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

@page
@node Input and Output
@chapter 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.
* I/O Extensions::              Using and extending ports in C.
@end menu


@node Ports
@section Ports

[Concept of the port abstraction.]

Sequential input/output in Scheme is represented by operations on a
@dfn{port}.  Characters can be read from an input port and
written to an output port.  This chapter explains the operations
that Guile provides for working with ports.

The formal definition of a port is very generic: an input port is
simply ``an object which can deliver characters on command,'' 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.

@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


@node Reading
@section Reading

[Generic procedures for reading from ports.]

@rnindex eof-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}.
@footnote{@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.
@end deffn

@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.@footnote{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.}
@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 in 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 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},
using the current input port if none is specified.  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}, using the
current input port if none is specified.
@end deffn

@node Writing
@section Writing

[Generic procedures for writing to ports.]

@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 display
@deffn {Scheme Procedure} display obj [port]
Send a representation of @var{obj} to @var{current-output-port}.
Optional second arg @var{port} specifies an alternative output port.
The representation is similar to that produced by @code{write} (REFFIXME),
the differences being strings are not quoted (and their characters are not
escaped), and characters are rendered as if with @code{write-char}.
@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}.
@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

@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
@section Closing

@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

@rnindex close-input-port
@deffn {Scheme Procedure} close-input-port port
@deffnx {C Function} scm_close_input_port (port)
Close the specified input port object.  The routine has no effect if
the file has already been closed.  An exception may be raised if an
error occurs.  The value returned is unspecified.

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

@rnindex close-output-port
@deffn {Scheme Procedure} close-output-port port
@deffnx {C Function} scm_close_output_port (port)
Close the specified output port object.  The routine has no effect if
the file has already been closed.  An exception may be raised if an
error occurs.  The value returned 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
@section 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 object [length]
@deffnx {C Function} scm_truncate_file (object, length)
Truncates the object referred to by @var{object} to at most
@var{length} bytes.  @var{object} can be a string containing a
file name or an integer file descriptor or a port.
@var{length} may be omitted if @var{object} is not a file name,
in which case the truncation occurs at the current port.
position.  The return value is unspecified.
@end deffn

@node Line/Delimited
@section Line Oriented and Delimited Text

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

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

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
@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} and return the number of
characters added to @var{buf} (subject to @var{handle-delim}, which takes
the same values specified for @code{read-line}.  If @var{buf} is filled,
@code{#f} is returned for both the number of characters read and the
delimiter.  Also terminates if 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)}.
@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 abovementioned 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
@section Block reading and writing

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

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

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
@section Default Ports for Input, Output and Errors

@rnindex current-input-port
@deffn {Scheme Procedure} current-input-port
@deffnx {C Function} scm_current_input_port ()
Return the current input port.  This is the default port used
by many input procedures.  Initially, @code{current-input-port}
returns the @dfn{standard input} in Unix and C terminology.
@end deffn

@rnindex current-output-port
@deffn {Scheme Procedure} current-output-port
@deffnx {C Function} scm_current_output_port ()
Return the current output port.  This is the default port used
by many output procedures.  Initially,
@code{current-output-port} returns the @dfn{standard output} in
Unix and C terminology.
@end deffn

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

@deffn {Scheme Procedure} set-current-output-port port
Set the current default output port to PORT.
@end deffn

@deffn {Scheme Procedure} set-current-error-port port
Set the current default error port to PORT.
@end deffn


@node Port Types
@section Types of Port

[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
@subsection File Ports

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.

@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.
@end table
In theory we could create read/write ports which were buffered
in one direction only.  However this isn't included in the
current interfaces.  If a file cannot be opened with the access
requested, @code{open-file} throws an exception.
@end deffn

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

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

@rnindex call-with-input-file
@deffn {Scheme Procedure} call-with-input-file file proc
@var{proc} should be a procedure of one argument, and @var{file} should
be a string naming a file.  The file must already exist. These
procedures call @var{proc} with one argument: the port obtained by
opening the named file for input or output.  If the file cannot be
opened, an error is signalled.  If the procedure returns, then the port
is closed automatically and the value yielded by the procedure is
returned.  If the procedure does not return, then the port will not be
closed automatically unless it is possible to prove that the port will
never again be used for a read or write operation.
@end deffn

@rnindex call-with-output-file
@deffn {Scheme Procedure} call-with-output-file file proc
@var{proc} should be a procedure of one argument, and @var{file} should
be a string naming a file.  The behaviour is unspecified if the file
already exists. These procedures call @var{proc} with one argument: the
port obtained by opening the named file for input or output.  If the
file cannot be opened, an error is signalled.  If the procedure returns,
then the port is closed automatically and the value yielded by the
procedure is returned.  If the procedure does not return, then the port
will not be closed automatically unless it is possible to prove that the
port will never again be used for a read or write operation.
@end deffn

@rnindex with-input-from-file
@deffn {Scheme Procedure} with-input-from-file file thunk
@var{thunk} must be a procedure of no arguments, and @var{file} must be
a string naming a file.  The file must already exist. The file is opened
for input, an input port connected to it is made the default value
returned by @code{current-input-port}, and the @var{thunk} is called
with no arguments.  When the @var{thunk} returns, the port is closed and
the previous default is restored.  Returns the value yielded by
@var{thunk}.  If an escape procedure is used to escape from the
continuation of these procedures, their behavior is implementation
dependent.
@end deffn

@rnindex with-output-to-file
@deffn {Scheme Procedure} with-output-to-file file thunk
@var{thunk} must be a procedure of no arguments, and @var{file} must be
a string naming a file.  The effect is unspecified if the file already
exists.  The file is opened for output, an output port connected to it
is made the default value returned by @code{current-output-port}, and
the @var{thunk} is called with no arguments.  When the @var{thunk}
returns, the port is closed and the previous default is restored.
Returns the value yielded by @var{thunk}.  If an escape procedure is
used to escape from the continuation of these procedures, their behavior
is implementation dependent.
@end deffn

@deffn {Scheme Procedure} with-error-to-file file thunk
@var{thunk} must be a procedure of no arguments, and @var{file} must be
a string naming a file.  The effect is unspecified if the file already
exists.  The file is opened for output, an output port connected to it
is made the default value returned by @code{current-error-port}, and the
@var{thunk} is called with no arguments.  When the @var{thunk} returns,
the port is closed and the previous default is restored.  Returns the
value yielded by @var{thunk}.  If an escape procedure is used to escape
from the continuation of these procedures, their behavior is
implementation dependent.
@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.
@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
@subsection String Ports

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

@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.
@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.
@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.
@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
@subsection Soft Ports

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.  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)
@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
@subsection Void Ports

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
/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 I/O Extensions
@section 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
@subsection C Port Interface

This section describes how to use Scheme ports from C.

@subsubsection Port basics

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.

@subsubsection 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.

@subsubsection 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.

@subsubsection 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

@subsubsection 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

@subsubsection 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
@subsection 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}.

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.

@code{scm_make_port_type} initializes three elements of the structure
(@code{name}, @code{fill_input} and @code{write}) from its arguments.
The remaining elements are initialized with default values and can be
set later if required.

@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 @code{scm_set_port_mark}.

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

@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 @code{scm_set_port_print}.

@item equalp
Not used at present.  Set using @code{scm_set_port_equalp}.

@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 @code{scm_set_port_close}.

@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 @code{scm_set_port_flush}.

@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 @code{scm_set_port_end_input}.

@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 @code{scm_set_port_input_waiting}.

@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 (object);
      else if (pt->rw_active == SCM_PORT_WRITE)
	ptob->flush (object);
@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 @code{scm_set_port_seek}.

@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 @code{scm_set_port_truncate}.

@end table


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