type' in the same file.  The `c-define-type' special form
does not return a value.  It can only appear at top level.

   If only the two parameters name and type are supplied then after
this definition, the use of name in a type specification is synonymous
to type.

   For example:

     (c-define-type FILE "FILE")
     (c-define-type FILE* (pointer FILE))
     (c-define-type time-struct-ptr (pointer (struct "tms")))
     (define fopen (c-lambda (char-string char-string) FILE* "fopen"))
     (define fgetc (c-lambda (FILE*) int "fgetc"))

   Note that identifiers are not case-sensitive in standard Scheme but
it is good programming practice to use a name with the same case as in
C.

   If four or more parameters are supplied, then type must be a string
naming the C type, c-to-scheme and scheme-to-c must be strings
suffixing the C macros that convert data of that type between C and
Scheme.  If cleanup is supplied it must be a boolean indicating whether
it is necessary to perform a cleanup operation (such as freeing memory)
when data of that type is converted from Scheme to C (it defaults to
`#t').  The cleanup information is used when the C stack is unwound due
to a continuation invocation (see *Note continuations::).  Although it
is safe to always specify `#t', it is more efficient in time and space
to specify `#f' because the unwinding mechanism can skip C-interface
frames which only contain conversions of data types requiring no
cleanup.  Two pairs of C macros need to be defined for conversions
performed by `c-lambda' forms and two pairs for conversions performed
by `c-define' forms:

     ___BEGIN_CFUN_scheme-to-c(___SCMOBJ, type, int)
     ___END_CFUN_scheme-to-c(___SCMOBJ, type, int)

     ___BEGIN_CFUN_c-to-scheme(type, ___SCMOBJ)
     ___END_CFUN_c-to-scheme(type, ___SCMOBJ)

     ___BEGIN_SFUN_c-to-scheme(type, ___SCMOBJ, int)
     ___END_SFUN_c-to-scheme(type, ___SCMOBJ, int)

     ___BEGIN_SFUN_scheme-to-c(___SCMOBJ, type)
     ___END_SFUN_scheme-to-c(___SCMOBJ, type)

   The macros prefixed with `___BEGIN' perform the conversion and those
prefixed with `___END' perform any cleanup necessary (such as freeing
memory temporarily allocated for the conversion).  The macro
`___END_CFUN_scheme-to-c' must free the result of the conversion if it
is memory allocated, and `___END_SFUN_scheme-to-c' must not (i.e. it is
the responsibility of the caller to free the result).

   The first parameter of these macros is the C variable that contains
the value to be converted, and the second parameter is the C variable in
which to store the converted value.  The third parameter, when present,
is the index (starting at 1) of the parameter of the `c-lambda' or
`c-define' form that is being converted (this is useful for reporting
precise error information when a conversion is impossible).

   To allow for type checking, the first three `___BEGIN' macros must
expand to an unterminated compound statement prefixed by an `if',
conditional on the absence of type check error:

     if ((___err = CONVERSION_OPERATION) == ___FIX(___NO_ERR)) {

   The last `___BEGIN' macro must expand to an unterminated compound
statement:

     { ___err = CONVERSION_OPERATION;

   If type check errors are impossible then a `___BEGIN' macro can
simply expand to an unterminated compound statement performing the
conversion:

     { CONVERSION_OPERATION;

   The `___END' macros must expand to a statement, or to nothing if no
cleanup is required, followed by a closing brace (to terminate the
compound statement started at the corresponding `___BEGIN' macro).

   The CONVERSION_OPERATION is typically a function call that returns
an error code value of type `___SCMOBJ' (the error codes are defined in
`gambit.h', and the error code `___FIX(___UNKNOWN_ERR)' is available
for generic errors).  CONVERSION_OPERATION can also set the variable
`___errmsg' of type `___SCMOBJ' to a specific Scheme string error
message.

   Below is a simple example showing how to interface to an `EBCDIC'
character type.  Memory allocation is not needed for conversion and type
check errors are impossible when converting EBCDIC to Scheme characters,
but they are possible when converting from Scheme characters to EBCDIC
since Gambit supports Unicode characters.

     (c-declare #<<c-declare-end

     typedef char EBCDIC; /* EBCDIC encoded characters */

     void put_char (EBCDIC c) { ... } /* EBCDIC I/O functions */
     EBCDIC get_char (void) { ... }

     char EBCDIC_to_ISO_8859_1[256] = { ... }; /* conversion tables */
     char ISO_8859_1_to_EBCDIC[256] = { ... };

     ___SCMOBJ SCMOBJ_to_EBCDIC (___SCMOBJ src, EBCDIC *dst)
     {
       int x = ___INT(src); /* convert from Scheme character to int */
       if (x > 255) return ___FIX(___UNKNOWN_ERR);
       *dst = ISO_8859_1_to_EBCDIC[x];
       return ___FIX(___NO_ERR);
     }

     #define ___BEGIN_CFUN_SCMOBJ_to_EBCDIC(src,dst,i) \
     if ((___err = SCMOBJ_to_EBCDIC (src, &dst)) == ___FIX(___NO_ERR)) {
     #define ___END_CFUN_SCMOBJ_to_EBCDIC(src,dst,i) }

     #define ___BEGIN_CFUN_EBCDIC_to_SCMOBJ(src,dst) \
     { dst = ___CHR(EBCDIC_to_ISO_8859_1[src]);
     #define ___END_CFUN_EBCDIC_to_SCMOBJ(src,dst) }

     #define ___BEGIN_SFUN_EBCDIC_to_SCMOBJ(src,dst,i) \
     { dst = ___CHR(EBCDIC_to_ISO_8859_1[src]);
     #define ___END_SFUN_EBCDIC_to_SCMOBJ(src,dst,i) }

     #define ___BEGIN_SFUN_SCMOBJ_to_EBCDIC(src,dst) \
     { ___err = SCMOBJ_to_EBCDIC (src, &dst);
     #define ___END_SFUN_SCMOBJ_to_EBCDIC(src,dst) }

     c-declare-end
     )

     (c-define-type EBCDIC "EBCDIC" "EBCDIC_to_SCMOBJ" "SCMOBJ_to_EBCDIC" #f)

     (define put-char (c-lambda (EBCDIC) void "put_char"))
     (define get-char (c-lambda () EBCDIC "get_char"))

     (c-define (write-EBCDIC c) (EBCDIC) void "write_EBCDIC" ""
       (write-char c))

     (c-define (read-EBCDIC) () EBCDIC "read_EBCDIC" ""
       (read-char))

   Below is a more complex example that requires memory allocation when
converting from C to Scheme.  It is an interface to a 2D `point' type
which is represented in Scheme by a pair of integers.  The conversion
of the `x' and `y' components is done by calls to the conversion macros
for the `int' type (defined in `gambit.h').  Note that no cleanup is
necessary when converting from Scheme to C (i.e. the last parameter of
the `c-define-type' is `#f').

     (c-declare #<<c-declare-end

     typedef struct { int x, y; } point;

     void line_to (point p) { ... }
     point get_mouse (void) { ... }
     point add_points (point p1, point p2) { ... }

     ___SCMOBJ SCMOBJ_to_POINT (___SCMOBJ src, point *dst, int arg_num)
     {
       ___SCMOBJ ___err = ___FIX(___NO_ERR);
       if (!___PAIRP(src))
         ___err = ___FIX(___UNKNOWN_ERR);
       else
         {
           ___SCMOBJ car = ___CAR(src);
           ___SCMOBJ cdr = ___CDR(src);
           ___BEGIN_CFUN_SCMOBJ_TO_INT(car,dst->x,arg_num)
           ___BEGIN_CFUN_SCMOBJ_TO_INT(cdr,dst->y,arg_num)
           ___END_CFUN_SCMOBJ_TO_INT(cdr,dst->y,arg_num)
           ___END_CFUN_SCMOBJ_TO_INT(car,dst->x,arg_num)
         }
       return ___err;
     }

     ___SCMOBJ POINT_to_SCMOBJ (point src, ___SCMOBJ *dst, int arg_num)
     {
       ___SCMOBJ ___err = ___FIX(___NO_ERR);
       ___SCMOBJ x_scmobj;
       ___SCMOBJ y_scmobj;
       ___BEGIN_SFUN_INT_TO_SCMOBJ(src.x,x_scmobj,arg_num)
       ___BEGIN_SFUN_INT_TO_SCMOBJ(src.y,y_scmobj,arg_num)
       *dst = ___EXT(___make_pair) (x_scmobj, y_scmobj, ___STILL);
       if (___FIXNUMP(*dst))
         ___err = *dst; /* return allocation error */
       ___END_SFUN_INT_TO_SCMOBJ(src.y,y_scmobj,arg_num)
       ___END_SFUN_INT_TO_SCMOBJ(src.x,x_scmobj,arg_num)
       return ___err;
     }

     #define ___BEGIN_CFUN_SCMOBJ_to_POINT(src,dst,i) \
     if ((___err = SCMOBJ_to_POINT (src, &dst, i)) == ___FIX(___NO_ERR)) {
     #define ___END_CFUN_SCMOBJ_to_POINT(src,dst,i) }

     #define ___BEGIN_CFUN_POINT_to_SCMOBJ(src,dst) \
     if ((___err = POINT_to_SCMOBJ (src, &dst, ___RETURN_POS)) == ___FIX(___NO_ERR)) {
     #define ___END_CFUN_POINT_to_SCMOBJ(src,dst) \
     ___EXT(___release_scmobj) (dst); }

     #define ___BEGIN_SFUN_POINT_to_SCMOBJ(src,dst,i) \
     if ((___err = POINT_to_SCMOBJ (src, &dst, i)) == ___FIX(___NO_ERR)) {
     #define ___END_SFUN_POINT_to_SCMOBJ(src,dst,i) \
     ___EXT(___release_scmobj) (dst); }

     #define ___BEGIN_SFUN_SCMOBJ_to_POINT(src,dst) \
     { ___err = SCMOBJ_to_POINT (src, &dst, ___RETURN_POS);
     #define ___END_SFUN_SCMOBJ_to_POINT(src,dst) }

     c-declare-end
     )

     (c-define-type point "point" "POINT_to_SCMOBJ" "SCMOBJ_to_POINT" #f)

     (define line-to (c-lambda (point) void "line_to"))
     (define get-mouse (c-lambda () point "get_mouse"))
     (define add-points (c-lambda (point point) point "add_points"))

     (c-define (write-point p) (point) void "write_point" ""
       (write p))

     (c-define (read-point) () point "read_point" ""
       (read))

   An example that requires memory allocation when converting from C to
Scheme and Scheme to C is shown below.  It is an interface to a
"null-terminated array of strings" type which is represented in Scheme
by a list of strings.  Note that some cleanup is necessary when
converting from Scheme to C.

     (c-declare #<<c-declare-end

     #include <stdlib.h>
     #include <unistd.h>

     extern char **environ;

     char **get_environ (void) { return environ; }

     void free_strings (char **strings)
     {
       char **ptr = strings;
       while (*ptr != NULL)
         {
           ___EXT(___release_string) (*ptr);
           ptr++;
         }
       free (strings);
     }

     ___SCMOBJ SCMOBJ_to_STRINGS (___SCMOBJ src, char ***dst, int arg_num)
     {
       /*
        * Src is a list of Scheme strings.  Dst will be a null terminated
        * array of C strings.
        */

       int i;
       ___SCMOBJ lst = src;
       int len = 4; /* start with a small result array */
       char **result = (char**) malloc (len * sizeof (char*));

       if (result == NULL)
         return ___FIX(___HEAP_OVERFLOW_ERR);

       i = 0;
       result[i] = NULL; /* always keep array null terminated */

       while (___PAIRP(lst))
         {
           ___SCMOBJ scm_str = ___CAR(lst);
           char *c_str;
           ___SCMOBJ ___err;

           if (i >= len-1) /* need to grow the result array? */
             {
               char **new_result;
               int j;

               len = len * 3 / 2;
               new_result = (char**) malloc (len * sizeof (char*));
               if (new_result == NULL)
                 {
                   free_strings (result);
                   return ___FIX(___HEAP_OVERFLOW_ERR);
                 }
               for (j=i; j>=0; j--)
                 new_result[j] = result[j];
               free (result);
               result = new_result;
             }

           ___err = ___EXT(___SCMOBJ_to_CHARSTRING) (scm_str, &c_str, arg_num);

           if (___err != ___FIX(___NO_ERR))
             {
               free_strings (result);
               return ___err;
             }

           result[i++] = c_str;
           result[i] = NULL;
           lst = ___CDR(lst);
         }

       if (!___NULLP(lst))
         {
           free_strings (result);
           return ___FIX(___UNKNOWN_ERR);
         }

       /*
        * Note that the caller is responsible for calling free_strings
        * when it is done with the result.
        */

       *dst = result;
       return ___FIX(___NO_ERR);
     }

     ___SCMOBJ STRINGS_to_SCMOBJ (char **src, ___SCMOBJ *dst, int arg_num)
     {
       ___SCMOBJ ___err = ___FIX(___NO_ERR);
       ___SCMOBJ result = ___NUL; /* start with the empty list */
       int i = 0;

       while (src[i] != NULL)
         i++;

       /* build the list of strings starting at the tail */

       while (--i >= 0)
         {
           ___SCMOBJ scm_str;
           ___SCMOBJ new_result;

           /*
            * Invariant: result is either the empty list or a ___STILL pair
            * with reference count equal to 1.  This is important because
            * it is possible that ___CHARSTRING_to_SCMOBJ and ___make_pair
            * will invoke the garbage collector and we don't want the
            * reference in result to become invalid (which would be the
            * case if result was a ___MOVABLE pair or if it had a zero
            * reference count).
            */

           ___err = ___EXT(___CHARSTRING_to_SCMOBJ) (src[i], &scm_str, arg_num);

           if (___err != ___FIX(___NO_ERR))
             {
               ___EXT(___release_scmobj) (result); /* allow GC to reclaim result */
               return ___FIX(___UNKNOWN_ERR);
             }

           /*
            * Note that scm_str will be a ___STILL object with reference
            * count equal to 1, so there is no risk that it will be
            * reclaimed or moved if ___make_pair invokes the garbage
            * collector.
            */

           new_result = ___EXT(___make_pair) (scm_str, result, ___STILL);

           /*
            * We can zero the reference count of scm_str and result (if
            * not the empty list) because the pair now references these
            * objects and the pair is reachable (it can't be reclaimed
            * or moved by the garbage collector).
            */

           ___EXT(___release_scmobj) (scm_str);
           ___EXT(___release_scmobj) (result);

           result = new_result;

           if (___FIXNUMP(result))
             return result; /* allocation failed */
         }

       /*
        * Note that result is either the empty list or a ___STILL pair
        * with a reference count equal to 1.  There will be a call to
        * ___release_scmobj later on (in ___END_CFUN_STRINGS_to_SCMOBJ
        * or ___END_SFUN_STRINGS_to_SCMOBJ) that will allow the garbage
        * collector to reclaim the whole list of strings when the Scheme
        * world no longer references it.
        */

       *dst = result;
       return ___FIX(___NO_ERR);
     }

     #define ___BEGIN_CFUN_SCMOBJ_to_STRINGS(src,dst,i) \
     if ((___err = SCMOBJ_to_STRINGS (src, &dst, i)) == ___FIX(___NO_ERR)) {
     #define ___END_CFUN_SCMOBJ_to_STRINGS(src,dst,i) \
     free_strings (dst); }

     #define ___BEGIN_CFUN_STRINGS_to_SCMOBJ(src,dst) \
     if ((___err = STRINGS_to_SCMOBJ (src, &dst, ___RETURN_POS)) == ___FIX(___NO_ERR)) {
     #define ___END_CFUN_STRINGS_to_SCMOBJ(src,dst) \
     ___EXT(___release_scmobj) (dst); }

     #define ___BEGIN_SFUN_STRINGS_to_SCMOBJ(src,dst,i) \
     if ((___err = STRINGS_to_SCMOBJ (src, &dst, i)) == ___FIX(___NO_ERR)) {
     #define ___END_SFUN_STRINGS_to_SCMOBJ(src,dst,i) \
     ___EXT(___release_scmobj) (dst); }

     #define ___BEGIN_SFUN_SCMOBJ_to_STRINGS(src,dst) \
     { ___err = SCMOBJ_to_STRINGS (src, &dst, ___RETURN_POS);
     #define ___END_SFUN_SCMOBJ_to_STRINGS(src,dst) }

     c-declare-end
     )

     (c-define-type char** "char**" "STRINGS_to_SCMOBJ" "SCMOBJ_to_STRINGS")

     (define execv (c-lambda (char-string char**) int "execv"))
     (define get-environ (c-lambda () char** "get_environ"))

     (c-define (write-strings x) (char**) void "write_strings" ""
       (write x))

     (c-define (read-strings) () char** "read_strings" ""
       (read))


File: gambit-c.info,  Node: continuations,  Prev: c-define-type,  Up: C-interface

19.7 Continuations and the C-interface
======================================

The C-interface allows C to Scheme calls to be nested.  This means that
during a call from C to Scheme another call from C to Scheme can be
performed.  This case occurs in the following program:

     (c-declare #<<c-declare-end

     int p (char *); /* forward declarations */
     int q (void);

     int a (char *x) { return 2 * p (x+1); }
     int b (short y) { return y + q (); }

     c-declare-end
     )

     (define a (c-lambda (char-string) int "a"))
     (define b (c-lambda (short) int "b"))

     (c-define (p z) (char-string) int "p" ""
       (+ (b 10) (string-length z)))

     (c-define (q) () int "q" ""
       123)

     (write (a "hello"))

   In this example, the main Scheme program calls the C function `a'
which calls the Scheme procedure `p' which in turn calls the C function
`b' which finally calls the Scheme procedure `q'.

   Gambit-C maintains the Scheme continuation separately from the C
stack, thus allowing the Scheme continuation to be unwound
independently from the C stack.  The C stack frame created for the C
function `f' is only removed from the C stack when control returns from
`f' or when control returns to a C function "above" `f'.  Special care
is required for programs which escape to Scheme (using first-class
continuations) from a Scheme to C (to Scheme) call because the C stack
frame will remain on the stack.  The C stack may overflow if this
happens in a loop with no intervening return to a C function.  To avoid
this problem make sure the C stack gets cleaned up by executing a normal
return from a Scheme to C call.


File: gambit-c.info,  Node: System limitations,  Next: Copyright and license,  Prev: C-interface,  Up: Top

20 System limitations
*********************

   * On some systems floating point overflows will cause the program to
     terminate with a floating point exception.

   * On some systems floating point operations involving `+nan.0'
     `+inf.0', `-inf.0', or `-0.' do not return the value required by
     the IEEE 754 floating point standard.

   * The compiler will not properly compile files with more than one
     definition (with `define') of the same procedure.  Replace all but
     the first `define' with assignments (`set!').

   * The maximum number of arguments that can be passed to a procedure
     by the `apply' procedure is 8192.



File: gambit-c.info,  Node: Copyright and license,  Next: General index,  Prev: System limitations,  Up: Top

21 Copyright and license
************************

The Gambit-C system is Copyright (C) 1994-2006 by Marc Feeley, all
rights reserved.  The Gambit-C system Version 4.0 beta 20 is licensed
under two licenses: the Apache License, Version 2.0, and the GNU LESSER
GENERAL PUBLIC LICENSE, Version 2.1.  You have the option to choose
which of these two licenses to abide by.  The licenses are copied below.


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                How to Apply These Terms to Your New Libraries

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     Also add information on how to contact you by electronic and paper mail.

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       <signature of Ty Coon>, 1 April 1990
       Ty Coon, President of Vice

     That's all there is to it!