2 @c This is part of the GNU Guile Reference Manual.
3 @c Copyright (C) 1996, 1997, 2000, 2001, 2002, 2003, 2004, 2007, 2009,
4 @c 2010, 2011 Free Software Foundation, Inc.
5 @c See the file guile.texi for copying conditions.
8 @section Input and Output
11 * Ports:: The idea of the port abstraction.
12 * Reading:: Procedures for reading from a port.
13 * Writing:: Procedures for writing to a port.
14 * Closing:: Procedures to close a port.
15 * Random Access:: Moving around a random access port.
16 * Line/Delimited:: Read and write lines or delimited text.
17 * Block Reading and Writing:: Reading and writing blocks of text.
18 * Default Ports:: Defaults for input, output and errors.
19 * Port Types:: Types of port and how to make them.
20 * R6RS I/O Ports:: The R6RS port API.
21 * I/O Extensions:: Using and extending ports in C.
29 Sequential input/output in Scheme is represented by operations on a
30 @dfn{port}. This chapter explains the operations that Guile provides
31 for working with ports.
33 Ports are created by opening, for instance @code{open-file} for a file
34 (@pxref{File Ports}). Characters can be read from an input port and
35 written to an output port, or both on an input/output port. A port
36 can be closed (@pxref{Closing}) when no longer required, after which
37 any attempt to read or write is an error.
39 The formal definition of a port is very generic: an input port is
40 simply ``an object which can deliver characters on demand,'' and an
41 output port is ``an object which can accept characters.'' Because
42 this definition is so loose, it is easy to write functions that
43 simulate ports in software. @dfn{Soft ports} and @dfn{string ports}
44 are two interesting and powerful examples of this technique.
45 (@pxref{Soft Ports}, and @ref{String Ports}.)
47 Ports are garbage collected in the usual way (@pxref{Memory
48 Management}), and will be closed at that time if not already closed.
49 In this case any errors occurring in the close will not be reported.
50 Usually a program will want to explicitly close so as to be sure all
51 its operations have been successful. Of course if a program has
52 abandoned something due to an error or other condition then closing
53 problems are probably not of interest.
55 It is strongly recommended that file ports be closed explicitly when
56 no longer required. Most systems have limits on how many files can be
57 open, both on a per-process and a system-wide basis. A program that
58 uses many files should take care not to hit those limits. The same
59 applies to similar system resources such as pipes and sockets.
61 Note that automatic garbage collection is triggered only by memory
62 consumption, not by file or other resource usage, so a program cannot
63 rely on that to keep it away from system limits. An explicit call to
64 @code{gc} can of course be relied on to pick up unreferenced ports.
65 If program flow makes it hard to be certain when to close then this
66 may be an acceptable way to control resource usage.
68 All file access uses the ``LFS'' large file support functions when
69 available, so files bigger than 2 Gbytes (@math{2^31} bytes) can be
70 read and written on a 32-bit system.
72 Each port has an associated character encoding that controls how bytes
73 read from the port are converted to characters and string and controls
74 how characters and strings written to the port are converted to bytes.
75 When ports are created, they inherit their character encoding from the
76 current locale, but, that can be modified after the port is created.
78 Currently, the ports only work with @emph{non-modal} encodings. Most
79 encodings are non-modal, meaning that the conversion of bytes to a
80 string doesn't depend on its context: the same byte sequence will always
81 return the same string. A couple of modal encodings are in common use,
82 like ISO-2022-JP and ISO-2022-KR, and they are not yet supported.
84 Each port also has an associated conversion strategy: what to do when
85 a Guile character can't be converted to the port's encoded character
86 representation for output. There are three possible strategies: to
87 raise an error, to replace the character with a hex escape, or to
88 replace the character with a substitute character.
91 @deffn {Scheme Procedure} input-port? x
92 @deffnx {C Function} scm_input_port_p (x)
93 Return @code{#t} if @var{x} is an input port, otherwise return
94 @code{#f}. Any object satisfying this predicate also satisfies
99 @deffn {Scheme Procedure} output-port? x
100 @deffnx {C Function} scm_output_port_p (x)
101 Return @code{#t} if @var{x} is an output port, otherwise return
102 @code{#f}. Any object satisfying this predicate also satisfies
106 @deffn {Scheme Procedure} port? x
107 @deffnx {C Function} scm_port_p (x)
108 Return a boolean indicating whether @var{x} is a port.
109 Equivalent to @code{(or (input-port? @var{x}) (output-port?
113 @deffn {Scheme Procedure} set-port-encoding! port enc
114 @deffnx {C Function} scm_set_port_encoding_x (port, enc)
115 Sets the character encoding that will be used to interpret all port I/O.
116 @var{enc} is a string containing the name of an encoding. Valid
117 encoding names are those
118 @url{http://www.iana.org/assignments/character-sets, defined by IANA}.
121 @defvr {Scheme Variable} %default-port-encoding
122 A fluid containing @code{#f} or the name of the encoding to
123 be used by default for newly created ports (@pxref{Fluids and Dynamic
124 States}). The value @code{#f} is equivalent to @code{"ISO-8859-1"}.
126 New ports are created with the encoding appropriate for the current
127 locale if @code{setlocale} has been called or the value specified by
128 this fluid otherwise.
131 @deffn {Scheme Procedure} port-encoding port
132 @deffnx {C Function} scm_port_encoding
133 Returns, as a string, the character encoding that @var{port} uses to interpret
134 its input and output. The value @code{#f} is equivalent to @code{"ISO-8859-1"}.
137 @deffn {Scheme Procedure} set-port-conversion-strategy! port sym
138 @deffnx {C Function} scm_set_port_conversion_strategy_x (port, sym)
139 Sets the behavior of the interpreter when outputting a character that
140 is not representable in the port's current encoding. @var{sym} can be
141 either @code{'error}, @code{'substitute}, or @code{'escape}. If it is
142 @code{'error}, an error will be thrown when an nonconvertible character
143 is encountered. If it is @code{'substitute}, then nonconvertible
144 characters will be replaced with approximate characters, or with
145 question marks if no approximately correct character is available. If
146 it is @code{'escape}, it will appear as a hex escape when output.
148 If @var{port} is an open port, the conversion error behavior
149 is set for that port. If it is @code{#f}, it is set as the
150 default behavior for any future ports that get created in
154 @deffn {Scheme Procedure} port-conversion-strategy port
155 @deffnx {C Function} scm_port_conversion_strategy (port)
156 Returns the behavior of the port when outputting a character that is
157 not representable in the port's current encoding. It returns the
158 symbol @code{error} if unrepresentable characters should cause
159 exceptions, @code{substitute} if the port should try to replace
160 unrepresentable characters with question marks or approximate
161 characters, or @code{escape} if unrepresentable characters should be
162 converted to string escapes.
164 If @var{port} is @code{#f}, then the current default behavior will be
165 returned. New ports will have this default behavior when they are
175 [Generic procedures for reading from ports.]
177 These procedures pertain to reading characters and strings from
178 ports. To read general S-expressions from ports, @xref{Scheme Read}.
181 @cindex End of file object
182 @deffn {Scheme Procedure} eof-object? x
183 @deffnx {C Function} scm_eof_object_p (x)
184 Return @code{#t} if @var{x} is an end-of-file object; otherwise
189 @deffn {Scheme Procedure} char-ready? [port]
190 @deffnx {C Function} scm_char_ready_p (port)
191 Return @code{#t} if a character is ready on input @var{port}
192 and return @code{#f} otherwise. If @code{char-ready?} returns
193 @code{#t} then the next @code{read-char} operation on
194 @var{port} is guaranteed not to hang. If @var{port} is a file
195 port at end of file then @code{char-ready?} returns @code{#t}.
197 @code{char-ready?} exists to make it possible for a
198 program to accept characters from interactive ports without
199 getting stuck waiting for input. Any input editors associated
200 with such ports must make sure that characters whose existence
201 has been asserted by @code{char-ready?} cannot be rubbed out.
202 If @code{char-ready?} were to return @code{#f} at end of file,
203 a port at end of file would be indistinguishable from an
204 interactive port that has no ready characters.
208 @deffn {Scheme Procedure} read-char [port]
209 @deffnx {C Function} scm_read_char (port)
210 Return the next character available from @var{port}, updating
211 @var{port} to point to the following character. If no more
212 characters are available, the end-of-file object is returned.
214 When @var{port}'s data cannot be decoded according to its
215 character encoding, a @code{decoding-error} is raised and
216 @var{port} points past the erroneous byte sequence.
219 @deftypefn {C Function} size_t scm_c_read (SCM port, void *buffer, size_t size)
220 Read up to @var{size} bytes from @var{port} and store them in
221 @var{buffer}. The return value is the number of bytes actually read,
222 which can be less than @var{size} if end-of-file has been reached.
224 Note that this function does not update @code{port-line} and
225 @code{port-column} below.
229 @deffn {Scheme Procedure} peek-char [port]
230 @deffnx {C Function} scm_peek_char (port)
231 Return the next character available from @var{port},
232 @emph{without} updating @var{port} to point to the following
233 character. If no more characters are available, the
234 end-of-file object is returned.
236 The value returned by
237 a call to @code{peek-char} is the same as the value that would
238 have been returned by a call to @code{read-char} on the same
239 port. The only difference is that the very next call to
240 @code{read-char} or @code{peek-char} on that @var{port} will
241 return the value returned by the preceding call to
242 @code{peek-char}. In particular, a call to @code{peek-char} on
243 an interactive port will hang waiting for input whenever a call
244 to @code{read-char} would have hung.
246 As for @code{read-char}, a @code{decoding-error} may be raised
247 if such a situation occurs. However, unlike with @code{read-char},
248 @var{port} still points at the beginning of the erroneous byte
249 sequence when the error is raised.
252 @deffn {Scheme Procedure} unread-char cobj [port]
253 @deffnx {C Function} scm_unread_char (cobj, port)
254 Place @var{char} in @var{port} so that it will be read by the
255 next read operation. If called multiple times, the unread characters
256 will be read again in last-in first-out order. If @var{port} is
257 not supplied, the current input port is used.
260 @deffn {Scheme Procedure} unread-string str port
261 @deffnx {C Function} scm_unread_string (str, port)
262 Place the string @var{str} in @var{port} so that its characters will
263 be read from left-to-right as the next characters from @var{port}
264 during subsequent read operations. If called multiple times, the
265 unread characters will be read again in last-in first-out order. If
266 @var{port} is not supplied, the @code{current-input-port} is used.
269 @deffn {Scheme Procedure} drain-input port
270 @deffnx {C Function} scm_drain_input (port)
271 This procedure clears a port's input buffers, similar
272 to the way that force-output clears the output buffer. The
273 contents of the buffers are returned as a single string, e.g.,
276 (define p (open-input-file ...))
277 (drain-input p) => empty string, nothing buffered yet.
278 (unread-char (read-char p) p)
279 (drain-input p) => initial chars from p, up to the buffer size.
282 Draining the buffers may be useful for cleanly finishing
283 buffered I/O so that the file descriptor can be used directly
287 @deffn {Scheme Procedure} port-column port
288 @deffnx {Scheme Procedure} port-line port
289 @deffnx {C Function} scm_port_column (port)
290 @deffnx {C Function} scm_port_line (port)
291 Return the current column number or line number of @var{port}.
293 unknown, the result is #f. Otherwise, the result is a 0-origin integer
294 - i.e.@: the first character of the first line is line 0, column 0.
295 (However, when you display a file position, for example in an error
296 message, we recommend you add 1 to get 1-origin integers. This is
297 because lines and column numbers traditionally start with 1, and that is
298 what non-programmers will find most natural.)
301 @deffn {Scheme Procedure} set-port-column! port column
302 @deffnx {Scheme Procedure} set-port-line! port line
303 @deffnx {C Function} scm_set_port_column_x (port, column)
304 @deffnx {C Function} scm_set_port_line_x (port, line)
305 Set the current column or line number of @var{port}.
312 [Generic procedures for writing to ports.]
314 These procedures are for writing characters and strings to
315 ports. For more information on writing arbitrary Scheme objects to
316 ports, @xref{Scheme Write}.
318 @deffn {Scheme Procedure} get-print-state port
319 @deffnx {C Function} scm_get_print_state (port)
320 Return the print state of the port @var{port}. If @var{port}
321 has no associated print state, @code{#f} is returned.
325 @deffn {Scheme Procedure} newline [port]
326 @deffnx {C Function} scm_newline (port)
327 Send a newline to @var{port}.
328 If @var{port} is omitted, send to the current output port.
331 @deffn {Scheme Procedure} port-with-print-state port [pstate]
332 @deffnx {C Function} scm_port_with_print_state (port, pstate)
333 Create a new port which behaves like @var{port}, but with an
334 included print state @var{pstate}. @var{pstate} is optional.
335 If @var{pstate} isn't supplied and @var{port} already has
336 a print state, the old print state is reused.
339 @deffn {Scheme Procedure} simple-format destination message . args
340 @deffnx {C Function} scm_simple_format (destination, message, args)
341 Write @var{message} to @var{destination}, defaulting to
342 the current output port.
343 @var{message} can contain @code{~A} (was @code{%s}) and
344 @code{~S} (was @code{%S}) escapes. When printed,
345 the escapes are replaced with corresponding members of
347 @code{~A} formats using @code{display} and @code{~S} formats
349 If @var{destination} is @code{#t}, then use the current output
350 port, if @var{destination} is @code{#f}, then return a string
351 containing the formatted text. Does not add a trailing newline.
355 @deffn {Scheme Procedure} write-char chr [port]
356 @deffnx {C Function} scm_write_char (chr, port)
357 Send character @var{chr} to @var{port}.
360 @deftypefn {C Function} void scm_c_write (SCM port, const void *buffer, size_t size)
361 Write @var{size} bytes at @var{buffer} to @var{port}.
363 Note that this function does not update @code{port-line} and
364 @code{port-column} (@pxref{Reading}).
368 @deffn {Scheme Procedure} force-output [port]
369 @deffnx {C Function} scm_force_output (port)
370 Flush the specified output port, or the current output port if @var{port}
371 is omitted. The current output buffer contents are passed to the
372 underlying port implementation (e.g., in the case of fports, the
373 data will be written to the file and the output buffer will be cleared.)
374 It has no effect on an unbuffered port.
376 The return value is unspecified.
379 @deffn {Scheme Procedure} flush-all-ports
380 @deffnx {C Function} scm_flush_all_ports ()
381 Equivalent to calling @code{force-output} on
382 all open output ports. The return value is unspecified.
388 @cindex Closing ports
391 @deffn {Scheme Procedure} close-port port
392 @deffnx {C Function} scm_close_port (port)
393 Close the specified port object. Return @code{#t} if it
394 successfully closes a port or @code{#f} if it was already
395 closed. An exception may be raised if an error occurs, for
396 example when flushing buffered output. See also @ref{Ports and
397 File Descriptors, close}, for a procedure which can close file
401 @deffn {Scheme Procedure} close-input-port port
402 @deffnx {Scheme Procedure} close-output-port port
403 @deffnx {C Function} scm_close_input_port (port)
404 @deffnx {C Function} scm_close_output_port (port)
405 @rnindex close-input-port
406 @rnindex close-output-port
407 Close the specified input or output @var{port}. An exception may be
408 raised if an error occurs while closing. If @var{port} is already
409 closed, nothing is done. The return value is unspecified.
411 See also @ref{Ports and File Descriptors, close}, for a procedure
412 which can close file descriptors.
415 @deffn {Scheme Procedure} port-closed? port
416 @deffnx {C Function} scm_port_closed_p (port)
417 Return @code{#t} if @var{port} is closed or @code{#f} if it is
423 @subsection Random Access
424 @cindex Random access, ports
425 @cindex Port, random access
427 @deffn {Scheme Procedure} seek fd_port offset whence
428 @deffnx {C Function} scm_seek (fd_port, offset, whence)
429 Sets the current position of @var{fd/port} to the integer
430 @var{offset}, which is interpreted according to the value of
433 One of the following variables should be supplied for
436 Seek from the beginning of the file.
439 Seek from the current position.
442 Seek from the end of the file.
444 If @var{fd/port} is a file descriptor, the underlying system
445 call is @code{lseek}. @var{port} may be a string port.
447 The value returned is the new position in the file. This means
448 that the current position of a port can be obtained using:
450 (seek port 0 SEEK_CUR)
454 @deffn {Scheme Procedure} ftell fd_port
455 @deffnx {C Function} scm_ftell (fd_port)
456 Return an integer representing the current position of
457 @var{fd/port}, measured from the beginning. Equivalent to:
460 (seek port 0 SEEK_CUR)
466 @deffn {Scheme Procedure} truncate-file file [length]
467 @deffnx {C Function} scm_truncate_file (file, length)
468 Truncate @var{file} to @var{length} bytes. @var{file} can be a
469 filename string, a port object, or an integer file descriptor. The
470 return value is unspecified.
472 For a port or file descriptor @var{length} can be omitted, in which
473 case the file is truncated at the current position (per @code{ftell}
476 On most systems a file can be extended by giving a length greater than
477 the current size, but this is not mandatory in the POSIX standard.
481 @subsection Line Oriented and Delimited Text
482 @cindex Line input/output
483 @cindex Port, line input/output
485 The delimited-I/O module can be accessed with:
488 (use-modules (ice-9 rdelim))
491 It can be used to read or write lines of text, or read text delimited by
492 a specified set of characters. It's similar to the @code{(scsh rdelim)}
493 module from guile-scsh, but does not use multiple values or character
494 sets and has an extra procedure @code{write-line}.
496 @c begin (scm-doc-string "rdelim.scm" "read-line")
497 @deffn {Scheme Procedure} read-line [port] [handle-delim]
498 Return a line of text from @var{port} if specified, otherwise from the
499 value returned by @code{(current-input-port)}. Under Unix, a line of text
500 is terminated by the first end-of-line character or by end-of-file.
502 If @var{handle-delim} is specified, it should be one of the following
506 Discard the terminating delimiter. This is the default, but it will
507 be impossible to tell whether the read terminated with a delimiter or
510 Append the terminating delimiter (if any) to the returned string.
512 Push the terminating delimiter (if any) back on to the port.
514 Return a pair containing the string read from the port and the
515 terminating delimiter or end-of-file object.
518 Like @code{read-char}, this procedure can throw to @code{decoding-error}
519 (@pxref{Reading, @code{read-char}}).
522 @c begin (scm-doc-string "rdelim.scm" "read-line!")
523 @deffn {Scheme Procedure} read-line! buf [port]
524 Read a line of text into the supplied string @var{buf} and return the
525 number of characters added to @var{buf}. If @var{buf} is filled, then
526 @code{#f} is returned.
527 Read from @var{port} if
528 specified, otherwise from the value returned by @code{(current-input-port)}.
531 @c begin (scm-doc-string "rdelim.scm" "read-delimited")
532 @deffn {Scheme Procedure} read-delimited delims [port] [handle-delim]
533 Read text until one of the characters in the string @var{delims} is found
534 or end-of-file is reached. Read from @var{port} if supplied, otherwise
535 from the value returned by @code{(current-input-port)}.
536 @var{handle-delim} takes the same values as described for @code{read-line}.
539 @c begin (scm-doc-string "rdelim.scm" "read-delimited!")
540 @deffn {Scheme Procedure} read-delimited! delims buf [port] [handle-delim] [start] [end]
541 Read text into the supplied string @var{buf}.
543 If a delimiter was found, return the number of characters written,
544 except if @var{handle-delim} is @code{split}, in which case the return
545 value is a pair, as noted above.
547 As a special case, if @var{port} was already at end-of-stream, the EOF
548 object is returned. Also, if no characters were written because the
549 buffer was full, @code{#f} is returned.
551 It's something of a wacky interface, to be honest.
554 @deffn {Scheme Procedure} write-line obj [port]
555 @deffnx {C Function} scm_write_line (obj, port)
556 Display @var{obj} and a newline character to @var{port}. If
557 @var{port} is not specified, @code{(current-output-port)} is
558 used. This function is equivalent to:
565 Some of the aforementioned I/O functions rely on the following C
566 primitives. These will mainly be of interest to people hacking Guile
569 @deffn {Scheme Procedure} %read-delimited! delims str gobble [port [start [end]]]
570 @deffnx {C Function} scm_read_delimited_x (delims, str, gobble, port, start, end)
571 Read characters from @var{port} into @var{str} until one of the
572 characters in the @var{delims} string is encountered. If
573 @var{gobble} is true, discard the delimiter character;
574 otherwise, leave it in the input stream for the next read. If
575 @var{port} is not specified, use the value of
576 @code{(current-input-port)}. If @var{start} or @var{end} are
577 specified, store data only into the substring of @var{str}
578 bounded by @var{start} and @var{end} (which default to the
579 beginning and end of the string, respectively).
581 Return a pair consisting of the delimiter that terminated the
582 string and the number of characters read. If reading stopped
583 at the end of file, the delimiter returned is the
584 @var{eof-object}; if the string was filled without encountering
585 a delimiter, this value is @code{#f}.
588 @deffn {Scheme Procedure} %read-line [port]
589 @deffnx {C Function} scm_read_line (port)
590 Read a newline-terminated line from @var{port}, allocating storage as
591 necessary. The newline terminator (if any) is removed from the string,
592 and a pair consisting of the line and its delimiter is returned. The
593 delimiter may be either a newline or the @var{eof-object}; if
594 @code{%read-line} is called at the end of file, it returns the pair
595 @code{(#<eof> . #<eof>)}.
598 @node Block Reading and Writing
599 @subsection Block reading and writing
600 @cindex Block read/write
601 @cindex Port, block read/write
603 The Block-string-I/O module can be accessed with:
606 (use-modules (ice-9 rw))
609 It currently contains procedures that help to implement the
610 @code{(scsh rw)} module in guile-scsh.
612 @deffn {Scheme Procedure} read-string!/partial str [port_or_fdes [start [end]]]
613 @deffnx {C Function} scm_read_string_x_partial (str, port_or_fdes, start, end)
614 Read characters from a port or file descriptor into a
615 string @var{str}. A port must have an underlying file
616 descriptor --- a so-called fport. This procedure is
617 scsh-compatible and can efficiently read large strings.
622 attempt to fill the entire string, unless the @var{start}
623 and/or @var{end} arguments are supplied. i.e., @var{start}
624 defaults to 0 and @var{end} defaults to
625 @code{(string-length str)}
627 use the current input port if @var{port_or_fdes} is not
630 return fewer than the requested number of characters in some
631 cases, e.g., on end of file, if interrupted by a signal, or if
632 not all the characters are immediately available.
634 wait indefinitely for some input if no characters are
636 unless the port is in non-blocking mode.
638 read characters from the port's input buffers if available,
639 instead from the underlying file descriptor.
641 return @code{#f} if end-of-file is encountered before reading
642 any characters, otherwise return the number of characters
645 return 0 if the port is in non-blocking mode and no characters
646 are immediately available.
648 return 0 if the request is for 0 bytes, with no
653 @deffn {Scheme Procedure} write-string/partial str [port_or_fdes [start [end]]]
654 @deffnx {C Function} scm_write_string_partial (str, port_or_fdes, start, end)
655 Write characters from a string @var{str} to a port or file
656 descriptor. A port must have an underlying file descriptor
657 --- a so-called fport. This procedure is
658 scsh-compatible and can efficiently write large strings.
663 attempt to write the entire string, unless the @var{start}
664 and/or @var{end} arguments are supplied. i.e., @var{start}
665 defaults to 0 and @var{end} defaults to
666 @code{(string-length str)}
668 use the current output port if @var{port_of_fdes} is not
671 in the case of a buffered port, store the characters in the
672 port's output buffer, if all will fit. If they will not fit
673 then any existing buffered characters will be flushed
675 to write the new characters directly to the underlying file
676 descriptor. If the port is in non-blocking mode and
677 buffered characters can not be flushed immediately, then an
678 @code{EAGAIN} system-error exception will be raised (Note:
679 scsh does not support the use of non-blocking buffered ports.)
681 write fewer than the requested number of
682 characters in some cases, e.g., if interrupted by a signal or
683 if not all of the output can be accepted immediately.
685 wait indefinitely for at least one character
686 from @var{str} to be accepted by the port, unless the port is
687 in non-blocking mode.
689 return the number of characters accepted by the port.
691 return 0 if the port is in non-blocking mode and can not accept
692 at least one character from @var{str} immediately
694 return 0 immediately if the request size is 0 bytes.
699 @subsection Default Ports for Input, Output and Errors
700 @cindex Default ports
701 @cindex Port, default
703 @rnindex current-input-port
704 @deffn {Scheme Procedure} current-input-port
705 @deffnx {C Function} scm_current_input_port ()
706 @cindex standard input
707 Return the current input port. This is the default port used
708 by many input procedures.
710 Initially this is the @dfn{standard input} in Unix and C terminology.
711 When the standard input is a tty the port is unbuffered, otherwise
714 Unbuffered input is good if an application runs an interactive
715 subprocess, since any type-ahead input won't go into Guile's buffer
716 and be unavailable to the subprocess.
718 Note that Guile buffering is completely separate from the tty ``line
719 discipline''. In the usual cooked mode on a tty Guile only sees a
720 line of input once the user presses @key{Return}.
723 @rnindex current-output-port
724 @deffn {Scheme Procedure} current-output-port
725 @deffnx {C Function} scm_current_output_port ()
726 @cindex standard output
727 Return the current output port. This is the default port used
728 by many output procedures.
730 Initially this is the @dfn{standard output} in Unix and C terminology.
731 When the standard output is a tty this port is unbuffered, otherwise
734 Unbuffered output to a tty is good for ensuring progress output or a
735 prompt is seen. But an application which always prints whole lines
736 could change to line buffered, or an application with a lot of output
737 could go fully buffered and perhaps make explicit @code{force-output}
738 calls (@pxref{Writing}) at selected points.
741 @deffn {Scheme Procedure} current-error-port
742 @deffnx {C Function} scm_current_error_port ()
743 @cindex standard error output
744 Return the port to which errors and warnings should be sent.
746 Initially this is the @dfn{standard error} in Unix and C terminology.
747 When the standard error is a tty this port is unbuffered, otherwise
751 @deffn {Scheme Procedure} set-current-input-port port
752 @deffnx {Scheme Procedure} set-current-output-port port
753 @deffnx {Scheme Procedure} set-current-error-port port
754 @deffnx {C Function} scm_set_current_input_port (port)
755 @deffnx {C Function} scm_set_current_output_port (port)
756 @deffnx {C Function} scm_set_current_error_port (port)
757 Change the ports returned by @code{current-input-port},
758 @code{current-output-port} and @code{current-error-port}, respectively,
759 so that they use the supplied @var{port} for input or output.
762 @deftypefn {C Function} void scm_dynwind_current_input_port (SCM port)
763 @deftypefnx {C Function} void scm_dynwind_current_output_port (SCM port)
764 @deftypefnx {C Function} void scm_dynwind_current_error_port (SCM port)
765 These functions must be used inside a pair of calls to
766 @code{scm_dynwind_begin} and @code{scm_dynwind_end} (@pxref{Dynamic
767 Wind}). During the dynwind context, the indicated port is set to
770 More precisely, the current port is swapped with a `backup' value
771 whenever the dynwind context is entered or left. The backup value is
772 initialized with the @var{port} argument.
776 @subsection Types of Port
777 @cindex Types of ports
780 [Types of port; how to make them.]
783 * File Ports:: Ports on an operating system file.
784 * String Ports:: Ports on a Scheme string.
785 * Soft Ports:: Ports on arbitrary Scheme procedures.
786 * Void Ports:: Ports on nothing at all.
791 @subsubsection File Ports
795 The following procedures are used to open file ports.
796 See also @ref{Ports and File Descriptors, open}, for an interface
797 to the Unix @code{open} system call.
799 Most systems have limits on how many files can be open, so it's
800 strongly recommended that file ports be closed explicitly when no
801 longer required (@pxref{Ports}).
803 @deffn {Scheme Procedure} open-file filename mode
804 @deffnx {C Function} scm_open_file (filename, mode)
805 Open the file whose name is @var{filename}, and return a port
806 representing that file. The attributes of the port are
807 determined by the @var{mode} string. The way in which this is
808 interpreted is similar to C stdio. The first character must be
809 one of the following:
813 Open an existing file for input.
815 Open a file for output, creating it if it doesn't already exist
816 or removing its contents if it does.
818 Open a file for output, creating it if it doesn't already
819 exist. All writes to the port will go to the end of the file.
820 The "append mode" can be turned off while the port is in use
821 @pxref{Ports and File Descriptors, fcntl}
824 The following additional characters can be appended:
828 Open the port for both input and output. E.g., @code{r+}: open
829 an existing file for both input and output.
831 Create an "unbuffered" port. In this case input and output
832 operations are passed directly to the underlying port
833 implementation without additional buffering. This is likely to
834 slow down I/O operations. The buffering mode can be changed
835 while a port is in use @pxref{Ports and File Descriptors,
838 Add line-buffering to the port. The port output buffer will be
839 automatically flushed whenever a newline character is written.
841 Use binary mode, ensuring that each byte in the file will be read as one
844 To provide this property, the file will be opened with the 8-bit
845 character encoding "ISO-8859-1", ignoring any coding declaration or port
846 encoding. @xref{Ports}, for more information on port encodings.
848 Note that while it is possible to read and write binary data as
849 characters or strings, it is usually better to treat bytes as octets,
850 and byte sequences as bytevectors. @xref{R6RS Binary Input}, and
851 @ref{R6RS Binary Output}, for more.
853 This option had another historical meaning, for DOS compatibility: in
854 the default (textual) mode, DOS reads a CR-LF sequence as one LF byte.
855 The @code{b} flag prevents this from happening, adding @code{O_BINARY}
856 to the underlying @code{open} call. Still, the flag is generally useful
857 because of its port encoding ramifications.
860 If a file cannot be opened with the access
861 requested, @code{open-file} throws an exception.
863 When the file is opened, this procedure will scan for a coding
864 declaration (@pxref{Character Encoding of Source Files}). If a coding
865 declaration is found, it will be used to interpret the file. Otherwise,
866 the port's encoding will be used. To suppress this behavior, open the
867 file in binary mode and then set the port encoding explicitly using
868 @code{set-port-encoding!}.
870 In theory we could create read/write ports which were buffered
871 in one direction only. However this isn't included in the
875 @rnindex open-input-file
876 @deffn {Scheme Procedure} open-input-file filename
877 Open @var{filename} for input. Equivalent to
879 (open-file @var{filename} "r")
883 @rnindex open-output-file
884 @deffn {Scheme Procedure} open-output-file filename
885 Open @var{filename} for output. Equivalent to
887 (open-file @var{filename} "w")
891 @deffn {Scheme Procedure} call-with-input-file filename proc
892 @deffnx {Scheme Procedure} call-with-output-file filename proc
893 @rnindex call-with-input-file
894 @rnindex call-with-output-file
895 Open @var{filename} for input or output, and call @code{(@var{proc}
896 port)} with the resulting port. Return the value returned by
897 @var{proc}. @var{filename} is opened as per @code{open-input-file} or
898 @code{open-output-file} respectively, and an error is signaled if it
901 When @var{proc} returns, the port is closed. If @var{proc} does not
902 return (e.g.@: if it throws an error), then the port might not be
903 closed automatically, though it will be garbage collected in the usual
904 way if not otherwise referenced.
907 @deffn {Scheme Procedure} with-input-from-file filename thunk
908 @deffnx {Scheme Procedure} with-output-to-file filename thunk
909 @deffnx {Scheme Procedure} with-error-to-file filename thunk
910 @rnindex with-input-from-file
911 @rnindex with-output-to-file
912 Open @var{filename} and call @code{(@var{thunk})} with the new port
913 setup as respectively the @code{current-input-port},
914 @code{current-output-port}, or @code{current-error-port}. Return the
915 value returned by @var{thunk}. @var{filename} is opened as per
916 @code{open-input-file} or @code{open-output-file} respectively, and an
917 error is signaled if it cannot be opened.
919 When @var{thunk} returns, the port is closed and the previous setting
920 of the respective current port is restored.
922 The current port setting is managed with @code{dynamic-wind}, so the
923 previous value is restored no matter how @var{thunk} exits (eg.@: an
924 exception), and if @var{thunk} is re-entered (via a captured
925 continuation) then it's set again to the @var{FILENAME} port.
927 The port is closed when @var{thunk} returns normally, but not when
928 exited via an exception or new continuation. This ensures it's still
929 ready for use if @var{thunk} is re-entered by a captured continuation.
930 Of course the port is always garbage collected and closed in the usual
931 way when no longer referenced anywhere.
934 @deffn {Scheme Procedure} port-mode port
935 @deffnx {C Function} scm_port_mode (port)
936 Return the port modes associated with the open port @var{port}.
937 These will not necessarily be identical to the modes used when
938 the port was opened, since modes such as "append" which are
939 used only during port creation are not retained.
942 @deffn {Scheme Procedure} port-filename port
943 @deffnx {C Function} scm_port_filename (port)
944 Return the filename associated with @var{port}, or @code{#f} if no
945 filename is associated with the port.
947 @var{port} must be open, @code{port-filename} cannot be used once the
951 @deffn {Scheme Procedure} set-port-filename! port filename
952 @deffnx {C Function} scm_set_port_filename_x (port, filename)
953 Change the filename associated with @var{port}, using the current input
954 port if none is specified. Note that this does not change the port's
955 source of data, but only the value that is returned by
956 @code{port-filename} and reported in diagnostic output.
959 @deffn {Scheme Procedure} file-port? obj
960 @deffnx {C Function} scm_file_port_p (obj)
961 Determine whether @var{obj} is a port that is related to a file.
966 @subsubsection String Ports
970 The following allow string ports to be opened by analogy to R4RS
971 file port facilities:
973 With string ports, the port-encoding is treated differently than other
974 types of ports. When string ports are created, they do not inherit a
975 character encoding from the current locale. They are given a
976 default locale that allows them to handle all valid string characters.
977 Typically one should not modify a string port's character encoding
978 away from its default.
980 @deffn {Scheme Procedure} call-with-output-string proc
981 @deffnx {C Function} scm_call_with_output_string (proc)
982 Calls the one-argument procedure @var{proc} with a newly created output
983 port. When the function returns, the string composed of the characters
984 written into the port is returned. @var{proc} should not close the port.
986 Note that which characters can be written to a string port depend on the port's
987 encoding. The default encoding of string ports is specified by the
988 @code{%default-port-encoding} fluid (@pxref{Ports,
989 @code{%default-port-encoding}}). For instance, it is an error to write Greek
990 letter alpha to an ISO-8859-1-encoded string port since this character cannot be
991 represented with ISO-8859-1:
994 (define alpha (integer->char #x03b1)) ; GREEK SMALL LETTER ALPHA
996 (with-fluids ((%default-port-encoding "ISO-8859-1"))
997 (call-with-output-string
1002 Throw to key `encoding-error'
1005 Changing the string port's encoding to a Unicode-capable encoding such as UTF-8
1009 @deffn {Scheme Procedure} call-with-input-string string proc
1010 @deffnx {C Function} scm_call_with_input_string (string, proc)
1011 Calls the one-argument procedure @var{proc} with a newly
1012 created input port from which @var{string}'s contents may be
1013 read. The value yielded by the @var{proc} is returned.
1016 @deffn {Scheme Procedure} with-output-to-string thunk
1017 Calls the zero-argument procedure @var{thunk} with the current output
1018 port set temporarily to a new string port. It returns a string
1019 composed of the characters written to the current output.
1021 See @code{call-with-output-string} above for character encoding considerations.
1024 @deffn {Scheme Procedure} with-input-from-string string thunk
1025 Calls the zero-argument procedure @var{thunk} with the current input
1026 port set temporarily to a string port opened on the specified
1027 @var{string}. The value yielded by @var{thunk} is returned.
1030 @deffn {Scheme Procedure} open-input-string str
1031 @deffnx {C Function} scm_open_input_string (str)
1032 Take a string and return an input port that delivers characters
1033 from the string. The port can be closed by
1034 @code{close-input-port}, though its storage will be reclaimed
1035 by the garbage collector if it becomes inaccessible.
1038 @deffn {Scheme Procedure} open-output-string
1039 @deffnx {C Function} scm_open_output_string ()
1040 Return an output port that will accumulate characters for
1041 retrieval by @code{get-output-string}. The port can be closed
1042 by the procedure @code{close-output-port}, though its storage
1043 will be reclaimed by the garbage collector if it becomes
1047 @deffn {Scheme Procedure} get-output-string port
1048 @deffnx {C Function} scm_get_output_string (port)
1049 Given an output port created by @code{open-output-string},
1050 return a string consisting of the characters that have been
1051 output to the port so far.
1053 @code{get-output-string} must be used before closing @var{port}, once
1054 closed the string cannot be obtained.
1057 A string port can be used in many procedures which accept a port
1058 but which are not dependent on implementation details of fports.
1059 E.g., seeking and truncating will work on a string port,
1060 but trying to extract the file descriptor number will fail.
1064 @subsubsection Soft Ports
1068 A @dfn{soft-port} is a port based on a vector of procedures capable of
1069 accepting or delivering characters. It allows emulation of I/O ports.
1071 @deffn {Scheme Procedure} make-soft-port pv modes
1072 @deffnx {C Function} scm_make_soft_port (pv, modes)
1073 Return a port capable of receiving or delivering characters as
1074 specified by the @var{modes} string (@pxref{File Ports,
1075 open-file}). @var{pv} must be a vector of length 5 or 6. Its
1076 components are as follows:
1080 procedure accepting one character for output
1082 procedure accepting a string for output
1084 thunk for flushing output
1086 thunk for getting one character
1088 thunk for closing port (not by garbage collection)
1090 (if present and not @code{#f}) thunk for computing the number of
1091 characters that can be read from the port without blocking.
1094 For an output-only port only elements 0, 1, 2, and 4 need be
1095 procedures. For an input-only port only elements 3 and 4 need
1096 be procedures. Thunks 2 and 4 can instead be @code{#f} if
1097 there is no useful operation for them to perform.
1099 If thunk 3 returns @code{#f} or an @code{eof-object}
1100 (@pxref{Input, eof-object?, ,r5rs, The Revised^5 Report on
1101 Scheme}) it indicates that the port has reached end-of-file.
1105 (define stdout (current-output-port))
1106 (define p (make-soft-port
1108 (lambda (c) (write c stdout))
1109 (lambda (s) (display s stdout))
1110 (lambda () (display "." stdout))
1111 (lambda () (char-upcase (read-char)))
1112 (lambda () (display "@@" stdout)))
1115 (write p p) @result{} #<input-output: soft 8081e20>
1121 @subsubsection Void Ports
1125 This kind of port causes any data to be discarded when written to, and
1126 always returns the end-of-file object when read from.
1128 @deffn {Scheme Procedure} %make-void-port mode
1129 @deffnx {C Function} scm_sys_make_void_port (mode)
1130 Create and return a new void port. A void port acts like
1131 @file{/dev/null}. The @var{mode} argument
1132 specifies the input/output modes for this port: see the
1133 documentation for @code{open-file} in @ref{File Ports}.
1137 @node R6RS I/O Ports
1138 @subsection R6RS I/O Ports
1143 The I/O port API of the @uref{http://www.r6rs.org/, Revised Report^6 on
1144 the Algorithmic Language Scheme (R6RS)} is provided by the @code{(rnrs
1145 io ports)} module. It provides features, such as binary I/O and Unicode
1146 string I/O, that complement or refine Guile's historical port API
1147 presented above (@pxref{Input and Output}). Note that R6RS ports are not
1148 disjoint from Guile's native ports, so Guile-specific procedures will
1149 work on ports created using the R6RS API, and vice versa.
1151 The text in this section is taken from the R6RS standard libraries
1152 document, with only minor adaptions for inclusion in this manual. The
1153 Guile developers offer their thanks to the R6RS editors for having
1154 provided the report's text under permissive conditions making this
1157 @c FIXME: Update description when implemented.
1158 @emph{Note}: The implementation of this R6RS API is not complete yet.
1161 * R6RS File Names:: File names.
1162 * R6RS File Options:: Options for opening files.
1163 * R6RS Buffer Modes:: Influencing buffering behavior.
1164 * R6RS Transcoders:: Influencing port encoding.
1165 * R6RS End-of-File:: The end-of-file object.
1166 * R6RS Port Manipulation:: Manipulating R6RS ports.
1167 * R6RS Input Ports:: Input Ports.
1168 * R6RS Binary Input:: Binary input.
1169 * R6RS Textual Input:: Textual input.
1170 * R6RS Output Ports:: Output Ports.
1171 * R6RS Binary Output:: Binary output.
1172 * R6RS Textual Output:: Textual output.
1175 A subset of the @code{(rnrs io ports)} module is provided by the
1176 @code{(ice-9 binary-ports)} module. It contains binary input/output
1177 procedures and does not rely on R6RS support.
1179 @node R6RS File Names
1180 @subsubsection File Names
1182 Some of the procedures described in this chapter accept a file name as an
1183 argument. Valid values for such a file name include strings that name a file
1184 using the native notation of file system paths on an implementation's
1185 underlying operating system, and may include implementation-dependent
1188 A @var{filename} parameter name means that the
1189 corresponding argument must be a file name.
1191 @node R6RS File Options
1192 @subsubsection File Options
1193 @cindex file options
1195 When opening a file, the various procedures in this library accept a
1196 @code{file-options} object that encapsulates flags to specify how the
1197 file is to be opened. A @code{file-options} object is an enum-set
1198 (@pxref{rnrs enums}) over the symbols constituting valid file options.
1200 A @var{file-options} parameter name means that the corresponding
1201 argument must be a file-options object.
1203 @deffn {Scheme Syntax} file-options @var{file-options-symbol} ...
1205 Each @var{file-options-symbol} must be a symbol.
1207 The @code{file-options} syntax returns a file-options object that
1208 encapsulates the specified options.
1210 When supplied to an operation that opens a file for output, the
1211 file-options object returned by @code{(file-options)} specifies that the
1212 file is created if it does not exist and an exception with condition
1213 type @code{&i/o-file-already-exists} is raised if it does exist. The
1214 following standard options can be included to modify the default
1219 If the file does not already exist, it is not created;
1220 instead, an exception with condition type @code{&i/o-file-does-not-exist}
1222 If the file already exists, the exception with condition type
1223 @code{&i/o-file-already-exists} is not raised
1224 and the file is truncated to zero length.
1226 If the file already exists, the exception with condition type
1227 @code{&i/o-file-already-exists} is not raised,
1228 even if @code{no-create} is not included,
1229 and the file is truncated to zero length.
1231 If the file already exists and the exception with condition type
1232 @code{&i/o-file-already-exists} has been inhibited by inclusion of
1233 @code{no-create} or @code{no-fail}, the file is not truncated, but
1234 the port's current position is still set to the beginning of the
1238 These options have no effect when a file is opened only for input.
1239 Symbols other than those listed above may be used as
1240 @var{file-options-symbol}s; they have implementation-specific meaning,
1244 Only the name of @var{file-options-symbol} is significant.
1248 @node R6RS Buffer Modes
1249 @subsubsection Buffer Modes
1251 Each port has an associated buffer mode. For an output port, the
1252 buffer mode defines when an output operation flushes the buffer
1253 associated with the output port. For an input port, the buffer mode
1254 defines how much data will be read to satisfy read operations. The
1255 possible buffer modes are the symbols @code{none} for no buffering,
1256 @code{line} for flushing upon line endings and reading up to line
1257 endings, or other implementation-dependent behavior,
1258 and @code{block} for arbitrary buffering. This section uses
1259 the parameter name @var{buffer-mode} for arguments that must be
1260 buffer-mode symbols.
1262 If two ports are connected to the same mutable source, both ports
1263 are unbuffered, and reading a byte or character from that shared
1264 source via one of the two ports would change the bytes or characters
1265 seen via the other port, a lookahead operation on one port will
1266 render the peeked byte or character inaccessible via the other port,
1267 while a subsequent read operation on the peeked port will see the
1268 peeked byte or character even though the port is otherwise unbuffered.
1270 In other words, the semantics of buffering is defined in terms of side
1271 effects on shared mutable sources, and a lookahead operation has the
1272 same side effect on the shared source as a read operation.
1274 @deffn {Scheme Syntax} buffer-mode @var{buffer-mode-symbol}
1276 @var{buffer-mode-symbol} must be a symbol whose name is one of
1277 @code{none}, @code{line}, and @code{block}. The result is the
1278 corresponding symbol, and specifies the associated buffer mode.
1281 Only the name of @var{buffer-mode-symbol} is significant.
1285 @deffn {Scheme Procedure} buffer-mode? obj
1286 Returns @code{#t} if the argument is a valid buffer-mode symbol, and
1287 returns @code{#f} otherwise.
1290 @node R6RS Transcoders
1291 @subsubsection Transcoders
1293 @cindex end-of-line style
1296 @cindex textual port
1298 Several different Unicode encoding schemes describe standard ways to
1299 encode characters and strings as byte sequences and to decode those
1300 sequences. Within this document, a @dfn{codec} is an immutable Scheme
1301 object that represents a Unicode or similar encoding scheme.
1303 An @dfn{end-of-line style} is a symbol that, if it is not @code{none},
1304 describes how a textual port transcodes representations of line endings.
1306 A @dfn{transcoder} is an immutable Scheme object that combines a codec
1307 with an end-of-line style and a method for handling decoding errors.
1308 Each transcoder represents some specific bidirectional (but not
1309 necessarily lossless), possibly stateful translation between byte
1310 sequences and Unicode characters and strings. Every transcoder can
1311 operate in the input direction (bytes to characters) or in the output
1312 direction (characters to bytes). A @var{transcoder} parameter name
1313 means that the corresponding argument must be a transcoder.
1315 A @dfn{binary port} is a port that supports binary I/O, does not have an
1316 associated transcoder and does not support textual I/O. A @dfn{textual
1317 port} is a port that supports textual I/O, and does not support binary
1318 I/O. A textual port may or may not have an associated transcoder.
1320 @deffn {Scheme Procedure} latin-1-codec
1321 @deffnx {Scheme Procedure} utf-8-codec
1322 @deffnx {Scheme Procedure} utf-16-codec
1324 These are predefined codecs for the ISO 8859-1, UTF-8, and UTF-16
1327 A call to any of these procedures returns a value that is equal in the
1328 sense of @code{eqv?} to the result of any other call to the same
1332 @deffn {Scheme Syntax} eol-style @var{eol-style-symbol}
1334 @var{eol-style-symbol} should be a symbol whose name is one of
1335 @code{lf}, @code{cr}, @code{crlf}, @code{nel}, @code{crnel}, @code{ls},
1338 The form evaluates to the corresponding symbol. If the name of
1339 @var{eol-style-symbol} is not one of these symbols, the effect and
1340 result are implementation-dependent; in particular, the result may be an
1341 eol-style symbol acceptable as an @var{eol-style} argument to
1342 @code{make-transcoder}. Otherwise, an exception is raised.
1344 All eol-style symbols except @code{none} describe a specific
1345 line-ending encoding:
1353 carriage return, linefeed
1357 carriage return, next line
1362 For a textual port with a transcoder, and whose transcoder has an
1363 eol-style symbol @code{none}, no conversion occurs. For a textual input
1364 port, any eol-style symbol other than @code{none} means that all of the
1365 above line-ending encodings are recognized and are translated into a
1366 single linefeed. For a textual output port, @code{none} and @code{lf}
1367 are equivalent. Linefeed characters are encoded according to the
1368 specified eol-style symbol, and all other characters that participate in
1369 possible line endings are encoded as is.
1372 Only the name of @var{eol-style-symbol} is significant.
1376 @deffn {Scheme Procedure} native-eol-style
1377 Returns the default end-of-line style of the underlying platform, e.g.,
1378 @code{lf} on Unix and @code{crlf} on Windows.
1381 @deffn {Condition Type} &i/o-decoding
1382 @deffnx {Scheme Procedure} make-i/o-decoding-error port
1383 @deffnx {Scheme Procedure} i/o-decoding-error? obj
1385 This condition type could be defined by
1388 (define-condition-type &i/o-decoding &i/o-port
1389 make-i/o-decoding-error i/o-decoding-error?)
1392 An exception with this type is raised when one of the operations for
1393 textual input from a port encounters a sequence of bytes that cannot be
1394 translated into a character or string by the input direction of the
1397 When such an exception is raised, the port's position is past the
1401 @deffn {Condition Type} &i/o-encoding
1402 @deffnx {Scheme Procedure} make-i/o-encoding-error port char
1403 @deffnx {Scheme Procedure} i/o-encoding-error? obj
1404 @deffnx {Scheme Procedure} i/o-encoding-error-char condition
1406 This condition type could be defined by
1409 (define-condition-type &i/o-encoding &i/o-port
1410 make-i/o-encoding-error i/o-encoding-error?
1411 (char i/o-encoding-error-char))
1414 An exception with this type is raised when one of the operations for
1415 textual output to a port encounters a character that cannot be
1416 translated into bytes by the output direction of the port's transcoder.
1417 @var{Char} is the character that could not be encoded.
1420 @deffn {Scheme Syntax} error-handling-mode @var{error-handling-mode-symbol}
1422 @var{error-handling-mode-symbol} should be a symbol whose name is one of
1423 @code{ignore}, @code{raise}, and @code{replace}. The form evaluates to
1424 the corresponding symbol. If @var{error-handling-mode-symbol} is not
1425 one of these identifiers, effect and result are
1426 implementation-dependent: The result may be an error-handling-mode
1427 symbol acceptable as a @var{handling-mode} argument to
1428 @code{make-transcoder}. If it is not acceptable as a
1429 @var{handling-mode} argument to @code{make-transcoder}, an exception is
1433 Only the name of @var{error-handling-style-symbol} is significant.
1436 The error-handling mode of a transcoder specifies the behavior
1437 of textual I/O operations in the presence of encoding or decoding
1440 If a textual input operation encounters an invalid or incomplete
1441 character encoding, and the error-handling mode is @code{ignore}, an
1442 appropriate number of bytes of the invalid encoding are ignored and
1443 decoding continues with the following bytes.
1445 If the error-handling mode is @code{replace}, the replacement
1446 character U+FFFD is injected into the data stream, an appropriate
1447 number of bytes are ignored, and decoding
1448 continues with the following bytes.
1450 If the error-handling mode is @code{raise}, an exception with condition
1451 type @code{&i/o-decoding} is raised.
1453 If a textual output operation encounters a character it cannot encode,
1454 and the error-handling mode is @code{ignore}, the character is ignored
1455 and encoding continues with the next character. If the error-handling
1456 mode is @code{replace}, a codec-specific replacement character is
1457 emitted by the transcoder, and encoding continues with the next
1458 character. The replacement character is U+FFFD for transcoders whose
1459 codec is one of the Unicode encodings, but is the @code{?} character
1460 for the Latin-1 encoding. If the error-handling mode is @code{raise},
1461 an exception with condition type @code{&i/o-encoding} is raised.
1464 @deffn {Scheme Procedure} make-transcoder codec
1465 @deffnx {Scheme Procedure} make-transcoder codec eol-style
1466 @deffnx {Scheme Procedure} make-transcoder codec eol-style handling-mode
1468 @var{codec} must be a codec; @var{eol-style}, if present, an eol-style
1469 symbol; and @var{handling-mode}, if present, an error-handling-mode
1472 @var{eol-style} may be omitted, in which case it defaults to the native
1473 end-of-line style of the underlying platform. @var{Handling-mode} may
1474 be omitted, in which case it defaults to @code{replace}. The result is
1475 a transcoder with the behavior specified by its arguments.
1478 @deffn {Scheme procedure} native-transcoder
1479 Returns an implementation-dependent transcoder that represents a
1480 possibly locale-dependent ``native'' transcoding.
1483 @deffn {Scheme Procedure} transcoder-codec transcoder
1484 @deffnx {Scheme Procedure} transcoder-eol-style transcoder
1485 @deffnx {Scheme Procedure} transcoder-error-handling-mode transcoder
1487 These are accessors for transcoder objects; when applied to a
1488 transcoder returned by @code{make-transcoder}, they return the
1489 @var{codec}, @var{eol-style}, and @var{handling-mode} arguments,
1493 @deffn {Scheme Procedure} bytevector->string bytevector transcoder
1495 Returns the string that results from transcoding the
1496 @var{bytevector} according to the input direction of the transcoder.
1499 @deffn {Scheme Procedure} string->bytevector string transcoder
1501 Returns the bytevector that results from transcoding the
1502 @var{string} according to the output direction of the transcoder.
1505 @node R6RS End-of-File
1506 @subsubsection The End-of-File Object
1511 R5RS' @code{eof-object?} procedure is provided by the @code{(rnrs io
1514 @deffn {Scheme Procedure} eof-object? obj
1515 @deffnx {C Function} scm_eof_object_p (obj)
1516 Return true if @var{obj} is the end-of-file (EOF) object.
1519 In addition, the following procedure is provided:
1521 @deffn {Scheme Procedure} eof-object
1522 @deffnx {C Function} scm_eof_object ()
1523 Return the end-of-file (EOF) object.
1526 (eof-object? (eof-object))
1532 @node R6RS Port Manipulation
1533 @subsubsection Port Manipulation
1535 The procedures listed below operate on any kind of R6RS I/O port.
1537 @deffn {Scheme Procedure} port? obj
1538 Returns @code{#t} if the argument is a port, and returns @code{#f}
1542 @deffn {Scheme Procedure} port-transcoder port
1543 Returns the transcoder associated with @var{port} if @var{port} is
1544 textual and has an associated transcoder, and returns @code{#f} if
1545 @var{port} is binary or does not have an associated transcoder.
1548 @deffn {Scheme Procedure} binary-port? port
1549 Return @code{#t} if @var{port} is a @dfn{binary port}, suitable for
1550 binary data input/output.
1552 Note that internally Guile does not differentiate between binary and
1553 textual ports, unlike the R6RS. Thus, this procedure returns true when
1554 @var{port} does not have an associated encoding---i.e., when
1555 @code{(port-encoding @var{port})} is @code{#f} (@pxref{Ports,
1556 port-encoding}). This is the case for ports returned by R6RS procedures
1557 such as @code{open-bytevector-input-port} and
1558 @code{make-custom-binary-output-port}.
1560 However, Guile currently does not prevent use of textual I/O procedures
1561 such as @code{display} or @code{read-char} with binary ports. Doing so
1562 ``upgrades'' the port from binary to textual, under the ISO-8859-1
1563 encoding. Likewise, Guile does not prevent use of
1564 @code{set-port-encoding!} on a binary port, which also turns it into a
1568 @deffn {Scheme Procedure} textual-port? port
1569 Always return @var{#t}, as all ports can be used for textual I/O in
1573 @deffn {Scheme Procedure} transcoded-port obj
1574 The @code{transcoded-port} procedure
1575 returns a new textual port with the specified @var{transcoder}.
1576 Otherwise the new textual port's state is largely the same as
1577 that of @var{binary-port}.
1578 If @var{binary-port} is an input port, the new textual
1579 port will be an input port and
1580 will transcode the bytes that have not yet been read from
1582 If @var{binary-port} is an output port, the new textual
1583 port will be an output port and
1584 will transcode output characters into bytes that are
1585 written to the byte sink represented by @var{binary-port}.
1587 As a side effect, however, @code{transcoded-port}
1588 closes @var{binary-port} in
1589 a special way that allows the new textual port to continue to
1590 use the byte source or sink represented by @var{binary-port},
1591 even though @var{binary-port} itself is closed and cannot
1592 be used by the input and output operations described in this
1596 @deffn {Scheme Procedure} port-position port
1597 If @var{port} supports it (see below), return the offset (an integer)
1598 indicating where the next octet will be read from/written to in
1599 @var{port}. If @var{port} does not support this operation, an error
1600 condition is raised.
1602 This is similar to Guile's @code{seek} procedure with the
1603 @code{SEEK_CUR} argument (@pxref{Random Access}).
1606 @deffn {Scheme Procedure} port-has-port-position? port
1607 Return @code{#t} is @var{port} supports @code{port-position}.
1610 @deffn {Scheme Procedure} set-port-position! port offset
1611 If @var{port} supports it (see below), set the position where the next
1612 octet will be read from/written to @var{port} to @var{offset} (an
1613 integer). If @var{port} does not support this operation, an error
1614 condition is raised.
1616 This is similar to Guile's @code{seek} procedure with the
1617 @code{SEEK_SET} argument (@pxref{Random Access}).
1620 @deffn {Scheme Procedure} port-has-set-port-position!? port
1621 Return @code{#t} is @var{port} supports @code{set-port-position!}.
1624 @deffn {Scheme Procedure} call-with-port port proc
1625 Call @var{proc}, passing it @var{port} and closing @var{port} upon exit
1626 of @var{proc}. Return the return values of @var{proc}.
1629 @node R6RS Input Ports
1630 @subsubsection Input Ports
1632 @deffn {Scheme Procedure} input-port? obj@
1633 Returns @code{#t} if the argument is an input port (or a combined input
1634 and output port), and returns @code{#f} otherwise.
1637 @deffn {Scheme Procedure} port-eof? port
1639 if the @code{lookahead-u8} procedure (if @var{input-port} is a binary port)
1640 or the @code{lookahead-char} procedure (if @var{input-port} is a textual port)
1642 the end-of-file object, and @code{#f} otherwise.
1643 The operation may block indefinitely if no data is available
1644 but the port cannot be determined to be at end of file.
1647 @deffn {Scheme Procedure} open-file-input-port filename
1648 @deffnx {Scheme Procedure} open-file-input-port filename file-options
1649 @deffnx {Scheme Procedure} open-file-input-port filename file-options buffer-mode
1650 @deffnx {Scheme Procedure} open-file-input-port filename file-options buffer-mode maybe-transcoder
1651 @var{Maybe-transcoder} must be either a transcoder or @code{#f}.
1653 The @code{open-file-input-port} procedure returns an
1654 input port for the named file. The @var{file-options} and
1655 @var{maybe-transcoder} arguments are optional.
1657 The @var{file-options} argument, which may determine
1658 various aspects of the returned port (@pxref{R6RS File Options}),
1659 defaults to the value of @code{(file-options)}.
1661 The @var{buffer-mode} argument, if supplied,
1662 must be one of the symbols that name a buffer mode.
1663 The @var{buffer-mode} argument defaults to @code{block}.
1665 If @var{maybe-transcoder} is a transcoder, it becomes the transcoder associated
1666 with the returned port.
1668 If @var{maybe-transcoder} is @code{#f} or absent,
1669 the port will be a binary port and will support the
1670 @code{port-position} and @code{set-port-position!} operations.
1671 Otherwise the port will be a textual port, and whether it supports
1672 the @code{port-position} and @code{set-port-position!} operations
1673 is implementation-dependent (and possibly transcoder-dependent).
1676 @deffn {Scheme Procedure} standard-input-port
1677 Returns a fresh binary input port connected to standard input. Whether
1678 the port supports the @code{port-position} and @code{set-port-position!}
1679 operations is implementation-dependent.
1682 @deffn {Scheme Procedure} current-input-port
1683 This returns a default textual port for input. Normally, this default
1684 port is associated with standard input, but can be dynamically
1685 re-assigned using the @code{with-input-from-file} procedure from the
1686 @code{io simple (6)} library (@pxref{rnrs io simple}). The port may or
1687 may not have an associated transcoder; if it does, the transcoder is
1688 implementation-dependent.
1691 @node R6RS Binary Input
1692 @subsubsection Binary Input
1694 @cindex binary input
1696 R6RS binary input ports can be created with the procedures described
1699 @deffn {Scheme Procedure} open-bytevector-input-port bv [transcoder]
1700 @deffnx {C Function} scm_open_bytevector_input_port (bv, transcoder)
1701 Return an input port whose contents are drawn from bytevector @var{bv}
1702 (@pxref{Bytevectors}).
1704 @c FIXME: Update description when implemented.
1705 The @var{transcoder} argument is currently not supported.
1708 @cindex custom binary input ports
1710 @deffn {Scheme Procedure} make-custom-binary-input-port id read! get-position set-position! close
1711 @deffnx {C Function} scm_make_custom_binary_input_port (id, read!, get-position, set-position!, close)
1712 Return a new custom binary input port@footnote{This is similar in spirit
1713 to Guile's @dfn{soft ports} (@pxref{Soft Ports}).} named @var{id} (a
1714 string) whose input is drained by invoking @var{read!} and passing it a
1715 bytevector, an index where bytes should be written, and the number of
1716 bytes to read. The @code{read!} procedure must return an integer
1717 indicating the number of bytes read, or @code{0} to indicate the
1720 Optionally, if @var{get-position} is not @code{#f}, it must be a thunk
1721 that will be called when @var{port-position} is invoked on the custom
1722 binary port and should return an integer indicating the position within
1723 the underlying data stream; if @var{get-position} was not supplied, the
1724 returned port does not support @var{port-position}.
1726 Likewise, if @var{set-position!} is not @code{#f}, it should be a
1727 one-argument procedure. When @var{set-port-position!} is invoked on the
1728 custom binary input port, @var{set-position!} is passed an integer
1729 indicating the position of the next byte is to read.
1731 Finally, if @var{close} is not @code{#f}, it must be a thunk. It is
1732 invoked when the custom binary input port is closed.
1734 Using a custom binary input port, the @code{open-bytevector-input-port}
1735 procedure could be implemented as follows:
1738 (define (open-bytevector-input-port source)
1740 (define length (bytevector-length source))
1742 (define (read! bv start count)
1743 (let ((count (min count (- length position))))
1744 (bytevector-copy! source position
1746 (set! position (+ position count))
1749 (define (get-position) position)
1751 (define (set-position! new-position)
1752 (set! position new-position))
1754 (make-custom-binary-input-port "the port" read!
1758 (read (open-bytevector-input-port (string->utf8 "hello")))
1763 @cindex binary input
1764 Binary input is achieved using the procedures below:
1766 @deffn {Scheme Procedure} get-u8 port
1767 @deffnx {C Function} scm_get_u8 (port)
1768 Return an octet read from @var{port}, a binary input port, blocking as
1769 necessary, or the end-of-file object.
1772 @deffn {Scheme Procedure} lookahead-u8 port
1773 @deffnx {C Function} scm_lookahead_u8 (port)
1774 Like @code{get-u8} but does not update @var{port}'s position to point
1778 @deffn {Scheme Procedure} get-bytevector-n port count
1779 @deffnx {C Function} scm_get_bytevector_n (port, count)
1780 Read @var{count} octets from @var{port}, blocking as necessary and
1781 return a bytevector containing the octets read. If fewer bytes are
1782 available, a bytevector smaller than @var{count} is returned.
1785 @deffn {Scheme Procedure} get-bytevector-n! port bv start count
1786 @deffnx {C Function} scm_get_bytevector_n_x (port, bv, start, count)
1787 Read @var{count} bytes from @var{port} and store them in @var{bv}
1788 starting at index @var{start}. Return either the number of bytes
1789 actually read or the end-of-file object.
1792 @deffn {Scheme Procedure} get-bytevector-some port
1793 @deffnx {C Function} scm_get_bytevector_some (port)
1794 Read from @var{port}, blocking as necessary, until data are available or
1795 and end-of-file is reached. Return either a new bytevector containing
1796 the data read or the end-of-file object.
1799 @deffn {Scheme Procedure} get-bytevector-all port
1800 @deffnx {C Function} scm_get_bytevector_all (port)
1801 Read from @var{port}, blocking as necessary, until the end-of-file is
1802 reached. Return either a new bytevector containing the data read or the
1803 end-of-file object (if no data were available).
1806 @node R6RS Textual Input
1807 @subsubsection Textual Input
1809 @deffn {Scheme Procedure} get-char port
1810 Reads from @var{textual-input-port}, blocking as necessary, until a
1811 complete character is available from @var{textual-input-port},
1812 or until an end of file is reached.
1814 If a complete character is available before the next end of file,
1815 @code{get-char} returns that character and updates the input port to
1816 point past the character. If an end of file is reached before any
1817 character is read, @code{get-char} returns the end-of-file object.
1820 @deffn {Scheme Procedure} lookahead-char port
1821 The @code{lookahead-char} procedure is like @code{get-char}, but it does
1822 not update @var{textual-input-port} to point past the character.
1825 @deffn {Scheme Procedure} get-string-n port count
1827 @var{Count} must be an exact, non-negative integer object, representing
1828 the number of characters to be read.
1830 The @code{get-string-n} procedure reads from @var{textual-input-port},
1831 blocking as necessary, until @var{count} characters are available, or
1832 until an end of file is reached.
1834 If @var{count} characters are available before end of file,
1835 @code{get-string-n} returns a string consisting of those @var{count}
1836 characters. If fewer characters are available before an end of file, but
1837 one or more characters can be read, @code{get-string-n} returns a string
1838 containing those characters. In either case, the input port is updated
1839 to point just past the characters read. If no characters can be read
1840 before an end of file, the end-of-file object is returned.
1843 @deffn {Scheme Procedure} get-string-n! port string start count
1845 @var{Start} and @var{count} must be exact, non-negative integer objects,
1846 with @var{count} representing the number of characters to be read.
1847 @var{String} must be a string with at least $@var{start} + @var{count}$
1850 The @code{get-string-n!} procedure reads from @var{textual-input-port}
1851 in the same manner as @code{get-string-n}. If @var{count} characters
1852 are available before an end of file, they are written into @var{string}
1853 starting at index @var{start}, and @var{count} is returned. If fewer
1854 characters are available before an end of file, but one or more can be
1855 read, those characters are written into @var{string} starting at index
1856 @var{start} and the number of characters actually read is returned as an
1857 exact integer object. If no characters can be read before an end of
1858 file, the end-of-file object is returned.
1861 @deffn {Scheme Procedure} get-string-all port count
1862 Reads from @var{textual-input-port} until an end of file, decoding
1863 characters in the same manner as @code{get-string-n} and
1864 @code{get-string-n!}.
1866 If characters are available before the end of file, a string containing
1867 all the characters decoded from that data are returned. If no character
1868 precedes the end of file, the end-of-file object is returned.
1871 @deffn {Scheme Procedure} get-line port
1872 Reads from @var{textual-input-port} up to and including the linefeed
1873 character or end of file, decoding characters in the same manner as
1874 @code{get-string-n} and @code{get-string-n!}.
1876 If a linefeed character is read, a string containing all of the text up
1877 to (but not including) the linefeed character is returned, and the port
1878 is updated to point just past the linefeed character. If an end of file
1879 is encountered before any linefeed character is read, but some
1880 characters have been read and decoded as characters, a string containing
1881 those characters is returned. If an end of file is encountered before
1882 any characters are read, the end-of-file object is returned.
1885 The end-of-line style, if not @code{none}, will cause all line endings
1886 to be read as linefeed characters. @xref{R6RS Transcoders}.
1890 @deffn {Scheme Procedure} get-datum port count
1891 Reads an external representation from @var{textual-input-port} and returns the
1892 datum it represents. The @code{get-datum} procedure returns the next
1893 datum that can be parsed from the given @var{textual-input-port}, updating
1894 @var{textual-input-port} to point exactly past the end of the external
1895 representation of the object.
1897 Any @emph{interlexeme space} (comment or whitespace, @pxref{Scheme
1898 Syntax}) in the input is first skipped. If an end of file occurs after
1899 the interlexeme space, the end-of-file object (@pxref{R6RS End-of-File})
1902 If a character inconsistent with an external representation is
1903 encountered in the input, an exception with condition types
1904 @code{&lexical} and @code{&i/o-read} is raised. Also, if the end of
1905 file is encountered after the beginning of an external representation,
1906 but the external representation is incomplete and therefore cannot be
1907 parsed, an exception with condition types @code{&lexical} and
1908 @code{&i/o-read} is raised.
1911 @node R6RS Output Ports
1912 @subsubsection Output Ports
1914 @deffn {Scheme Procedure} output-port? obj
1915 Returns @code{#t} if the argument is an output port (or a
1916 combined input and output port), @code{#f} otherwise.
1919 @deffn {Scheme Procedure} flush-output-port port
1920 Flushes any buffered output from the buffer of @var{output-port} to the
1921 underlying file, device, or object. The @code{flush-output-port}
1922 procedure returns an unspecified values.
1925 @deffn {Scheme Procedure} open-file-output-port filename
1926 @deffnx {Scheme Procedure} open-file-output-port filename file-options
1927 @deffnx {Scheme Procedure} open-file-output-port filename file-options buffer-mode
1928 @deffnx {Scheme Procedure} open-file-output-port filename file-options buffer-mode maybe-transcoder
1930 @var{maybe-transcoder} must be either a transcoder or @code{#f}.
1932 The @code{open-file-output-port} procedure returns an output port for the named file.
1934 The @var{file-options} argument, which may determine various aspects of
1935 the returned port (@pxref{R6RS File Options}), defaults to the value of
1936 @code{(file-options)}.
1938 The @var{buffer-mode} argument, if supplied,
1939 must be one of the symbols that name a buffer mode.
1940 The @var{buffer-mode} argument defaults to @code{block}.
1942 If @var{maybe-transcoder} is a transcoder, it becomes the transcoder
1943 associated with the port.
1945 If @var{maybe-transcoder} is @code{#f} or absent,
1946 the port will be a binary port and will support the
1947 @code{port-position} and @code{set-port-position!} operations.
1948 Otherwise the port will be a textual port, and whether it supports
1949 the @code{port-position} and @code{set-port-position!} operations
1950 is implementation-dependent (and possibly transcoder-dependent).
1953 @deffn {Scheme Procedure} standard-output-port
1954 @deffnx {Scheme Procedure} standard-error-port
1955 Returns a fresh binary output port connected to the standard output or
1956 standard error respectively. Whether the port supports the
1957 @code{port-position} and @code{set-port-position!} operations is
1958 implementation-dependent.
1961 @deffn {Scheme Procedure} current-output-port
1962 @deffnx {Scheme Procedure} current-error-port
1963 These return default textual ports for regular output and error output.
1964 Normally, these default ports are associated with standard output, and
1965 standard error, respectively. The return value of
1966 @code{current-output-port} can be dynamically re-assigned using the
1967 @code{with-output-to-file} procedure from the @code{io simple (6)}
1968 library (@pxref{rnrs io simple}). A port returned by one of these
1969 procedures may or may not have an associated transcoder; if it does, the
1970 transcoder is implementation-dependent.
1973 @node R6RS Binary Output
1974 @subsubsection Binary Output
1976 Binary output ports can be created with the procedures below.
1978 @deffn {Scheme Procedure} open-bytevector-output-port [transcoder]
1979 @deffnx {C Function} scm_open_bytevector_output_port (transcoder)
1980 Return two values: a binary output port and a procedure. The latter
1981 should be called with zero arguments to obtain a bytevector containing
1982 the data accumulated by the port, as illustrated below.
1987 (open-bytevector-output-port))
1988 (lambda (port get-bytevector)
1989 (display "hello" port)
1992 @result{} #vu8(104 101 108 108 111)
1995 @c FIXME: Update description when implemented.
1996 The @var{transcoder} argument is currently not supported.
1999 @cindex custom binary output ports
2001 @deffn {Scheme Procedure} make-custom-binary-output-port id write! get-position set-position! close
2002 @deffnx {C Function} scm_make_custom_binary_output_port (id, write!, get-position, set-position!, close)
2003 Return a new custom binary output port named @var{id} (a string) whose
2004 output is sunk by invoking @var{write!} and passing it a bytevector, an
2005 index where bytes should be read from this bytevector, and the number of
2006 bytes to be ``written''. The @code{write!} procedure must return an
2007 integer indicating the number of bytes actually written; when it is
2008 passed @code{0} as the number of bytes to write, it should behave as
2009 though an end-of-file was sent to the byte sink.
2011 The other arguments are as for @code{make-custom-binary-input-port}
2012 (@pxref{R6RS Binary Input, @code{make-custom-binary-input-port}}).
2015 @cindex binary output
2016 Writing to a binary output port can be done using the following
2019 @deffn {Scheme Procedure} put-u8 port octet
2020 @deffnx {C Function} scm_put_u8 (port, octet)
2021 Write @var{octet}, an integer in the 0--255 range, to @var{port}, a
2025 @deffn {Scheme Procedure} put-bytevector port bv [start [count]]
2026 @deffnx {C Function} scm_put_bytevector (port, bv, start, count)
2027 Write the contents of @var{bv} to @var{port}, optionally starting at
2028 index @var{start} and limiting to @var{count} octets.
2031 @node R6RS Textual Output
2032 @subsubsection Textual Output
2034 @deffn {Scheme Procedure} put-char port char
2035 Writes @var{char} to the port. The @code{put-char} procedure returns
2038 @deffn {Scheme Procedure} put-string port string
2039 @deffnx {Scheme Procedure} put-string port string start
2040 @deffnx {Scheme Procedure} put-string port string start count
2042 @var{start} and @var{count} must be non-negative exact integer objects.
2043 @var{string} must have a length of at least @math{@var{start} +
2044 @var{count}}. @var{start} defaults to 0. @var{count} defaults to
2045 @math{@code{(string-length @var{string})} - @var{start}}$. The
2046 @code{put-string} procedure writes the @var{count} characters of
2047 @var{string} starting at index @var{start} to the port. The
2048 @code{put-string} procedure returns an unspecified value.
2051 @deffn {Scheme Procedure} put-datum port datum
2052 @var{datum} should be a datum value. The @code{put-datum} procedure
2053 writes an external representation of @var{datum} to
2054 @var{textual-output-port}. The specific external representation is
2055 implementation-dependent. However, whenever possible, an implementation
2056 should produce a representation for which @code{get-datum}, when reading
2057 the representation, will return an object equal (in the sense of
2058 @code{equal?}) to @var{datum}.
2061 Not all datums may allow producing an external representation for which
2062 @code{get-datum} will produce an object that is equal to the
2063 original. Specifically, NaNs contained in @var{datum} may make
2068 The @code{put-datum} procedure merely writes the external
2069 representation, but no trailing delimiter. If @code{put-datum} is
2070 used to write several subsequent external representations to an
2071 output port, care should be taken to delimit them properly so they can
2072 be read back in by subsequent calls to @code{get-datum}.
2076 @node I/O Extensions
2077 @subsection Using and Extending Ports in C
2080 * C Port Interface:: Using ports from C.
2081 * Port Implementation:: How to implement a new port type in C.
2085 @node C Port Interface
2086 @subsubsection C Port Interface
2087 @cindex C port interface
2088 @cindex Port, C interface
2090 This section describes how to use Scheme ports from C.
2092 @subsubheading Port basics
2095 @tindex scm_ptob_descriptor
2097 @findex SCM_PTAB_ENTRY
2100 There are two main data structures. A port type object (ptob) is of
2101 type @code{scm_ptob_descriptor}. A port instance is of type
2102 @code{scm_port}. Given an @code{SCM} variable which points to a port,
2103 the corresponding C port object can be obtained using the
2104 @code{SCM_PTAB_ENTRY} macro. The ptob can be obtained by using
2105 @code{SCM_PTOBNUM} to give an index into the @code{scm_ptobs}
2108 @subsubheading Port buffers
2110 An input port always has a read buffer and an output port always has a
2111 write buffer. However the size of these buffers is not guaranteed to be
2112 more than one byte (e.g., the @code{shortbuf} field in @code{scm_port}
2113 which is used when no other buffer is allocated). The way in which the
2114 buffers are allocated depends on the implementation of the ptob. For
2115 example in the case of an fport, buffers may be allocated with malloc
2116 when the port is created, but in the case of an strport the underlying
2117 string is used as the buffer.
2119 @subsubheading The @code{rw_random} flag
2121 Special treatment is required for ports which can be seeked at random.
2122 Before various operations, such as seeking the port or changing from
2123 input to output on a bidirectional port or vice versa, the port
2124 implementation must be given a chance to update its state. The write
2125 buffer is updated by calling the @code{flush} ptob procedure and the
2126 input buffer is updated by calling the @code{end_input} ptob procedure.
2127 In the case of an fport, @code{flush} causes buffered output to be
2128 written to the file descriptor, while @code{end_input} causes the
2129 descriptor position to be adjusted to account for buffered input which
2132 The special treatment must be performed if the @code{rw_random} flag in
2133 the port is non-zero.
2135 @subsubheading The @code{rw_active} variable
2137 The @code{rw_active} variable in the port is only used if
2138 @code{rw_random} is set. It's defined as an enum with the following
2143 the read buffer may have unread data.
2145 @item SCM_PORT_WRITE
2146 the write buffer may have unwritten data.
2148 @item SCM_PORT_NEITHER
2149 neither the write nor the read buffer has data.
2152 @subsubheading Reading from a port.
2154 To read from a port, it's possible to either call existing libguile
2155 procedures such as @code{scm_getc} and @code{scm_read_line} or to read
2156 data from the read buffer directly. Reading from the buffer involves
2157 the following steps:
2161 Flush output on the port, if @code{rw_active} is @code{SCM_PORT_WRITE}.
2164 Fill the read buffer, if it's empty, using @code{scm_fill_input}.
2166 @item Read the data from the buffer and update the read position in
2167 the buffer. Steps 2) and 3) may be repeated as many times as required.
2169 @item Set rw_active to @code{SCM_PORT_READ} if @code{rw_random} is set.
2171 @item update the port's line and column counts.
2174 @subsubheading Writing to a port.
2176 To write data to a port, calling @code{scm_lfwrite} should be sufficient for
2177 most purposes. This takes care of the following steps:
2181 End input on the port, if @code{rw_active} is @code{SCM_PORT_READ}.
2184 Pass the data to the ptob implementation using the @code{write} ptob
2185 procedure. The advantage of using the ptob @code{write} instead of
2186 manipulating the write buffer directly is that it allows the data to be
2187 written in one operation even if the port is using the single-byte
2191 Set @code{rw_active} to @code{SCM_PORT_WRITE} if @code{rw_random}
2196 @node Port Implementation
2197 @subsubsection Port Implementation
2198 @cindex Port implementation
2200 This section describes how to implement a new port type in C.
2202 As described in the previous section, a port type object (ptob) is
2203 a structure of type @code{scm_ptob_descriptor}. A ptob is created by
2204 calling @code{scm_make_port_type}.
2206 @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))
2207 Return a new port type object. The @var{name}, @var{fill_input} and
2208 @var{write} parameters are initial values for those port type fields,
2209 as described below. The other fields are initialized with default
2210 values and can be changed later.
2213 All of the elements of the ptob, apart from @code{name}, are procedures
2214 which collectively implement the port behaviour. Creating a new port
2215 type mostly involves writing these procedures.
2219 A pointer to a NUL terminated string: the name of the port type. This
2220 is the only element of @code{scm_ptob_descriptor} which is not
2221 a procedure. Set via the first argument to @code{scm_make_port_type}.
2224 Called during garbage collection to mark any SCM objects that a port
2225 object may contain. It doesn't need to be set unless the port has
2226 @code{SCM} components. Set using
2228 @deftypefun void scm_set_port_mark (scm_t_bits tc, SCM (*mark) (SCM port))
2232 Called when the port is collected during gc. It
2233 should free any resources used by the port.
2236 @deftypefun void scm_set_port_free (scm_t_bits tc, size_t (*free) (SCM port))
2240 Called when @code{write} is called on the port object, to print a
2241 port description. E.g., for an fport it may produce something like:
2242 @code{#<input: /etc/passwd 3>}. Set using
2244 @deftypefun void scm_set_port_print (scm_t_bits tc, int (*print) (SCM port, SCM dest_port, scm_print_state *pstate))
2245 The first argument @var{port} is the object being printed, the second
2246 argument @var{dest_port} is where its description should go.
2250 Not used at present. Set using
2252 @deftypefun void scm_set_port_equalp (scm_t_bits tc, SCM (*equalp) (SCM, SCM))
2256 Called when the port is closed, unless it was collected during gc. It
2257 should free any resources used by the port.
2260 @deftypefun void scm_set_port_close (scm_t_bits tc, int (*close) (SCM port))
2264 Accept data which is to be written using the port. The port implementation
2265 may choose to buffer the data instead of processing it directly.
2266 Set via the third argument to @code{scm_make_port_type}.
2269 Complete the processing of buffered output data. Reset the value of
2270 @code{rw_active} to @code{SCM_PORT_NEITHER}.
2273 @deftypefun void scm_set_port_flush (scm_t_bits tc, void (*flush) (SCM port))
2277 Perform any synchronization required when switching from input to output
2278 on the port. Reset the value of @code{rw_active} to @code{SCM_PORT_NEITHER}.
2281 @deftypefun void scm_set_port_end_input (scm_t_bits tc, void (*end_input) (SCM port, int offset))
2285 Read new data into the read buffer and return the first character. It
2286 can be assumed that the read buffer is empty when this procedure is called.
2287 Set via the second argument to @code{scm_make_port_type}.
2290 Return a lower bound on the number of bytes that could be read from the
2291 port without blocking. It can be assumed that the current state of
2292 @code{rw_active} is @code{SCM_PORT_NEITHER}.
2295 @deftypefun void scm_set_port_input_waiting (scm_t_bits tc, int (*input_waiting) (SCM port))
2299 Set the current position of the port. The procedure can not make
2300 any assumptions about the value of @code{rw_active} when it's
2301 called. It can reset the buffers first if desired by using something
2305 if (pt->rw_active == SCM_PORT_READ)
2306 scm_end_input (port);
2307 else if (pt->rw_active == SCM_PORT_WRITE)
2311 However note that this will have the side effect of discarding any data
2312 in the unread-char buffer, in addition to any side effects from the
2313 @code{end_input} and @code{flush} ptob procedures. This is undesirable
2314 when seek is called to measure the current position of the port, i.e.,
2315 @code{(seek p 0 SEEK_CUR)}. The libguile fport and string port
2316 implementations take care to avoid this problem.
2318 The procedure is set using
2320 @deftypefun void scm_set_port_seek (scm_t_bits tc, scm_t_off (*seek) (SCM port, scm_t_off offset, int whence))
2324 Truncate the port data to be specified length. It can be assumed that the
2325 current state of @code{rw_active} is @code{SCM_PORT_NEITHER}.
2328 @deftypefun void scm_set_port_truncate (scm_t_bits tc, void (*truncate) (SCM port, scm_t_off length))
2334 @c TeX-master: "guile.texi"