Have `read-char' & co. throw to `decoding-error'.

[bpt/guile.git] / doc / ref / api-io.texi

@c -*-texinfo-*-
@c This is part of the GNU Guile Reference Manual.
@c Copyright (C)  1996, 1997, 2000, 2001, 2002, 2003, 2004, 2007, 2009,
@c   2010, 2011  Free Software Foundation, Inc.
@c See the file guile.texi for copying conditions.

@node Input and Output
@section Input and Output

@menu
* Ports::                       The idea of the port abstraction.
* Reading::                     Procedures for reading from a port.
* Writing::                     Procedures for writing to a port.
* Closing::                     Procedures to close a port.
* Random Access::               Moving around a random access port.
* Line/Delimited::              Read and write lines or delimited text.
* Block Reading and Writing::   Reading and writing blocks of text.
* Default Ports::               Defaults for input, output and errors.
* Port Types::                  Types of port and how to make them.
* R6RS I/O Ports::              The R6RS port API.
* I/O Extensions::              Using and extending ports in C.
@end menu


@node Ports
@subsection Ports
@cindex Port

Sequential input/output in Scheme is represented by operations on a
@dfn{port}.  This chapter explains the operations that Guile provides
for working with ports.

Ports are created by opening, for instance @code{open-file} for a file
(@pxref{File Ports}).  Characters can be read from an input port and
written to an output port, or both on an input/output port.  A port
can be closed (@pxref{Closing}) when no longer required, after which
any attempt to read or write is an error.

The formal definition of a port is very generic: an input port is
simply ``an object which can deliver characters on demand,'' and an
output port is ``an object which can accept characters.''  Because
this definition is so loose, it is easy to write functions that
simulate ports in software.  @dfn{Soft ports} and @dfn{string ports}
are two interesting and powerful examples of this technique.
(@pxref{Soft Ports}, and @ref{String Ports}.)

Ports are garbage collected in the usual way (@pxref{Memory
Management}), and will be closed at that time if not already closed.
In this case any errors occurring in the close will not be reported.
Usually a program will want to explicitly close so as to be sure all
its operations have been successful.  Of course if a program has
abandoned something due to an error or other condition then closing
problems are probably not of interest.

It is strongly recommended that file ports be closed explicitly when
no longer required.  Most systems have limits on how many files can be
open, both on a per-process and a system-wide basis.  A program that
uses many files should take care not to hit those limits.  The same
applies to similar system resources such as pipes and sockets.

Note that automatic garbage collection is triggered only by memory
consumption, not by file or other resource usage, so a program cannot
rely on that to keep it away from system limits.  An explicit call to
@code{gc} can of course be relied on to pick up unreferenced ports.
If program flow makes it hard to be certain when to close then this
may be an acceptable way to control resource usage.

All file access uses the ``LFS'' large file support functions when
available, so files bigger than 2 Gbytes (@math{2^31} bytes) can be
read and written on a 32-bit system.

Each port has an associated character encoding that controls how bytes
read from the port are converted to characters and string and controls
how characters and strings written to the port are converted to bytes.
When ports are created, they inherit their character encoding from the
current locale, but, that can be modified after the port is created.

Currently, the ports only work with @emph{non-modal} encodings.  Most
encodings are non-modal, meaning that the conversion of bytes to a
string doesn't depend on its context: the same byte sequence will always
return the same string.  A couple of modal encodings are in common use,
like ISO-2022-JP and ISO-2022-KR, and they are not yet supported.

Each port also has an associated conversion strategy: what to do when
a Guile character can't be converted to the port's encoded character
representation for output. There are three possible strategies: to
raise an error, to replace the character with a hex escape, or to
replace the character with a substitute character.

@rnindex input-port?
@deffn {Scheme Procedure} input-port? x
@deffnx {C Function} scm_input_port_p (x)
Return @code{#t} if @var{x} is an input port, otherwise return
@code{#f}.  Any object satisfying this predicate also satisfies
@code{port?}.
@end deffn

@rnindex output-port?
@deffn {Scheme Procedure} output-port? x
@deffnx {C Function} scm_output_port_p (x)
Return @code{#t} if @var{x} is an output port, otherwise return
@code{#f}.  Any object satisfying this predicate also satisfies
@code{port?}.
@end deffn

@deffn {Scheme Procedure} port? x
@deffnx {C Function} scm_port_p (x)
Return a boolean indicating whether @var{x} is a port.
Equivalent to @code{(or (input-port? @var{x}) (output-port?
@var{x}))}.
@end deffn

@deffn {Scheme Procedure} set-port-encoding! port enc
@deffnx {C Function} scm_set_port_encoding_x (port, enc)
Sets the character encoding that will be used to interpret all port I/O.
@var{enc} is a string containing the name of an encoding.  Valid
encoding names are those
@url{http://www.iana.org/assignments/character-sets, defined by IANA}.
@end deffn

@defvr {Scheme Variable} %default-port-encoding
A fluid containing @code{#f} or the name of the encoding to
be used by default for newly created ports (@pxref{Fluids and Dynamic
States}).  The value @code{#f} is equivalent to @code{"ISO-8859-1"}.

New ports are created with the encoding appropriate for the current
locale if @code{setlocale} has been called or the value specified by
this fluid otherwise.
@end defvr

@deffn {Scheme Procedure} port-encoding port
@deffnx {C Function} scm_port_encoding
Returns, as a string, the character encoding that @var{port} uses to interpret
its input and output.  The value @code{#f} is equivalent to @code{"ISO-8859-1"}.
@end deffn

@deffn {Scheme Procedure} set-port-conversion-strategy! port sym
@deffnx {C Function} scm_set_port_conversion_strategy_x (port, sym)
Sets the behavior of the interpreter when outputting a character that
is not representable in the port's current encoding.  @var{sym} can be
either @code{'error}, @code{'substitute}, or @code{'escape}.  If it is
@code{'error}, an error will be thrown when an nonconvertible character
is encountered.  If it is @code{'substitute}, then nonconvertible
characters will be replaced with approximate characters, or with
question marks if no approximately correct character is available.  If
it is @code{'escape}, it will appear as a hex escape when output.

If @var{port} is an open port, the conversion error behavior
is set for that port.  If it is @code{#f}, it is set as the
default behavior for any future ports that get created in
this thread.
@end deffn

@deffn {Scheme Procedure} port-conversion-strategy port
@deffnx {C Function} scm_port_conversion_strategy (port)
Returns the behavior of the port when outputting a character that is
not representable in the port's current encoding.  It returns the
symbol @code{error} if unrepresentable characters should cause
exceptions, @code{substitute} if the port should try to replace
unrepresentable characters with question marks or approximate
characters, or @code{escape} if unrepresentable characters should be
converted to string escapes.

If @var{port} is @code{#f}, then the current default behavior will be
returned.  New ports will have this default behavior when they are
created.
@end deffn



@node Reading
@subsection Reading
@cindex Reading

[Generic procedures for reading from ports.]

These procedures pertain to reading characters and strings from
ports. To read general S-expressions from ports, @xref{Scheme Read}.

@rnindex eof-object?
@cindex End of file object
@deffn {Scheme Procedure} eof-object? x
@deffnx {C Function} scm_eof_object_p (x)
Return @code{#t} if @var{x} is an end-of-file object; otherwise
return @code{#f}.
@end deffn

@rnindex char-ready?
@deffn {Scheme Procedure} char-ready? [port]
@deffnx {C Function} scm_char_ready_p (port)
Return @code{#t} if a character is ready on input @var{port}
and return @code{#f} otherwise.  If @code{char-ready?} returns
@code{#t} then the next @code{read-char} operation on
@var{port} is guaranteed not to hang.  If @var{port} is a file
port at end of file then @code{char-ready?} returns @code{#t}.

@code{char-ready?} exists to make it possible for a
program to accept characters from interactive ports without
getting stuck waiting for input.  Any input editors associated
with such ports must make sure that characters whose existence
has been asserted by @code{char-ready?} cannot be rubbed out.
If @code{char-ready?} were to return @code{#f} at end of file,
a port at end of file would be indistinguishable from an
interactive port that has no ready characters.
@end deffn

@rnindex read-char
@deffn {Scheme Procedure} read-char [port]
@deffnx {C Function} scm_read_char (port)
Return the next character available from @var{port}, updating
@var{port} to point to the following character.  If no more
characters are available, the end-of-file object is returned.

When @var{port}'s data cannot be decoded according to its
character encoding, a @code{decoding-error} is raised and
@var{port} points past the erroneous byte sequence.
@end deffn

@deftypefn {C Function} size_t scm_c_read (SCM port, void *buffer, size_t size)
Read up to @var{size} bytes from @var{port} and store them in
@var{buffer}.  The return value is the number of bytes actually read,
which can be less than @var{size} if end-of-file has been reached.

Note that this function does not update @code{port-line} and
@code{port-column} below.
@end deftypefn

@rnindex peek-char
@deffn {Scheme Procedure} peek-char [port]
@deffnx {C Function} scm_peek_char (port)
Return the next character available from @var{port},
@emph{without} updating @var{port} to point to the following
character.  If no more characters are available, the
end-of-file object is returned.

The value returned by
a call to @code{peek-char} is the same as the value that would
have been returned by a call to @code{read-char} on the same
port.  The only difference is that the very next call to
@code{read-char} or @code{peek-char} on that @var{port} will
return the value returned by the preceding call to
@code{peek-char}.  In particular, a call to @code{peek-char} on
an interactive port will hang waiting for input whenever a call
to @code{read-char} would have hung.

As for @code{read-char}, a @code{decoding-error} may be raised
if such a situation occurs.  However, unlike with @code{read-char},
@var{port} still points at the beginning of the erroneous byte
sequence when the error is raised.
@end deffn

@deffn {Scheme Procedure} unread-char cobj [port]
@deffnx {C Function} scm_unread_char (cobj, port)
Place @var{char} in @var{port} so that it will be read by the
next read operation.  If called multiple times, the unread characters
will be read again in last-in first-out order.  If @var{port} is
not supplied, the current input port is used.
@end deffn

@deffn {Scheme Procedure} unread-string str port
@deffnx {C Function} scm_unread_string (str, port)
Place the string @var{str} in @var{port} so that its characters will
be read from left-to-right as the next characters from @var{port}
during subsequent read operations.  If called multiple times, the
unread characters will be read again in last-in first-out order.  If
@var{port} is not supplied, the @code{current-input-port} is used.
@end deffn

@deffn {Scheme Procedure} drain-input port
@deffnx {C Function} scm_drain_input (port)
This procedure clears a port's input buffers, similar
to the way that force-output clears the output buffer.  The
contents of the buffers are returned as a single string, e.g.,

@lisp
(define p (open-input-file ...))
(drain-input p) => empty string, nothing buffered yet.
(unread-char (read-char p) p)
(drain-input p) => initial chars from p, up to the buffer size.
@end lisp

Draining the buffers may be useful for cleanly finishing
buffered I/O so that the file descriptor can be used directly
for further input.
@end deffn

@deffn {Scheme Procedure} port-column port
@deffnx {Scheme Procedure} port-line port
@deffnx {C Function} scm_port_column (port)
@deffnx {C Function} scm_port_line (port)
Return the current column number or line number of @var{port}.
If the number is
unknown, the result is #f.  Otherwise, the result is a 0-origin integer
- i.e.@: the first character of the first line is line 0, column 0.
(However, when you display a file position, for example in an error
message, we recommend you add 1 to get 1-origin integers.  This is
because lines and column numbers traditionally start with 1, and that is
what non-programmers will find most natural.)
@end deffn

@deffn {Scheme Procedure} set-port-column! port column
@deffnx {Scheme Procedure} set-port-line! port line
@deffnx {C Function} scm_set_port_column_x (port, column)
@deffnx {C Function} scm_set_port_line_x (port, line)
Set the current column or line number of @var{port}.
@end deffn

@node Writing
@subsection Writing
@cindex Writing

[Generic procedures for writing to ports.]

These procedures are for writing characters and strings to
ports. For more information on writing arbitrary Scheme objects to
ports, @xref{Scheme Write}.

@deffn {Scheme Procedure} get-print-state port
@deffnx {C Function} scm_get_print_state (port)
Return the print state of the port @var{port}.  If @var{port}
has no associated print state, @code{#f} is returned.
@end deffn

@rnindex newline
@deffn {Scheme Procedure} newline [port]
@deffnx {C Function} scm_newline (port)
Send a newline to @var{port}.
If @var{port} is omitted, send to the current output port.
@end deffn

@deffn {Scheme Procedure} port-with-print-state port [pstate]
@deffnx {C Function} scm_port_with_print_state (port, pstate)
Create a new port which behaves like @var{port}, but with an
included print state @var{pstate}.  @var{pstate} is optional.
If @var{pstate} isn't supplied and @var{port} already has
a print state, the old print state is reused.
@end deffn

@deffn {Scheme Procedure} print-options-interface [setting]
@deffnx {C Function} scm_print_options (setting)
Option interface for the print options. Instead of using
this procedure directly, use the procedures
@code{print-enable}, @code{print-disable}, @code{print-set!}
and @code{print-options}.
@end deffn

@deffn {Scheme Procedure} simple-format destination message . args
@deffnx {C Function} scm_simple_format (destination, message, args)
Write @var{message} to @var{destination}, defaulting to
the current output port.
@var{message} can contain @code{~A} (was @code{%s}) and
@code{~S} (was @code{%S}) escapes.  When printed,
the escapes are replaced with corresponding members of
@var{ARGS}:
@code{~A} formats using @code{display} and @code{~S} formats
using @code{write}.
If @var{destination} is @code{#t}, then use the current output
port, if @var{destination} is @code{#f}, then return a string
containing the formatted text. Does not add a trailing newline.
@end deffn

@rnindex write-char
@deffn {Scheme Procedure} write-char chr [port]
@deffnx {C Function} scm_write_char (chr, port)
Send character @var{chr} to @var{port}.
@end deffn

@deftypefn {C Function} void scm_c_write (SCM port, const void *buffer, size_t size)
Write @var{size} bytes at @var{buffer} to @var{port}.

Note that this function does not update @code{port-line} and
@code{port-column} (@pxref{Reading}).
@end deftypefn

@findex fflush
@deffn {Scheme Procedure} force-output [port]
@deffnx {C Function} scm_force_output (port)
Flush the specified output port, or the current output port if @var{port}
is omitted.  The current output buffer contents are passed to the
underlying port implementation (e.g., in the case of fports, the
data will be written to the file and the output buffer will be cleared.)
It has no effect on an unbuffered port.

The return value is unspecified.
@end deffn

@deffn {Scheme Procedure} flush-all-ports
@deffnx {C Function} scm_flush_all_ports ()
Equivalent to calling @code{force-output} on
all open output ports.  The return value is unspecified.
@end deffn


@node Closing
@subsection Closing
@cindex Closing ports
@cindex Port, close

@deffn {Scheme Procedure} close-port port
@deffnx {C Function} scm_close_port (port)
Close the specified port object.  Return @code{#t} if it
successfully closes a port or @code{#f} if it was already
closed.  An exception may be raised if an error occurs, for
example when flushing buffered output.  See also @ref{Ports and
File Descriptors, close}, for a procedure which can close file
descriptors.
@end deffn

@deffn {Scheme Procedure} close-input-port port
@deffnx {Scheme Procedure} close-output-port port
@deffnx {C Function} scm_close_input_port (port)
@deffnx {C Function} scm_close_output_port (port)
@rnindex close-input-port
@rnindex close-output-port
Close the specified input or output @var{port}.  An exception may be
raised if an error occurs while closing.  If @var{port} is already
closed, nothing is done.  The return value is unspecified.

See also @ref{Ports and File Descriptors, close}, for a procedure
which can close file descriptors.
@end deffn

@deffn {Scheme Procedure} port-closed? port
@deffnx {C Function} scm_port_closed_p (port)
Return @code{#t} if @var{port} is closed or @code{#f} if it is
open.
@end deffn


@node Random Access
@subsection Random Access
@cindex Random access, ports
@cindex Port, random access

@deffn {Scheme Procedure} seek fd_port offset whence
@deffnx {C Function} scm_seek (fd_port, offset, whence)
Sets the current position of @var{fd/port} to the integer
@var{offset}, which is interpreted according to the value of
@var{whence}.

One of the following variables should be supplied for
@var{whence}:
@defvar SEEK_SET
Seek from the beginning of the file.
@end defvar
@defvar SEEK_CUR
Seek from the current position.
@end defvar
@defvar SEEK_END
Seek from the end of the file.
@end defvar
If @var{fd/port} is a file descriptor, the underlying system
call is @code{lseek}.  @var{port} may be a string port.

The value returned is the new position in the file.  This means
that the current position of a port can be obtained using:
@lisp
(seek port 0 SEEK_CUR)
@end lisp
@end deffn

@deffn {Scheme Procedure} ftell fd_port
@deffnx {C Function} scm_ftell (fd_port)
Return an integer representing the current position of
@var{fd/port}, measured from the beginning.  Equivalent to:

@lisp
(seek port 0 SEEK_CUR)
@end lisp
@end deffn

@findex truncate
@findex ftruncate
@deffn {Scheme Procedure} truncate-file file [length]
@deffnx {C Function} scm_truncate_file (file, length)
Truncate @var{file} to @var{length} bytes.  @var{file} can be a
filename string, a port object, or an integer file descriptor.  The
return value is unspecified.

For a port or file descriptor @var{length} can be omitted, in which
case the file is truncated at the current position (per @code{ftell}
above).

On most systems a file can be extended by giving a length greater than
the current size, but this is not mandatory in the POSIX standard.
@end deffn

@node Line/Delimited
@subsection Line Oriented and Delimited Text
@cindex Line input/output
@cindex Port, line input/output

The delimited-I/O module can be accessed with:

@lisp
(use-modules (ice-9 rdelim))
@end lisp

It can be used to read or write lines of text, or read text delimited by
a specified set of characters.  It's similar to the @code{(scsh rdelim)}
module from guile-scsh, but does not use multiple values or character
sets and has an extra procedure @code{write-line}.

@c begin (scm-doc-string "rdelim.scm" "read-line")
@deffn {Scheme Procedure} read-line [port] [handle-delim]
Return a line of text from @var{port} if specified, otherwise from the
value returned by @code{(current-input-port)}.  Under Unix, a line of text
is terminated by the first end-of-line character or by end-of-file.

If @var{handle-delim} is specified, it should be one of the following
symbols:
@table @code
@item trim
Discard the terminating delimiter.  This is the default, but it will
be impossible to tell whether the read terminated with a delimiter or
end-of-file.
@item concat
Append the terminating delimiter (if any) to the returned string.
@item peek
Push the terminating delimiter (if any) back on to the port.
@item split
Return a pair containing the string read from the port and the
terminating delimiter or end-of-file object.
@end table

Like @code{read-char}, this procedure can throw to @code{decoding-error}
(@pxref{Reading, @code{read-char}}).
@end deffn

@c begin (scm-doc-string "rdelim.scm" "read-line!")
@deffn {Scheme Procedure} read-line! buf [port]
Read a line of text into the supplied string @var{buf} and return the
number of characters added to @var{buf}.  If @var{buf} is filled, then
@code{#f} is returned.
Read from @var{port} if
specified, otherwise from the value returned by @code{(current-input-port)}.
@end deffn

@c begin (scm-doc-string "rdelim.scm" "read-delimited")
@deffn {Scheme Procedure} read-delimited delims [port] [handle-delim]
Read text until one of the characters in the string @var{delims} is found
or end-of-file is reached.  Read from @var{port} if supplied, otherwise
from the value returned by @code{(current-input-port)}.
@var{handle-delim} takes the same values as described for @code{read-line}.
@end deffn

@c begin (scm-doc-string "rdelim.scm" "read-delimited!")
@deffn {Scheme Procedure} read-delimited! delims buf [port] [handle-delim] [start] [end]
Read text into the supplied string @var{buf}.

If a delimiter was found, return the number of characters written,
except if @var{handle-delim} is @code{split}, in which case the return
value is a pair, as noted above.

As a special case, if @var{port} was already at end-of-stream, the EOF
object is returned. Also, if no characters were written because the
buffer was full, @code{#f} is returned.

It's something of a wacky interface, to be honest.
@end deffn

@deffn {Scheme Procedure} write-line obj [port]
@deffnx {C Function} scm_write_line (obj, port)
Display @var{obj} and a newline character to @var{port}.  If
@var{port} is not specified, @code{(current-output-port)} is
used.  This function is equivalent to:
@lisp
(display obj [port])
(newline [port])
@end lisp
@end deffn

Some of the aforementioned I/O functions rely on the following C
primitives.  These will mainly be of interest to people hacking Guile
internals.

@deffn {Scheme Procedure} %read-delimited! delims str gobble [port [start [end]]]
@deffnx {C Function} scm_read_delimited_x (delims, str, gobble, port, start, end)
Read characters from @var{port} into @var{str} until one of the
characters in the @var{delims} string is encountered.  If
@var{gobble} is true, discard the delimiter character;
otherwise, leave it in the input stream for the next read.  If
@var{port} is not specified, use the value of
@code{(current-input-port)}.  If @var{start} or @var{end} are
specified, store data only into the substring of @var{str}
bounded by @var{start} and @var{end} (which default to the
beginning and end of the string, respectively).

 Return a pair consisting of the delimiter that terminated the
string and the number of characters read.  If reading stopped
at the end of file, the delimiter returned is the
@var{eof-object}; if the string was filled without encountering
a delimiter, this value is @code{#f}.
@end deffn

@deffn {Scheme Procedure} %read-line [port]
@deffnx {C Function} scm_read_line (port)
Read a newline-terminated line from @var{port}, allocating storage as
necessary.  The newline terminator (if any) is removed from the string,
and a pair consisting of the line and its delimiter is returned.  The
delimiter may be either a newline or the @var{eof-object}; if
@code{%read-line} is called at the end of file, it returns the pair
@code{(#<eof> . #<eof>)}.
@end deffn

@node Block Reading and Writing
@subsection Block reading and writing
@cindex Block read/write
@cindex Port, block read/write

The Block-string-I/O module can be accessed with:

@lisp
(use-modules (ice-9 rw))
@end lisp

It currently contains procedures that help to implement the
@code{(scsh rw)} module in guile-scsh.

@deffn {Scheme Procedure} read-string!/partial str [port_or_fdes [start [end]]]
@deffnx {C Function} scm_read_string_x_partial (str, port_or_fdes, start, end)
Read characters from a port or file descriptor into a
string @var{str}.  A port must have an underlying file
descriptor --- a so-called fport.  This procedure is
scsh-compatible and can efficiently read large strings.
It will:

@itemize
@item
attempt to fill the entire string, unless the @var{start}
and/or @var{end} arguments are supplied.  i.e., @var{start}
defaults to 0 and @var{end} defaults to
@code{(string-length str)}
@item
use the current input port if @var{port_or_fdes} is not
supplied.
@item
return fewer than the requested number of characters in some
cases, e.g., on end of file, if interrupted by a signal, or if
not all the characters are immediately available.
@item
wait indefinitely for some input if no characters are
currently available,
unless the port is in non-blocking mode.
@item
read characters from the port's input buffers if available,
instead from the underlying file descriptor.
@item
return @code{#f} if end-of-file is encountered before reading
any characters, otherwise return the number of characters
read.
@item
return 0 if the port is in non-blocking mode and no characters
are immediately available.
@item
return 0 if the request is for 0 bytes, with no
end-of-file check.
@end itemize
@end deffn

@deffn {Scheme Procedure} write-string/partial str [port_or_fdes [start [end]]]
@deffnx {C Function} scm_write_string_partial (str, port_or_fdes, start, end)
Write characters from a string @var{str} to a port or file
descriptor.  A port must have an underlying file descriptor
--- a so-called fport.  This procedure is
scsh-compatible and can efficiently write large strings.
It will:

@itemize
@item
attempt to write the entire string, unless the @var{start}
and/or @var{end} arguments are supplied.  i.e., @var{start}
defaults to 0 and @var{end} defaults to
@code{(string-length str)}
@item
use the current output port if @var{port_of_fdes} is not
supplied.
@item
in the case of a buffered port, store the characters in the
port's output buffer, if all will fit.  If they will not fit
then any existing buffered characters will be flushed
before attempting
to write the new characters directly to the underlying file
descriptor.  If the port is in non-blocking mode and
buffered characters can not be flushed immediately, then an
@code{EAGAIN} system-error exception will be raised (Note:
scsh does not support the use of non-blocking buffered ports.)
@item
write fewer than the requested number of
characters in some cases, e.g., if interrupted by a signal or
if not all of the output can be accepted immediately.
@item
wait indefinitely for at least one character
from @var{str} to be accepted by the port, unless the port is
in non-blocking mode.
@item
return the number of characters accepted by the port.
@item
return 0 if the port is in non-blocking mode and can not accept
at least one character from @var{str} immediately
@item
return 0 immediately if the request size is 0 bytes.
@end itemize
@end deffn

@node Default Ports
@subsection Default Ports for Input, Output and Errors
@cindex Default ports
@cindex Port, default

@rnindex current-input-port
@deffn {Scheme Procedure} current-input-port
@deffnx {C Function} scm_current_input_port ()
@cindex standard input
Return the current input port.  This is the default port used
by many input procedures.

Initially this is the @dfn{standard input} in Unix and C terminology.
When the standard input is a tty the port is unbuffered, otherwise
it's fully buffered.

Unbuffered input is good if an application runs an interactive
subprocess, since any type-ahead input won't go into Guile's buffer
and be unavailable to the subprocess.

Note that Guile buffering is completely separate from the tty ``line
discipline''.  In the usual cooked mode on a tty Guile only sees a
line of input once the user presses @key{Return}.
@end deffn

@rnindex current-output-port
@deffn {Scheme Procedure} current-output-port
@deffnx {C Function} scm_current_output_port ()
@cindex standard output
Return the current output port.  This is the default port used
by many output procedures.

Initially this is the @dfn{standard output} in Unix and C terminology.
When the standard output is a tty this port is unbuffered, otherwise
it's fully buffered.

Unbuffered output to a tty is good for ensuring progress output or a
prompt is seen.  But an application which always prints whole lines
could change to line buffered, or an application with a lot of output
could go fully buffered and perhaps make explicit @code{force-output}
calls (@pxref{Writing}) at selected points.
@end deffn

@deffn {Scheme Procedure} current-error-port
@deffnx {C Function} scm_current_error_port ()
@cindex standard error output
Return the port to which errors and warnings should be sent.

Initially this is the @dfn{standard error} in Unix and C terminology.
When the standard error is a tty this port is unbuffered, otherwise
it's fully buffered.
@end deffn

@deffn {Scheme Procedure} set-current-input-port port
@deffnx {Scheme Procedure} set-current-output-port port
@deffnx {Scheme Procedure} set-current-error-port port
@deffnx {C Function} scm_set_current_input_port (port)
@deffnx {C Function} scm_set_current_output_port (port)
@deffnx {C Function} scm_set_current_error_port (port)
Change the ports returned by @code{current-input-port},
@code{current-output-port} and @code{current-error-port}, respectively,
so that they use the supplied @var{port} for input or output.
@end deffn

@deftypefn {C Function} void scm_dynwind_current_input_port (SCM port)
@deftypefnx {C Function} void scm_dynwind_current_output_port (SCM port)
@deftypefnx {C Function} void scm_dynwind_current_error_port (SCM port)
These functions must be used inside a pair of calls to
@code{scm_dynwind_begin} and @code{scm_dynwind_end} (@pxref{Dynamic
Wind}).  During the dynwind context, the indicated port is set to
@var{port}.

More precisely, the current port is swapped with a `backup' value
whenever the dynwind context is entered or left.  The backup value is
initialized with the @var{port} argument.
@end deftypefn

@node Port Types
@subsection Types of Port
@cindex Types of ports
@cindex Port, types

[Types of port; how to make them.]

@menu
* File Ports:: Ports on an operating system file.
* String Ports:: Ports on a Scheme string.
* Soft Ports:: Ports on arbitrary Scheme procedures.
* Void Ports:: Ports on nothing at all.
@end menu


@node File Ports
@subsubsection File Ports
@cindex File port
@cindex Port, file

The following procedures are used to open file ports.
See also @ref{Ports and File Descriptors, open}, for an interface
to the Unix @code{open} system call.

Most systems have limits on how many files can be open, so it's
strongly recommended that file ports be closed explicitly when no
longer required (@pxref{Ports}).

@deffn {Scheme Procedure} open-file filename mode
@deffnx {C Function} scm_open_file (filename, mode)
Open the file whose name is @var{filename}, and return a port
representing that file.  The attributes of the port are
determined by the @var{mode} string.  The way in which this is
interpreted is similar to C stdio.  The first character must be
one of the following:

@table @samp
@item r
Open an existing file for input.
@item w
Open a file for output, creating it if it doesn't already exist
or removing its contents if it does.
@item a
Open a file for output, creating it if it doesn't already
exist.  All writes to the port will go to the end of the file.
The "append mode" can be turned off while the port is in use
@pxref{Ports and File Descriptors, fcntl}
@end table

The following additional characters can be appended:

@table @samp
@item +
Open the port for both input and output.  E.g., @code{r+}: open
an existing file for both input and output.
@item 0
Create an "unbuffered" port.  In this case input and output
operations are passed directly to the underlying port
implementation without additional buffering.  This is likely to
slow down I/O operations.  The buffering mode can be changed
while a port is in use @pxref{Ports and File Descriptors,
setvbuf}
@item l
Add line-buffering to the port.  The port output buffer will be
automatically flushed whenever a newline character is written.
@item b
Use binary mode.  On DOS systems the default text mode converts CR+LF
in the file to newline for the program, whereas binary mode reads and
writes all bytes unchanged.  On Unix-like systems there is no such
distinction, text files already contain just newlines and no
conversion is ever made.  The @code{b} flag is accepted on all
systems, but has no effect on Unix-like systems.

(For reference, Guile leaves text versus binary up to the C library,
@code{b} here just adds @code{O_BINARY} to the underlying @code{open}
call, when that flag is available.)

Also, open the file using the 8-bit character encoding "ISO-8859-1",
ignoring any coding declaration or port encoding.

Note that, when reading or writing binary data with ports, the
bytevector ports in the @code{(rnrs io ports)} module are preferred,
as they return vectors, and not strings (@pxref{R6RS I/O Ports}).
@end table

If a file cannot be opened with the access
requested, @code{open-file} throws an exception.

When the file is opened, this procedure will scan for a coding
declaration (@pxref{Character Encoding of Source Files}). If present
will use that encoding for interpreting the file.  Otherwise, the
port's encoding will be used.  To supress this behavior, open
the file in binary mode and then set the port encoding explicitly
using @code{set-port-encoding!}.

In theory we could create read/write ports which were buffered
in one direction only.  However this isn't included in the
current interfaces.
@end deffn

@rnindex open-input-file
@deffn {Scheme Procedure} open-input-file filename
Open @var{filename} for input.  Equivalent to
@lisp
(open-file @var{filename} "r")
@end lisp
@end deffn

@rnindex open-output-file
@deffn {Scheme Procedure} open-output-file filename
Open @var{filename} for output.  Equivalent to
@lisp
(open-file @var{filename} "w")
@end lisp
@end deffn

@deffn {Scheme Procedure} call-with-input-file filename proc
@deffnx {Scheme Procedure} call-with-output-file filename proc
@rnindex call-with-input-file
@rnindex call-with-output-file
Open @var{filename} for input or output, and call @code{(@var{proc}
port)} with the resulting port.  Return the value returned by
@var{proc}.  @var{filename} is opened as per @code{open-input-file} or
@code{open-output-file} respectively, and an error is signaled if it
cannot be opened.

When @var{proc} returns, the port is closed.  If @var{proc} does not
return (e.g.@: if it throws an error), then the port might not be
closed automatically, though it will be garbage collected in the usual
way if not otherwise referenced.
@end deffn

@deffn {Scheme Procedure} with-input-from-file filename thunk
@deffnx {Scheme Procedure} with-output-to-file filename thunk
@deffnx {Scheme Procedure} with-error-to-file filename thunk
@rnindex with-input-from-file
@rnindex with-output-to-file
Open @var{filename} and call @code{(@var{thunk})} with the new port
setup as respectively the @code{current-input-port},
@code{current-output-port}, or @code{current-error-port}.  Return the
value returned by @var{thunk}.  @var{filename} is opened as per
@code{open-input-file} or @code{open-output-file} respectively, and an
error is signaled if it cannot be opened.

When @var{thunk} returns, the port is closed and the previous setting
of the respective current port is restored.

The current port setting is managed with @code{dynamic-wind}, so the
previous value is restored no matter how @var{thunk} exits (eg.@: an
exception), and if @var{thunk} is re-entered (via a captured
continuation) then it's set again to the @var{FILENAME} port.

The port is closed when @var{thunk} returns normally, but not when
exited via an exception or new continuation.  This ensures it's still
ready for use if @var{thunk} is re-entered by a captured continuation.
Of course the port is always garbage collected and closed in the usual
way when no longer referenced anywhere.
@end deffn

@deffn {Scheme Procedure} port-mode port
@deffnx {C Function} scm_port_mode (port)
Return the port modes associated with the open port @var{port}.
These will not necessarily be identical to the modes used when
the port was opened, since modes such as "append" which are
used only during port creation are not retained.
@end deffn

@deffn {Scheme Procedure} port-filename port
@deffnx {C Function} scm_port_filename (port)
Return the filename associated with @var{port}.  This function returns
the strings "standard input", "standard output" and "standard error"
when called on the current input, output and error ports respectively.

@var{port} must be open, @code{port-filename} cannot be used once the
port is closed.
@end deffn

@deffn {Scheme Procedure} set-port-filename! port filename
@deffnx {C Function} scm_set_port_filename_x (port, filename)
Change the filename associated with @var{port}, using the current input
port if none is specified.  Note that this does not change the port's
source of data, but only the value that is returned by
@code{port-filename} and reported in diagnostic output.
@end deffn

@deffn {Scheme Procedure} file-port? obj
@deffnx {C Function} scm_file_port_p (obj)
Determine whether @var{obj} is a port that is related to a file.
@end deffn


@node String Ports
@subsubsection String Ports
@cindex String port
@cindex Port, string

The following allow string ports to be opened by analogy to R4R*
file port facilities:

With string ports, the port-encoding is treated differently than other
types of ports.  When string ports are created, they do not inherit a
character encoding from the current locale.  They are given a
default locale that allows them to handle all valid string characters.
Typically one should not modify a string port's character encoding
away from its default.

@deffn {Scheme Procedure} call-with-output-string proc
@deffnx {C Function} scm_call_with_output_string (proc)
Calls the one-argument procedure @var{proc} with a newly created output
port.  When the function returns, the string composed of the characters
written into the port is returned.  @var{proc} should not close the port.

Note that which characters can be written to a string port depend on the port's
encoding.  The default encoding of string ports is specified by the
@code{%default-port-encoding} fluid (@pxref{Ports,
@code{%default-port-encoding}}).  For instance, it is an error to write Greek
letter alpha to an ISO-8859-1-encoded string port since this character cannot be
represented with ISO-8859-1:

@example
(define alpha (integer->char #x03b1)) ; GREEK SMALL LETTER ALPHA

(with-fluids ((%default-port-encoding "ISO-8859-1"))
  (call-with-output-string
    (lambda (p)
      (display alpha p))))

@result{}
Throw to key `encoding-error'
@end example

Changing the string port's encoding to a Unicode-capable encoding such as UTF-8
solves the problem.
@end deffn

@deffn {Scheme Procedure} call-with-input-string string proc
@deffnx {C Function} scm_call_with_input_string (string, proc)
Calls the one-argument procedure @var{proc} with a newly
created input port from which @var{string}'s contents may be
read.  The value yielded by the @var{proc} is returned.
@end deffn

@deffn {Scheme Procedure} with-output-to-string thunk
Calls the zero-argument procedure @var{thunk} with the current output
port set temporarily to a new string port.  It returns a string
composed of the characters written to the current output.

See @code{call-with-output-string} above for character encoding considerations.
@end deffn

@deffn {Scheme Procedure} with-input-from-string string thunk
Calls the zero-argument procedure @var{thunk} with the current input
port set temporarily to a string port opened on the specified
@var{string}.  The value yielded by @var{thunk} is returned.
@end deffn

@deffn {Scheme Procedure} open-input-string str
@deffnx {C Function} scm_open_input_string (str)
Take a string and return an input port that delivers characters
from the string. The port can be closed by
@code{close-input-port}, though its storage will be reclaimed
by the garbage collector if it becomes inaccessible.
@end deffn

@deffn {Scheme Procedure} open-output-string
@deffnx {C Function} scm_open_output_string ()
Return an output port that will accumulate characters for
retrieval by @code{get-output-string}. The port can be closed
by the procedure @code{close-output-port}, though its storage
will be reclaimed by the garbage collector if it becomes
inaccessible.
@end deffn

@deffn {Scheme Procedure} get-output-string port
@deffnx {C Function} scm_get_output_string (port)
Given an output port created by @code{open-output-string},
return a string consisting of the characters that have been
output to the port so far.

@code{get-output-string} must be used before closing @var{port}, once
closed the string cannot be obtained.
@end deffn

A string port can be used in many procedures which accept a port
but which are not dependent on implementation details of fports.
E.g., seeking and truncating will work on a string port,
but trying to extract the file descriptor number will fail.


@node Soft Ports
@subsubsection Soft Ports
@cindex Soft port
@cindex Port, soft

A @dfn{soft-port} is a port based on a vector of procedures capable of
accepting or delivering characters.  It allows emulation of I/O ports.

@deffn {Scheme Procedure} make-soft-port pv modes
@deffnx {C Function} scm_make_soft_port (pv, modes)
Return a port capable of receiving or delivering characters as
specified by the @var{modes} string (@pxref{File Ports,
open-file}).  @var{pv} must be a vector of length 5 or 6.  Its
components are as follows:

@enumerate 0
@item
procedure accepting one character for output
@item
procedure accepting a string for output
@item
thunk for flushing output
@item
thunk for getting one character
@item
thunk for closing port (not by garbage collection)
@item
(if present and not @code{#f}) thunk for computing the number of
characters that can be read from the port without blocking.
@end enumerate

For an output-only port only elements 0, 1, 2, and 4 need be
procedures.  For an input-only port only elements 3 and 4 need
be procedures.  Thunks 2 and 4 can instead be @code{#f} if
there is no useful operation for them to perform.

If thunk 3 returns @code{#f} or an @code{eof-object}
(@pxref{Input, eof-object?, ,r5rs, The Revised^5 Report on
Scheme}) it indicates that the port has reached end-of-file.
For example:

@lisp
(define stdout (current-output-port))
(define p (make-soft-port
           (vector
            (lambda (c) (write c stdout))
            (lambda (s) (display s stdout))
            (lambda () (display "." stdout))
            (lambda () (char-upcase (read-char)))
            (lambda () (display "@@" stdout)))
           "rw"))

(write p p) @result{} #<input-output: soft 8081e20>
@end lisp
@end deffn


@node Void Ports
@subsubsection Void Ports
@cindex Void port
@cindex Port, void

This kind of port causes any data to be discarded when written to, and
always returns the end-of-file object when read from.

@deffn {Scheme Procedure} %make-void-port mode
@deffnx {C Function} scm_sys_make_void_port (mode)
Create and return a new void port.  A void port acts like
@file{/dev/null}.  The @var{mode} argument
specifies the input/output modes for this port: see the
documentation for @code{open-file} in @ref{File Ports}.
@end deffn


@node R6RS I/O Ports
@subsection R6RS I/O Ports

@cindex R6RS
@cindex R6RS ports

The I/O port API of the @uref{http://www.r6rs.org/, Revised Report^6 on
the Algorithmic Language Scheme (R6RS)} is provided by the @code{(rnrs
io ports)} module.  It provides features, such as binary I/O and Unicode
string I/O, that complement or refine Guile's historical port API
presented above (@pxref{Input and Output}).

@c FIXME: Update description when implemented.
@emph{Note}: The implementation of this R6RS API is currently far from
complete, notably due to the lack of support for Unicode I/O and strings.

@menu
* R6RS End-of-File::            The end-of-file object.
* R6RS Port Manipulation::      Manipulating R6RS ports.
* R6RS Binary Input::           Binary input.
* R6RS Binary Output::          Binary output.
@end menu

@node R6RS End-of-File
@subsubsection The End-of-File Object

@cindex EOF
@cindex end-of-file

R5RS' @code{eof-object?} procedure is provided by the @code{(rnrs io
ports)} module:

@deffn {Scheme Procedure} eof-object? obj
@deffnx {C Function} scm_eof_object_p (obj)
Return true if @var{obj} is the end-of-file (EOF) object.
@end deffn

In addition, the following procedure is provided:

@deffn {Scheme Procedure} eof-object
@deffnx {C Function} scm_eof_object ()
Return the end-of-file (EOF) object.

@lisp
(eof-object? (eof-object))
@result{} #t
@end lisp
@end deffn


@node R6RS Port Manipulation
@subsubsection Port Manipulation

The procedures listed below operate on any kind of R6RS I/O port.

@deffn {Scheme Procedure} port-position port
If @var{port} supports it (see below), return the offset (an integer)
indicating where the next octet will be read from/written to in
@var{port}.  If @var{port} does not support this operation, an error
condition is raised.

This is similar to Guile's @code{seek} procedure with the
@code{SEEK_CUR} argument (@pxref{Random Access}).
@end deffn

@deffn {Scheme Procedure} port-has-port-position? port
Return @code{#t} is @var{port} supports @code{port-position}.
@end deffn

@deffn {Scheme Procedure} set-port-position! port offset
If @var{port} supports it (see below), set the position where the next
octet will be read from/written to @var{port} to @var{offset} (an
integer).  If @var{port} does not support this operation, an error
condition is raised.

This is similar to Guile's @code{seek} procedure with the
@code{SEEK_SET} argument (@pxref{Random Access}).
@end deffn

@deffn {Scheme Procedure} port-has-set-port-position!? port
Return @code{#t} is @var{port} supports @code{set-port-position!}.
@end deffn

@deffn {Scheme Procedure} call-with-port port proc
Call @var{proc}, passing it @var{port} and closing @var{port} upon exit
of @var{proc}.  Return the return values of @var{proc}.
@end deffn


@node R6RS Binary Input
@subsubsection Binary Input

@cindex binary input

R6RS binary input ports can be created with the procedures described
below.

@deffn {Scheme Procedure} open-bytevector-input-port bv [transcoder]
@deffnx {C Function} scm_open_bytevector_input_port (bv, transcoder)
Return an input port whose contents are drawn from bytevector @var{bv}
(@pxref{Bytevectors}).

@c FIXME: Update description when implemented.
The @var{transcoder} argument is currently not supported.
@end deffn

@cindex custom binary input ports

@deffn {Scheme Procedure} make-custom-binary-input-port id read! get-position set-position! close
@deffnx {C Function} scm_make_custom_binary_input_port (id, read!, get-position, set-position!, close)
Return a new custom binary input port@footnote{This is similar in spirit
to Guile's @dfn{soft ports} (@pxref{Soft Ports}).} named @var{id} (a
string) whose input is drained by invoking @var{read!} and passing it a
bytevector, an index where bytes should be written, and the number of
bytes to read.  The @code{read!}  procedure must return an integer
indicating the number of bytes read, or @code{0} to indicate the
end-of-file.

Optionally, if @var{get-position} is not @code{#f}, it must be a thunk
that will be called when @var{port-position} is invoked on the custom
binary port and should return an integer indicating the position within
the underlying data stream; if @var{get-position} was not supplied, the
returned port does not support @var{port-position}.

Likewise, if @var{set-position!} is not @code{#f}, it should be a
one-argument procedure.  When @var{set-port-position!} is invoked on the
custom binary input port, @var{set-position!} is passed an integer
indicating the position of the next byte is to read.

Finally, if @var{close} is not @code{#f}, it must be a thunk.  It is
invoked when the custom binary input port is closed.

Using a custom binary input port, the @code{open-bytevector-input-port}
procedure could be implemented as follows:

@lisp
(define (open-bytevector-input-port source)
  (define position 0)
  (define length (bytevector-length source))

  (define (read! bv start count)
    (let ((count (min count (- length position))))
      (bytevector-copy! source position
                        bv start count)
      (set! position (+ position count))
      count))

  (define (get-position) position)

  (define (set-position! new-position)
    (set! position new-position))

  (make-custom-binary-input-port "the port" read!
                                  get-position
                                  set-position!))

(read (open-bytevector-input-port (string->utf8 "hello")))
@result{} hello
@end lisp
@end deffn

@cindex binary input
Binary input is achieved using the procedures below:

@deffn {Scheme Procedure} get-u8 port
@deffnx {C Function} scm_get_u8 (port)
Return an octet read from @var{port}, a binary input port, blocking as
necessary, or the end-of-file object.
@end deffn

@deffn {Scheme Procedure} lookahead-u8 port
@deffnx {C Function} scm_lookahead_u8 (port)
Like @code{get-u8} but does not update @var{port}'s position to point
past the octet.
@end deffn

@deffn {Scheme Procedure} get-bytevector-n port count
@deffnx {C Function} scm_get_bytevector_n (port, count)
Read @var{count} octets from @var{port}, blocking as necessary and
return a bytevector containing the octets read.  If fewer bytes are
available, a bytevector smaller than @var{count} is returned.
@end deffn

@deffn {Scheme Procedure} get-bytevector-n! port bv start count
@deffnx {C Function} scm_get_bytevector_n_x (port, bv, start, count)
Read @var{count} bytes from @var{port} and store them in @var{bv}
starting at index @var{start}.  Return either the number of bytes
actually read or the end-of-file object.
@end deffn

@deffn {Scheme Procedure} get-bytevector-some port
@deffnx {C Function} scm_get_bytevector_some (port)
Read from @var{port}, blocking as necessary, until data are available or
and end-of-file is reached.  Return either a new bytevector containing
the data read or the end-of-file object.
@end deffn

@deffn {Scheme Procedure} get-bytevector-all port
@deffnx {C Function} scm_get_bytevector_all (port)
Read from @var{port}, blocking as necessary, until the end-of-file is
reached.  Return either a new bytevector containing the data read or the
end-of-file object (if no data were available).
@end deffn

@node R6RS Binary Output
@subsubsection Binary Output

Binary output ports can be created with the procedures below.

@deffn {Scheme Procedure} open-bytevector-output-port [transcoder]
@deffnx {C Function} scm_open_bytevector_output_port (transcoder)
Return two values: a binary output port and a procedure.  The latter
should be called with zero arguments to obtain a bytevector containing
the data accumulated by the port, as illustrated below.

@lisp
(call-with-values
  (lambda ()
    (open-bytevector-output-port))
  (lambda (port get-bytevector)
    (display "hello" port)
    (get-bytevector)))

@result{} #vu8(104 101 108 108 111)
@end lisp

@c FIXME: Update description when implemented.
The @var{transcoder} argument is currently not supported.
@end deffn

@cindex custom binary output ports

@deffn {Scheme Procedure} make-custom-binary-output-port id write! get-position set-position! close
@deffnx {C Function} scm_make_custom_binary_output_port (id, write!, get-position, set-position!, close)
Return a new custom binary output port named @var{id} (a string) whose
output is sunk by invoking @var{write!} and passing it a bytevector, an
index where bytes should be read from this bytevector, and the number of
bytes to be ``written''.  The @code{write!}  procedure must return an
integer indicating the number of bytes actually written; when it is
passed @code{0} as the number of bytes to write, it should behave as
though an end-of-file was sent to the byte sink.

The other arguments are as for @code{make-custom-binary-input-port}
(@pxref{R6RS Binary Input, @code{make-custom-binary-input-port}}).
@end deffn

@cindex binary output
Writing to a binary output port can be done using the following
procedures:

@deffn {Scheme Procedure} put-u8 port octet
@deffnx {C Function} scm_put_u8 (port, octet)
Write @var{octet}, an integer in the 0--255 range, to @var{port}, a
binary output port.
@end deffn

@deffn {Scheme Procedure} put-bytevector port bv [start [count]]
@deffnx {C Function} scm_put_bytevector (port, bv, start, count)
Write the contents of @var{bv} to @var{port}, optionally starting at
index @var{start} and limiting to @var{count} octets.
@end deffn


@node I/O Extensions
@subsection Using and Extending Ports in C

@menu
* C Port Interface:: Using ports from C.
* Port Implementation:: How to implement a new port type in C.
@end menu


@node C Port Interface
@subsubsection C Port Interface
@cindex C port interface
@cindex Port, C interface

This section describes how to use Scheme ports from C.

@subsubheading Port basics

@cindex ptob
@tindex scm_ptob_descriptor
@tindex scm_port
@findex SCM_PTAB_ENTRY
@findex SCM_PTOBNUM
@vindex scm_ptobs
There are two main data structures.  A port type object (ptob) is of
type @code{scm_ptob_descriptor}.  A port instance is of type
@code{scm_port}.  Given an @code{SCM} variable which points to a port,
the corresponding C port object can be obtained using the
@code{SCM_PTAB_ENTRY} macro.  The ptob can be obtained by using
@code{SCM_PTOBNUM} to give an index into the @code{scm_ptobs}
global array.

@subsubheading Port buffers

An input port always has a read buffer and an output port always has a
write buffer.  However the size of these buffers is not guaranteed to be
more than one byte (e.g., the @code{shortbuf} field in @code{scm_port}
which is used when no other buffer is allocated).  The way in which the
buffers are allocated depends on the implementation of the ptob.  For
example in the case of an fport, buffers may be allocated with malloc
when the port is created, but in the case of an strport the underlying
string is used as the buffer.

@subsubheading The @code{rw_random} flag

Special treatment is required for ports which can be seeked at random.
Before various operations, such as seeking the port or changing from
input to output on a bidirectional port or vice versa, the port
implementation must be given a chance to update its state.  The write
buffer is updated by calling the @code{flush} ptob procedure and the
input buffer is updated by calling the @code{end_input} ptob procedure.
In the case of an fport, @code{flush} causes buffered output to be
written to the file descriptor, while @code{end_input} causes the
descriptor position to be adjusted to account for buffered input which
was never read.

The special treatment must be performed if the @code{rw_random} flag in
the port is non-zero.

@subsubheading The @code{rw_active} variable

The @code{rw_active} variable in the port is only used if
@code{rw_random} is set.  It's defined as an enum with the following
values:

@table @code
@item SCM_PORT_READ
the read buffer may have unread data.

@item SCM_PORT_WRITE
the write buffer may have unwritten data.

@item SCM_PORT_NEITHER
neither the write nor the read buffer has data.
@end table

@subsubheading Reading from a port.

To read from a port, it's possible to either call existing libguile
procedures such as @code{scm_getc} and @code{scm_read_line} or to read
data from the read buffer directly.  Reading from the buffer involves
the following steps:

@enumerate
@item
Flush output on the port, if @code{rw_active} is @code{SCM_PORT_WRITE}.

@item
Fill the read buffer, if it's empty, using @code{scm_fill_input}.

@item Read the data from the buffer and update the read position in
the buffer.  Steps 2) and 3) may be repeated as many times as required.

@item Set rw_active to @code{SCM_PORT_READ} if @code{rw_random} is set.

@item update the port's line and column counts.
@end enumerate

@subsubheading Writing to a port.

To write data to a port, calling @code{scm_lfwrite} should be sufficient for
most purposes.  This takes care of the following steps:

@enumerate
@item
End input on the port, if @code{rw_active} is @code{SCM_PORT_READ}.

@item
Pass the data to the ptob implementation using the @code{write} ptob
procedure.  The advantage of using the ptob @code{write} instead of
manipulating the write buffer directly is that it allows the data to be
written in one operation even if the port is using the single-byte
@code{shortbuf}.

@item
Set @code{rw_active} to @code{SCM_PORT_WRITE} if @code{rw_random}
is set.
@end enumerate


@node Port Implementation
@subsubsection Port Implementation
@cindex Port implementation

This section describes how to implement a new port type in C.

As described in the previous section, a port type object (ptob) is
a structure of type @code{scm_ptob_descriptor}.  A ptob is created by
calling @code{scm_make_port_type}.

@deftypefun scm_t_bits scm_make_port_type (char *name, int (*fill_input) (SCM port), void (*write) (SCM port, const void *data, size_t size))
Return a new port type object.  The @var{name}, @var{fill_input} and
@var{write} parameters are initial values for those port type fields,
as described below.  The other fields are initialized with default
values and can be changed later.
@end deftypefun

All of the elements of the ptob, apart from @code{name}, are procedures
which collectively implement the port behaviour.  Creating a new port
type mostly involves writing these procedures.

@table @code
@item name
A pointer to a NUL terminated string: the name of the port type.  This
is the only element of @code{scm_ptob_descriptor} which is not
a procedure.  Set via the first argument to @code{scm_make_port_type}.

@item mark
Called during garbage collection to mark any SCM objects that a port
object may contain.  It doesn't need to be set unless the port has
@code{SCM} components.  Set using

@deftypefun void scm_set_port_mark (scm_t_bits tc, SCM (*mark) (SCM port))
@end deftypefun

@item free
Called when the port is collected during gc.  It
should free any resources used by the port.
Set using

@deftypefun void scm_set_port_free (scm_t_bits tc, size_t (*free) (SCM port))
@end deftypefun

@item print
Called when @code{write} is called on the port object, to print a
port description.  E.g., for an fport it may produce something like:
@code{#<input: /etc/passwd 3>}.   Set using

@deftypefun void scm_set_port_print (scm_t_bits tc, int (*print) (SCM port, SCM dest_port, scm_print_state *pstate))
The first argument @var{port} is the object being printed, the second
argument @var{dest_port} is where its description should go.
@end deftypefun

@item equalp
Not used at present.  Set using

@deftypefun void scm_set_port_equalp (scm_t_bits tc, SCM (*equalp) (SCM, SCM))
@end deftypefun

@item close
Called when the port is closed, unless it was collected during gc.  It
should free any resources used by the port.
Set using

@deftypefun void scm_set_port_close (scm_t_bits tc, int (*close) (SCM port))
@end deftypefun

@item write
Accept data which is to be written using the port.  The port implementation
may choose to buffer the data instead of processing it directly.
Set via the third argument to @code{scm_make_port_type}.

@item flush
Complete the processing of buffered output data.  Reset the value of
@code{rw_active} to @code{SCM_PORT_NEITHER}.
Set using

@deftypefun void scm_set_port_flush (scm_t_bits tc, void (*flush) (SCM port))
@end deftypefun

@item end_input
Perform any synchronization required when switching from input to output
on the port.  Reset the value of @code{rw_active} to @code{SCM_PORT_NEITHER}.
Set using

@deftypefun void scm_set_port_end_input (scm_t_bits tc, void (*end_input) (SCM port, int offset))
@end deftypefun

@item fill_input
Read new data into the read buffer and return the first character.  It
can be assumed that the read buffer is empty when this procedure is called.
Set via the second argument to @code{scm_make_port_type}.

@item input_waiting
Return a lower bound on the number of bytes that could be read from the
port without blocking.  It can be assumed that the current state of
@code{rw_active} is @code{SCM_PORT_NEITHER}.
Set using

@deftypefun void scm_set_port_input_waiting (scm_t_bits tc, int (*input_waiting) (SCM port))
@end deftypefun

@item seek
Set the current position of the port.  The procedure can not make
any assumptions about the value of @code{rw_active} when it's
called.  It can reset the buffers first if desired by using something
like:

@example
if (pt->rw_active == SCM_PORT_READ)
  scm_end_input (port);
else if (pt->rw_active == SCM_PORT_WRITE)
  ptob->flush (port);
@end example

However note that this will have the side effect of discarding any data
in the unread-char buffer, in addition to any side effects from the
@code{end_input} and @code{flush} ptob procedures.  This is undesirable
when seek is called to measure the current position of the port, i.e.,
@code{(seek p 0 SEEK_CUR)}.  The libguile fport and string port
implementations take care to avoid this problem.

The procedure is set using

@deftypefun void scm_set_port_seek (scm_t_bits tc, scm_t_off (*seek) (SCM port, scm_t_off offset, int whence))
@end deftypefun

@item truncate
Truncate the port data to be specified length.  It can be assumed that the
current state of @code{rw_active} is @code{SCM_PORT_NEITHER}.
Set using

@deftypefun void scm_set_port_truncate (scm_t_bits tc, void (*truncate) (SCM port, scm_t_off length))
@end deftypefun

@end table


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