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1 | @c -*-texinfo-*- |
2 | @c This is part of the GNU Guile Reference Manual. | |
3 | @c Copyright (C) 1996, 1997, 2000, 2001, 2002, 2003, 2004 | |
4 | @c Free Software Foundation, Inc. | |
5 | @c See the file guile.texi for copying conditions. | |
6 | ||
7 | @page | |
8 | @node Input and Output | |
9 | @section Input and Output | |
10 | ||
11 | @menu | |
12 | * Ports:: The idea of the port abstraction. | |
13 | * Reading:: Procedures for reading from a port. | |
14 | * Writing:: Procedures for writing to a port. | |
15 | * Closing:: Procedures to close a port. | |
16 | * Random Access:: Moving around a random access port. | |
17 | * Line/Delimited:: Read and write lines or delimited text. | |
18 | * Block Reading and Writing:: Reading and writing blocks of text. | |
19 | * Default Ports:: Defaults for input, output and errors. | |
20 | * Port Types:: Types of port and how to make them. | |
21 | * I/O Extensions:: Using and extending ports in C. | |
22 | @end menu | |
23 | ||
24 | ||
25 | @node Ports | |
26 | @subsection Ports | |
27 | ||
28 | Sequential input/output in Scheme is represented by operations on a | |
29 | @dfn{port}. This chapter explains the operations that Guile provides | |
30 | for working with ports. | |
31 | ||
32 | Ports are created by opening, for instance @code{open-file} for a file | |
33 | (@pxref{File Ports}). Characters can be read from an input port and | |
34 | written to an output port, or both on an input/output port. A port | |
35 | can be closed (@pxref{Closing}) when no longer required, after which | |
36 | any attempt to read or write is an error. | |
37 | ||
38 | The formal definition of a port is very generic: an input port is | |
39 | simply ``an object which can deliver characters on demand,'' and an | |
40 | output port is ``an object which can accept characters.'' Because | |
41 | this definition is so loose, it is easy to write functions that | |
42 | simulate ports in software. @dfn{Soft ports} and @dfn{string ports} | |
43 | are two interesting and powerful examples of this technique. | |
44 | (@pxref{Soft Ports}, and @ref{String Ports}.) | |
45 | ||
46 | Ports are garbage collected in the usual way (@pxref{Memory | |
47 | Management}), and will be closed at that time if not already closed. | |
48 | In this case any errors occuring in the close will not be reported. | |
49 | Usually a program will want to explicitly close so as to be sure all | |
50 | its operations have been successful. Of course if a program has | |
51 | abandoned something due to an error or other condition then closing | |
52 | problems are probably not of interest. | |
53 | ||
54 | It is strongly recommended that file ports be closed explicitly when | |
55 | no longer required. Most systems have limits on how many files can be | |
56 | open, both on a per-process and a system-wide basis. A program that | |
57 | uses many files should take care not to hit those limits. The same | |
58 | applies to similar system resources such as pipes and sockets. | |
59 | ||
60 | Note that automatic garbage collection is triggered only by memory | |
61 | consumption, not by file or other resource usage, so a program cannot | |
62 | rely on that to keep it away from system limits. An explicit call to | |
63 | @code{gc} can of course be relied on to pick up unreferenced ports. | |
64 | If program flow makes it hard to be certain when to close then this | |
65 | may be an acceptable way to control resource usage. | |
66 | ||
67 | @rnindex input-port? | |
68 | @deffn {Scheme Procedure} input-port? x | |
69 | @deffnx {C Function} scm_input_port_p (x) | |
70 | Return @code{#t} if @var{x} is an input port, otherwise return | |
71 | @code{#f}. Any object satisfying this predicate also satisfies | |
72 | @code{port?}. | |
73 | @end deffn | |
74 | ||
75 | @rnindex output-port? | |
76 | @deffn {Scheme Procedure} output-port? x | |
77 | @deffnx {C Function} scm_output_port_p (x) | |
78 | Return @code{#t} if @var{x} is an output port, otherwise return | |
79 | @code{#f}. Any object satisfying this predicate also satisfies | |
80 | @code{port?}. | |
81 | @end deffn | |
82 | ||
83 | @deffn {Scheme Procedure} port? x | |
84 | @deffnx {C Function} scm_port_p (x) | |
85 | Return a boolean indicating whether @var{x} is a port. | |
86 | Equivalent to @code{(or (input-port? @var{x}) (output-port? | |
87 | @var{x}))}. | |
88 | @end deffn | |
89 | ||
90 | ||
91 | @node Reading | |
92 | @subsection Reading | |
93 | ||
94 | [Generic procedures for reading from ports.] | |
95 | ||
96 | @rnindex eof-object? | |
97 | @deffn {Scheme Procedure} eof-object? x | |
98 | @deffnx {C Function} scm_eof_object_p (x) | |
99 | Return @code{#t} if @var{x} is an end-of-file object; otherwise | |
100 | return @code{#f}. | |
101 | @end deffn | |
102 | ||
103 | @rnindex char-ready? | |
104 | @deffn {Scheme Procedure} char-ready? [port] | |
105 | @deffnx {C Function} scm_char_ready_p (port) | |
106 | Return @code{#t} if a character is ready on input @var{port} | |
107 | and return @code{#f} otherwise. If @code{char-ready?} returns | |
108 | @code{#t} then the next @code{read-char} operation on | |
109 | @var{port} is guaranteed not to hang. If @var{port} is a file | |
110 | port at end of file then @code{char-ready?} returns @code{#t}. | |
111 | @footnote{@code{char-ready?} exists to make it possible for a | |
112 | program to accept characters from interactive ports without | |
113 | getting stuck waiting for input. Any input editors associated | |
114 | with such ports must make sure that characters whose existence | |
115 | has been asserted by @code{char-ready?} cannot be rubbed out. | |
116 | If @code{char-ready?} were to return @code{#f} at end of file, | |
117 | a port at end of file would be indistinguishable from an | |
118 | interactive port that has no ready characters.} | |
119 | @end deffn | |
120 | ||
121 | @rnindex read-char | |
122 | @deffn {Scheme Procedure} read-char [port] | |
123 | @deffnx {C Function} scm_read_char (port) | |
124 | Return the next character available from @var{port}, updating | |
125 | @var{port} to point to the following character. If no more | |
126 | characters are available, the end-of-file object is returned. | |
127 | @end deffn | |
128 | ||
129 | @deftypefn {C Function} size_t scm_c_read (SCM port, void *buffer, size_t size) | |
130 | Read up to @var{size} bytes from @var{port} and store them in | |
131 | @var{buffer}. The return value is the number of bytes actually read, | |
132 | which can be less than @var{size} if end-of-file has been reached. | |
133 | ||
134 | Note that this function does not update @code{port-line} and | |
135 | @code{port-column} below. | |
136 | @end deftypefn | |
137 | ||
138 | @rnindex peek-char | |
139 | @deffn {Scheme Procedure} peek-char [port] | |
140 | @deffnx {C Function} scm_peek_char (port) | |
141 | Return the next character available from @var{port}, | |
142 | @emph{without} updating @var{port} to point to the following | |
143 | character. If no more characters are available, the | |
144 | end-of-file object is returned.@footnote{The value returned by | |
145 | a call to @code{peek-char} is the same as the value that would | |
146 | have been returned by a call to @code{read-char} on the same | |
147 | port. The only difference is that the very next call to | |
148 | @code{read-char} or @code{peek-char} on that @var{port} will | |
149 | return the value returned by the preceding call to | |
150 | @code{peek-char}. In particular, a call to @code{peek-char} on | |
151 | an interactive port will hang waiting for input whenever a call | |
152 | to @code{read-char} would have hung.} | |
153 | @end deffn | |
154 | ||
155 | @deffn {Scheme Procedure} unread-char cobj [port] | |
156 | @deffnx {C Function} scm_unread_char (cobj, port) | |
157 | Place @var{char} in @var{port} so that it will be read by the | |
158 | next read operation. If called multiple times, the unread characters | |
159 | will be read again in last-in first-out order. If @var{port} is | |
160 | not supplied, the current input port is used. | |
161 | @end deffn | |
162 | ||
163 | @deffn {Scheme Procedure} unread-string str port | |
164 | @deffnx {C Function} scm_unread_string (str, port) | |
165 | Place the string @var{str} in @var{port} so that its characters will | |
166 | be read from left-to-right as the next characters from @var{port} | |
167 | during subsequent read operations. If called multiple times, the | |
168 | unread characters will be read again in last-in first-out order. If | |
169 | @var{port} is not supplied, the current-input-port is used. | |
170 | @end deffn | |
171 | ||
172 | @deffn {Scheme Procedure} drain-input port | |
173 | @deffnx {C Function} scm_drain_input (port) | |
174 | This procedure clears a port's input buffers, similar | |
175 | to the way that force-output clears the output buffer. The | |
176 | contents of the buffers are returned as a single string, e.g., | |
177 | ||
178 | @lisp | |
179 | (define p (open-input-file ...)) | |
180 | (drain-input p) => empty string, nothing buffered yet. | |
181 | (unread-char (read-char p) p) | |
182 | (drain-input p) => initial chars from p, up to the buffer size. | |
183 | @end lisp | |
184 | ||
185 | Draining the buffers may be useful for cleanly finishing | |
186 | buffered I/O so that the file descriptor can be used directly | |
187 | for further input. | |
188 | @end deffn | |
189 | ||
190 | @deffn {Scheme Procedure} port-column port | |
191 | @deffnx {Scheme Procedure} port-line port | |
192 | @deffnx {C Function} scm_port_column (port) | |
193 | @deffnx {C Function} scm_port_line (port) | |
194 | Return the current column number or line number of @var{port}. | |
195 | If the number is | |
196 | unknown, the result is #f. Otherwise, the result is a 0-origin integer | |
197 | - i.e.@: the first character of the first line is line 0, column 0. | |
198 | (However, when you display a file position, for example in an error | |
199 | message, we recommend you add 1 to get 1-origin integers. This is | |
200 | because lines and column numbers traditionally start with 1, and that is | |
201 | what non-programmers will find most natural.) | |
202 | @end deffn | |
203 | ||
204 | @deffn {Scheme Procedure} set-port-column! port column | |
205 | @deffnx {Scheme Procedure} set-port-line! port line | |
206 | @deffnx {C Function} scm_set_port_column_x (port, column) | |
207 | @deffnx {C Function} scm_set_port_line_x (port, line) | |
208 | Set the current column or line number of @var{port}. | |
209 | @end deffn | |
210 | ||
211 | @node Writing | |
212 | @subsection Writing | |
213 | ||
214 | [Generic procedures for writing to ports.] | |
215 | ||
216 | @deffn {Scheme Procedure} get-print-state port | |
217 | @deffnx {C Function} scm_get_print_state (port) | |
218 | Return the print state of the port @var{port}. If @var{port} | |
219 | has no associated print state, @code{#f} is returned. | |
220 | @end deffn | |
221 | ||
222 | @rnindex write | |
223 | @deffn {Scheme Procedure} write obj [port] | |
224 | Send a representation of @var{obj} to @var{port} or to the current | |
225 | output port if not given. | |
226 | ||
227 | The output is designed to be machine readable, and can be read back | |
228 | with @code{read} (@pxref{Reading}). Strings are printed in | |
229 | doublequotes, with escapes if necessary, and characters are printed in | |
230 | @samp{#\} notation. | |
231 | @end deffn | |
232 | ||
233 | @rnindex display | |
234 | @deffn {Scheme Procedure} display obj [port] | |
235 | Send a representation of @var{obj} to @var{port} or to the current | |
236 | output port if not given. | |
237 | ||
238 | The output is designed for human readability, it differs from | |
239 | @code{write} in that strings are printed without doublequotes and | |
240 | escapes, and characters are printed as per @code{write-char}, not in | |
241 | @samp{#\} form. | |
242 | @end deffn | |
243 | ||
244 | @rnindex newline | |
245 | @deffn {Scheme Procedure} newline [port] | |
246 | @deffnx {C Function} scm_newline (port) | |
247 | Send a newline to @var{port}. | |
248 | If @var{port} is omitted, send to the current output port. | |
249 | @end deffn | |
250 | ||
251 | @deffn {Scheme Procedure} port-with-print-state port pstate | |
252 | @deffnx {C Function} scm_port_with_print_state (port, pstate) | |
253 | Create a new port which behaves like @var{port}, but with an | |
254 | included print state @var{pstate}. | |
255 | @end deffn | |
256 | ||
257 | @deffn {Scheme Procedure} print-options-interface [setting] | |
258 | @deffnx {C Function} scm_print_options (setting) | |
259 | Option interface for the print options. Instead of using | |
260 | this procedure directly, use the procedures | |
261 | @code{print-enable}, @code{print-disable}, @code{print-set!} | |
262 | and @code{print-options}. | |
263 | @end deffn | |
264 | ||
265 | @deffn {Scheme Procedure} simple-format destination message . args | |
266 | @deffnx {C Function} scm_simple_format (destination, message, args) | |
267 | Write @var{message} to @var{destination}, defaulting to | |
268 | the current output port. | |
269 | @var{message} can contain @code{~A} (was @code{%s}) and | |
270 | @code{~S} (was @code{%S}) escapes. When printed, | |
271 | the escapes are replaced with corresponding members of | |
272 | @var{ARGS}: | |
273 | @code{~A} formats using @code{display} and @code{~S} formats | |
274 | using @code{write}. | |
275 | If @var{destination} is @code{#t}, then use the current output | |
276 | port, if @var{destination} is @code{#f}, then return a string | |
277 | containing the formatted text. Does not add a trailing newline. | |
278 | @end deffn | |
279 | ||
280 | @rnindex write-char | |
281 | @deffn {Scheme Procedure} write-char chr [port] | |
282 | @deffnx {C Function} scm_write_char (chr, port) | |
283 | Send character @var{chr} to @var{port}. | |
284 | @end deffn | |
285 | ||
286 | @deftypefn {C Function} void scm_c_write (SCM port, const void *buffer, size_t size) | |
287 | Write @var{size} bytes at @var{buffer} to @var{port}. | |
288 | ||
289 | Note that this function does not update @code{port-line} and | |
290 | @code{port-column} (@pxref{Reading}). | |
291 | @end deftypefn | |
292 | ||
293 | @findex fflush | |
294 | @deffn {Scheme Procedure} force-output [port] | |
295 | @deffnx {C Function} scm_force_output (port) | |
296 | Flush the specified output port, or the current output port if @var{port} | |
297 | is omitted. The current output buffer contents are passed to the | |
298 | underlying port implementation (e.g., in the case of fports, the | |
299 | data will be written to the file and the output buffer will be cleared.) | |
300 | It has no effect on an unbuffered port. | |
301 | ||
302 | The return value is unspecified. | |
303 | @end deffn | |
304 | ||
305 | @deffn {Scheme Procedure} flush-all-ports | |
306 | @deffnx {C Function} scm_flush_all_ports () | |
307 | Equivalent to calling @code{force-output} on | |
308 | all open output ports. The return value is unspecified. | |
309 | @end deffn | |
310 | ||
311 | ||
312 | @node Closing | |
313 | @subsection Closing | |
314 | ||
315 | @deffn {Scheme Procedure} close-port port | |
316 | @deffnx {C Function} scm_close_port (port) | |
317 | Close the specified port object. Return @code{#t} if it | |
318 | successfully closes a port or @code{#f} if it was already | |
319 | closed. An exception may be raised if an error occurs, for | |
320 | example when flushing buffered output. See also @ref{Ports and | |
321 | File Descriptors, close}, for a procedure which can close file | |
322 | descriptors. | |
323 | @end deffn | |
324 | ||
325 | @deffn {Scheme Procedure} close-input-port port | |
326 | @deffnx {Scheme Procedure} close-output-port port | |
327 | @deffnx {C Function} scm_close_input_port (port) | |
328 | @deffnx {C Function} scm_close_output_port (port) | |
329 | @rnindex close-input-port | |
330 | @rnindex close-output-port | |
331 | Close the specified input or output @var{port}. An exception may be | |
332 | raised if an error occurs while closing. If @var{port} is already | |
333 | closed, nothing is done. The return value is unspecified. | |
334 | ||
335 | See also @ref{Ports and File Descriptors, close}, for a procedure | |
336 | which can close file descriptors. | |
337 | @end deffn | |
338 | ||
339 | @deffn {Scheme Procedure} port-closed? port | |
340 | @deffnx {C Function} scm_port_closed_p (port) | |
341 | Return @code{#t} if @var{port} is closed or @code{#f} if it is | |
342 | open. | |
343 | @end deffn | |
344 | ||
345 | ||
346 | @node Random Access | |
347 | @subsection Random Access | |
348 | ||
349 | @deffn {Scheme Procedure} seek fd_port offset whence | |
350 | @deffnx {C Function} scm_seek (fd_port, offset, whence) | |
351 | Sets the current position of @var{fd/port} to the integer | |
352 | @var{offset}, which is interpreted according to the value of | |
353 | @var{whence}. | |
354 | ||
355 | One of the following variables should be supplied for | |
356 | @var{whence}: | |
357 | @defvar SEEK_SET | |
358 | Seek from the beginning of the file. | |
359 | @end defvar | |
360 | @defvar SEEK_CUR | |
361 | Seek from the current position. | |
362 | @end defvar | |
363 | @defvar SEEK_END | |
364 | Seek from the end of the file. | |
365 | @end defvar | |
366 | If @var{fd/port} is a file descriptor, the underlying system | |
367 | call is @code{lseek}. @var{port} may be a string port. | |
368 | ||
369 | The value returned is the new position in the file. This means | |
370 | that the current position of a port can be obtained using: | |
371 | @lisp | |
372 | (seek port 0 SEEK_CUR) | |
373 | @end lisp | |
374 | @end deffn | |
375 | ||
376 | @deffn {Scheme Procedure} ftell fd_port | |
377 | @deffnx {C Function} scm_ftell (fd_port) | |
378 | Return an integer representing the current position of | |
379 | @var{fd/port}, measured from the beginning. Equivalent to: | |
380 | ||
381 | @lisp | |
382 | (seek port 0 SEEK_CUR) | |
383 | @end lisp | |
384 | @end deffn | |
385 | ||
386 | @findex truncate | |
387 | @findex ftruncate | |
388 | @deffn {Scheme Procedure} truncate-file object [length] | |
389 | @deffnx {C Function} scm_truncate_file (object, length) | |
390 | Truncates the object referred to by @var{object} to at most | |
391 | @var{length} bytes. @var{object} can be a string containing a | |
392 | file name or an integer file descriptor or a port. | |
393 | @var{length} may be omitted if @var{object} is not a file name, | |
394 | in which case the truncation occurs at the current port. | |
395 | position. The return value is unspecified. | |
396 | @end deffn | |
397 | ||
398 | @node Line/Delimited | |
399 | @subsection Line Oriented and Delimited Text | |
400 | ||
401 | The delimited-I/O module can be accessed with: | |
402 | ||
403 | @smalllisp | |
404 | (use-modules (ice-9 rdelim)) | |
405 | @end smalllisp | |
406 | ||
407 | It can be used to read or write lines of text, or read text delimited by | |
408 | a specified set of characters. It's similar to the @code{(scsh rdelim)} | |
409 | module from guile-scsh, but does not use multiple values or character | |
410 | sets and has an extra procedure @code{write-line}. | |
411 | ||
412 | @c begin (scm-doc-string "rdelim.scm" "read-line") | |
413 | @deffn {Scheme Procedure} read-line [port] [handle-delim] | |
414 | Return a line of text from @var{port} if specified, otherwise from the | |
415 | value returned by @code{(current-input-port)}. Under Unix, a line of text | |
416 | is terminated by the first end-of-line character or by end-of-file. | |
417 | ||
418 | If @var{handle-delim} is specified, it should be one of the following | |
419 | symbols: | |
420 | @table @code | |
421 | @item trim | |
422 | Discard the terminating delimiter. This is the default, but it will | |
423 | be impossible to tell whether the read terminated with a delimiter or | |
424 | end-of-file. | |
425 | @item concat | |
426 | Append the terminating delimiter (if any) to the returned string. | |
427 | @item peek | |
428 | Push the terminating delimiter (if any) back on to the port. | |
429 | @item split | |
430 | Return a pair containing the string read from the port and the | |
431 | terminating delimiter or end-of-file object. | |
432 | @end table | |
433 | @end deffn | |
434 | ||
435 | @c begin (scm-doc-string "rdelim.scm" "read-line!") | |
436 | @deffn {Scheme Procedure} read-line! buf [port] | |
437 | Read a line of text into the supplied string @var{buf} and return the | |
438 | number of characters added to @var{buf}. If @var{buf} is filled, then | |
439 | @code{#f} is returned. | |
440 | Read from @var{port} if | |
441 | specified, otherwise from the value returned by @code{(current-input-port)}. | |
442 | @end deffn | |
443 | ||
444 | @c begin (scm-doc-string "rdelim.scm" "read-delimited") | |
445 | @deffn {Scheme Procedure} read-delimited delims [port] [handle-delim] | |
446 | Read text until one of the characters in the string @var{delims} is found | |
447 | or end-of-file is reached. Read from @var{port} if supplied, otherwise | |
448 | from the value returned by @code{(current-input-port)}. | |
449 | @var{handle-delim} takes the same values as described for @code{read-line}. | |
450 | @end deffn | |
451 | ||
452 | @c begin (scm-doc-string "rdelim.scm" "read-delimited!") | |
453 | @deffn {Scheme Procedure} read-delimited! delims buf [port] [handle-delim] [start] [end] | |
454 | Read text into the supplied string @var{buf} and return the number of | |
455 | characters added to @var{buf} (subject to @var{handle-delim}, which takes | |
456 | the same values specified for @code{read-line}. If @var{buf} is filled, | |
457 | @code{#f} is returned for both the number of characters read and the | |
458 | delimiter. Also terminates if one of the characters in the string | |
459 | @var{delims} is found | |
460 | or end-of-file is reached. Read from @var{port} if supplied, otherwise | |
461 | from the value returned by @code{(current-input-port)}. | |
462 | @end deffn | |
463 | ||
464 | @deffn {Scheme Procedure} write-line obj [port] | |
465 | @deffnx {C Function} scm_write_line (obj, port) | |
466 | Display @var{obj} and a newline character to @var{port}. If | |
467 | @var{port} is not specified, @code{(current-output-port)} is | |
468 | used. This function is equivalent to: | |
469 | @lisp | |
470 | (display obj [port]) | |
471 | (newline [port]) | |
472 | @end lisp | |
473 | @end deffn | |
474 | ||
475 | Some of the abovementioned I/O functions rely on the following C | |
476 | primitives. These will mainly be of interest to people hacking Guile | |
477 | internals. | |
478 | ||
479 | @deffn {Scheme Procedure} %read-delimited! delims str gobble [port [start [end]]] | |
480 | @deffnx {C Function} scm_read_delimited_x (delims, str, gobble, port, start, end) | |
481 | Read characters from @var{port} into @var{str} until one of the | |
482 | characters in the @var{delims} string is encountered. If | |
483 | @var{gobble} is true, discard the delimiter character; | |
484 | otherwise, leave it in the input stream for the next read. If | |
485 | @var{port} is not specified, use the value of | |
486 | @code{(current-input-port)}. If @var{start} or @var{end} are | |
487 | specified, store data only into the substring of @var{str} | |
488 | bounded by @var{start} and @var{end} (which default to the | |
489 | beginning and end of the string, respectively). | |
490 | ||
491 | Return a pair consisting of the delimiter that terminated the | |
492 | string and the number of characters read. If reading stopped | |
493 | at the end of file, the delimiter returned is the | |
494 | @var{eof-object}; if the string was filled without encountering | |
495 | a delimiter, this value is @code{#f}. | |
496 | @end deffn | |
497 | ||
498 | @deffn {Scheme Procedure} %read-line [port] | |
499 | @deffnx {C Function} scm_read_line (port) | |
500 | Read a newline-terminated line from @var{port}, allocating storage as | |
501 | necessary. The newline terminator (if any) is removed from the string, | |
502 | and a pair consisting of the line and its delimiter is returned. The | |
503 | delimiter may be either a newline or the @var{eof-object}; if | |
504 | @code{%read-line} is called at the end of file, it returns the pair | |
505 | @code{(#<eof> . #<eof>)}. | |
506 | @end deffn | |
507 | ||
508 | @node Block Reading and Writing | |
509 | @subsection Block reading and writing | |
510 | ||
511 | The Block-string-I/O module can be accessed with: | |
512 | ||
513 | @smalllisp | |
514 | (use-modules (ice-9 rw)) | |
515 | @end smalllisp | |
516 | ||
517 | It currently contains procedures that help to implement the | |
518 | @code{(scsh rw)} module in guile-scsh. | |
519 | ||
520 | @deffn {Scheme Procedure} read-string!/partial str [port_or_fdes [start [end]]] | |
521 | @deffnx {C Function} scm_read_string_x_partial (str, port_or_fdes, start, end) | |
522 | Read characters from a port or file descriptor into a | |
523 | string @var{str}. A port must have an underlying file | |
524 | descriptor --- a so-called fport. This procedure is | |
525 | scsh-compatible and can efficiently read large strings. | |
526 | It will: | |
527 | ||
528 | @itemize | |
529 | @item | |
530 | attempt to fill the entire string, unless the @var{start} | |
531 | and/or @var{end} arguments are supplied. i.e., @var{start} | |
532 | defaults to 0 and @var{end} defaults to | |
533 | @code{(string-length str)} | |
534 | @item | |
535 | use the current input port if @var{port_or_fdes} is not | |
536 | supplied. | |
537 | @item | |
538 | return fewer than the requested number of characters in some | |
539 | cases, e.g., on end of file, if interrupted by a signal, or if | |
540 | not all the characters are immediately available. | |
541 | @item | |
542 | wait indefinitely for some input if no characters are | |
543 | currently available, | |
544 | unless the port is in non-blocking mode. | |
545 | @item | |
546 | read characters from the port's input buffers if available, | |
547 | instead from the underlying file descriptor. | |
548 | @item | |
549 | return @code{#f} if end-of-file is encountered before reading | |
550 | any characters, otherwise return the number of characters | |
551 | read. | |
552 | @item | |
553 | return 0 if the port is in non-blocking mode and no characters | |
554 | are immediately available. | |
555 | @item | |
556 | return 0 if the request is for 0 bytes, with no | |
557 | end-of-file check. | |
558 | @end itemize | |
559 | @end deffn | |
560 | ||
561 | @deffn {Scheme Procedure} write-string/partial str [port_or_fdes [start [end]]] | |
562 | @deffnx {C Function} scm_write_string_partial (str, port_or_fdes, start, end) | |
563 | Write characters from a string @var{str} to a port or file | |
564 | descriptor. A port must have an underlying file descriptor | |
565 | --- a so-called fport. This procedure is | |
566 | scsh-compatible and can efficiently write large strings. | |
567 | It will: | |
568 | ||
569 | @itemize | |
570 | @item | |
571 | attempt to write the entire string, unless the @var{start} | |
572 | and/or @var{end} arguments are supplied. i.e., @var{start} | |
573 | defaults to 0 and @var{end} defaults to | |
574 | @code{(string-length str)} | |
575 | @item | |
576 | use the current output port if @var{port_of_fdes} is not | |
577 | supplied. | |
578 | @item | |
579 | in the case of a buffered port, store the characters in the | |
580 | port's output buffer, if all will fit. If they will not fit | |
581 | then any existing buffered characters will be flushed | |
582 | before attempting | |
583 | to write the new characters directly to the underlying file | |
584 | descriptor. If the port is in non-blocking mode and | |
585 | buffered characters can not be flushed immediately, then an | |
586 | @code{EAGAIN} system-error exception will be raised (Note: | |
587 | scsh does not support the use of non-blocking buffered ports.) | |
588 | @item | |
589 | write fewer than the requested number of | |
590 | characters in some cases, e.g., if interrupted by a signal or | |
591 | if not all of the output can be accepted immediately. | |
592 | @item | |
593 | wait indefinitely for at least one character | |
594 | from @var{str} to be accepted by the port, unless the port is | |
595 | in non-blocking mode. | |
596 | @item | |
597 | return the number of characters accepted by the port. | |
598 | @item | |
599 | return 0 if the port is in non-blocking mode and can not accept | |
600 | at least one character from @var{str} immediately | |
601 | @item | |
602 | return 0 immediately if the request size is 0 bytes. | |
603 | @end itemize | |
604 | @end deffn | |
605 | ||
606 | @node Default Ports | |
607 | @subsection Default Ports for Input, Output and Errors | |
608 | ||
609 | @rnindex current-input-port | |
610 | @deffn {Scheme Procedure} current-input-port | |
611 | @deffnx {C Function} scm_current_input_port () | |
612 | Return the current input port. This is the default port used | |
613 | by many input procedures. Initially, @code{current-input-port} | |
614 | returns the @dfn{standard input} in Unix and C terminology. | |
615 | @end deffn | |
616 | ||
617 | @rnindex current-output-port | |
618 | @deffn {Scheme Procedure} current-output-port | |
619 | @deffnx {C Function} scm_current_output_port () | |
620 | Return the current output port. This is the default port used | |
621 | by many output procedures. Initially, | |
622 | @code{current-output-port} returns the @dfn{standard output} in | |
623 | Unix and C terminology. | |
624 | @end deffn | |
625 | ||
626 | @deffn {Scheme Procedure} current-error-port | |
627 | @deffnx {C Function} scm_current_error_port () | |
628 | Return the port to which errors and warnings should be sent (the | |
629 | @dfn{standard error} in Unix and C terminology). | |
630 | @end deffn | |
631 | ||
632 | @deffn {Scheme Procedure} set-current-input-port port | |
633 | @deffnx {Scheme Procedure} set-current-output-port port | |
634 | @deffnx {Scheme Procedure} set-current-error-port port | |
635 | @deffnx {C Function} scm_set_current_input_port (port) | |
636 | @deffnx {C Function} scm_set_current_output_port (port) | |
637 | @deffnx {C Function} scm_set_current_error_port (port) | |
638 | Change the ports returned by @code{current-input-port}, | |
639 | @code{current-output-port} and @code{current-error-port}, respectively, | |
640 | so that they use the supplied @var{port} for input or output. | |
641 | @end deffn | |
642 | ||
643 | @deftypefn {C Function} void scm_frame_current_input_port (SCM port) | |
644 | @deftypefnx {C Function} void scm_frame_current_output_port (SCM port) | |
645 | @deftypefnx {C Function} void scm_frame_current_error_port (SCM port) | |
646 | These functions must be used inside a pair of calls to | |
647 | @code{scm_frame_begin} and @code{scm_frame_end} (@pxref{Frames}). | |
648 | During the dynamic extent of the frame, the indicated port is set to | |
649 | @var{port}. | |
650 | ||
651 | More precisely, the current port is swapped with a `backup' value | |
652 | whenever the frame is entered or left. The backup value is | |
653 | initialized with the @var{port} argument. | |
654 | @end deftypefn | |
655 | ||
656 | @node Port Types | |
657 | @subsection Types of Port | |
658 | ||
659 | [Types of port; how to make them.] | |
660 | ||
661 | @menu | |
662 | * File Ports:: Ports on an operating system file. | |
663 | * String Ports:: Ports on a Scheme string. | |
664 | * Soft Ports:: Ports on arbitrary Scheme procedures. | |
665 | * Void Ports:: Ports on nothing at all. | |
666 | @end menu | |
667 | ||
668 | ||
669 | @node File Ports | |
670 | @subsubsection File Ports | |
671 | ||
672 | The following procedures are used to open file ports. | |
673 | See also @ref{Ports and File Descriptors, open}, for an interface | |
674 | to the Unix @code{open} system call. | |
675 | ||
676 | Most systems have limits on how many files can be open, so it's | |
677 | strongly recommended that file ports be closed explicitly when no | |
678 | longer required (@pxref{Ports}). | |
679 | ||
680 | @deffn {Scheme Procedure} open-file filename mode | |
681 | @deffnx {C Function} scm_open_file (filename, mode) | |
682 | Open the file whose name is @var{filename}, and return a port | |
683 | representing that file. The attributes of the port are | |
684 | determined by the @var{mode} string. The way in which this is | |
685 | interpreted is similar to C stdio. The first character must be | |
686 | one of the following: | |
c755b861 | 687 | |
07d83abe MV |
688 | @table @samp |
689 | @item r | |
690 | Open an existing file for input. | |
691 | @item w | |
692 | Open a file for output, creating it if it doesn't already exist | |
693 | or removing its contents if it does. | |
694 | @item a | |
695 | Open a file for output, creating it if it doesn't already | |
696 | exist. All writes to the port will go to the end of the file. | |
697 | The "append mode" can be turned off while the port is in use | |
698 | @pxref{Ports and File Descriptors, fcntl} | |
699 | @end table | |
c755b861 | 700 | |
07d83abe | 701 | The following additional characters can be appended: |
c755b861 | 702 | |
07d83abe MV |
703 | @table @samp |
704 | @item + | |
705 | Open the port for both input and output. E.g., @code{r+}: open | |
706 | an existing file for both input and output. | |
707 | @item 0 | |
708 | Create an "unbuffered" port. In this case input and output | |
709 | operations are passed directly to the underlying port | |
710 | implementation without additional buffering. This is likely to | |
711 | slow down I/O operations. The buffering mode can be changed | |
712 | while a port is in use @pxref{Ports and File Descriptors, | |
713 | setvbuf} | |
714 | @item l | |
715 | Add line-buffering to the port. The port output buffer will be | |
716 | automatically flushed whenever a newline character is written. | |
c755b861 KR |
717 | @item b |
718 | Use binary mode. On DOS systems the default text mode converts CR+LF | |
719 | in the file to newline for the program, whereas binary mode reads and | |
720 | writes all bytes unchanged. On Unix-like systems there is no such | |
721 | distinction, text files already contain just newlines and no | |
722 | conversion is ever made. The @code{b} flag is accepted on all | |
723 | systems, but has no effect on Unix-like systems. | |
724 | ||
725 | (For reference, Guile leaves text versus binary up to the C library, | |
726 | @code{b} here just adds @code{O_BINARY} to the underlying @code{open} | |
727 | call, when that flag is available.) | |
07d83abe | 728 | @end table |
c755b861 | 729 | |
07d83abe MV |
730 | In theory we could create read/write ports which were buffered |
731 | in one direction only. However this isn't included in the | |
732 | current interfaces. If a file cannot be opened with the access | |
733 | requested, @code{open-file} throws an exception. | |
734 | @end deffn | |
735 | ||
736 | @rnindex open-input-file | |
737 | @deffn {Scheme Procedure} open-input-file filename | |
738 | Open @var{filename} for input. Equivalent to | |
739 | @smalllisp | |
740 | (open-file @var{filename} "r") | |
741 | @end smalllisp | |
742 | @end deffn | |
743 | ||
744 | @rnindex open-output-file | |
745 | @deffn {Scheme Procedure} open-output-file filename | |
746 | Open @var{filename} for output. Equivalent to | |
747 | @smalllisp | |
748 | (open-file @var{filename} "w") | |
749 | @end smalllisp | |
750 | @end deffn | |
751 | ||
752 | @deffn {Scheme Procedure} call-with-input-file filename proc | |
753 | @deffnx {Scheme Procedure} call-with-output-file filename proc | |
754 | @rnindex call-with-input-file | |
755 | @rnindex call-with-output-file | |
756 | Open @var{filename} for input or output, and call @code{(@var{proc} | |
757 | port)} with the resulting port. Return the value returned by | |
758 | @var{proc}. @var{filename} is opened as per @code{open-input-file} or | |
759 | @code{open-output-file} respectively, and an error is signalled if it | |
760 | cannot be opened. | |
761 | ||
762 | When @var{proc} returns, the port is closed. If @var{proc} does not | |
763 | return (eg.@: if it throws an error), then the port might not be | |
764 | closed automatically, though it will be garbage collected in the usual | |
765 | way if not otherwise referenced. | |
766 | @end deffn | |
767 | ||
768 | @deffn {Scheme Procedure} with-input-from-file filename thunk | |
769 | @deffnx {Scheme Procedure} with-output-to-file filename thunk | |
770 | @deffnx {Scheme Procedure} with-error-to-file filename thunk | |
771 | @rnindex with-input-from-file | |
772 | @rnindex with-output-to-file | |
773 | Open @var{filename} and call @code{(@var{thunk})} with the new port | |
774 | setup as respectively the @code{current-input-port}, | |
775 | @code{current-output-port}, or @code{current-error-port}. Return the | |
776 | value returned by @var{thunk}. @var{filename} is opened as per | |
777 | @code{open-input-file} or @code{open-output-file} respectively, and an | |
778 | error is signalled if it cannot be opened. | |
779 | ||
780 | When @var{thunk} returns, the port is closed and the previous setting | |
781 | of the respective current port is restored. | |
782 | ||
783 | The current port setting is managed with @code{dynamic-wind}, so the | |
784 | previous value is restored no matter how @var{thunk} exits (eg.@: an | |
785 | exception), and if @var{thunk} is re-entered (via a captured | |
786 | continuation) then it's set again to the @var{FILENAME} port. | |
787 | ||
788 | The port is closed when @var{thunk} returns normally, but not when | |
789 | exited via an exception or new continuation. This ensures it's still | |
790 | ready for use if @var{thunk} is re-entered by a captured continuation. | |
791 | Of course the port is always garbage collected and closed in the usual | |
792 | way when no longer referenced anywhere. | |
793 | @end deffn | |
794 | ||
795 | @deffn {Scheme Procedure} port-mode port | |
796 | @deffnx {C Function} scm_port_mode (port) | |
797 | Return the port modes associated with the open port @var{port}. | |
798 | These will not necessarily be identical to the modes used when | |
799 | the port was opened, since modes such as "append" which are | |
800 | used only during port creation are not retained. | |
801 | @end deffn | |
802 | ||
803 | @deffn {Scheme Procedure} port-filename port | |
804 | @deffnx {C Function} scm_port_filename (port) | |
805 | Return the filename associated with @var{port}. This function returns | |
806 | the strings "standard input", "standard output" and "standard error" | |
807 | when called on the current input, output and error ports respectively. | |
808 | @end deffn | |
809 | ||
810 | @deffn {Scheme Procedure} set-port-filename! port filename | |
811 | @deffnx {C Function} scm_set_port_filename_x (port, filename) | |
812 | Change the filename associated with @var{port}, using the current input | |
813 | port if none is specified. Note that this does not change the port's | |
814 | source of data, but only the value that is returned by | |
815 | @code{port-filename} and reported in diagnostic output. | |
816 | @end deffn | |
817 | ||
818 | @deffn {Scheme Procedure} file-port? obj | |
819 | @deffnx {C Function} scm_file_port_p (obj) | |
820 | Determine whether @var{obj} is a port that is related to a file. | |
821 | @end deffn | |
822 | ||
823 | ||
824 | @node String Ports | |
825 | @subsubsection String Ports | |
826 | ||
827 | The following allow string ports to be opened by analogy to R4R* | |
828 | file port facilities: | |
829 | ||
830 | @deffn {Scheme Procedure} call-with-output-string proc | |
831 | @deffnx {C Function} scm_call_with_output_string (proc) | |
832 | Calls the one-argument procedure @var{proc} with a newly created output | |
833 | port. When the function returns, the string composed of the characters | |
834 | written into the port is returned. @var{proc} should not close the port. | |
835 | @end deffn | |
836 | ||
837 | @deffn {Scheme Procedure} call-with-input-string string proc | |
838 | @deffnx {C Function} scm_call_with_input_string (string, proc) | |
839 | Calls the one-argument procedure @var{proc} with a newly | |
840 | created input port from which @var{string}'s contents may be | |
841 | read. The value yielded by the @var{proc} is returned. | |
842 | @end deffn | |
843 | ||
844 | @deffn {Scheme Procedure} with-output-to-string thunk | |
845 | Calls the zero-argument procedure @var{thunk} with the current output | |
846 | port set temporarily to a new string port. It returns a string | |
847 | composed of the characters written to the current output. | |
848 | @end deffn | |
849 | ||
850 | @deffn {Scheme Procedure} with-input-from-string string thunk | |
851 | Calls the zero-argument procedure @var{thunk} with the current input | |
852 | port set temporarily to a string port opened on the specified | |
853 | @var{string}. The value yielded by @var{thunk} is returned. | |
854 | @end deffn | |
855 | ||
856 | @deffn {Scheme Procedure} open-input-string str | |
857 | @deffnx {C Function} scm_open_input_string (str) | |
858 | Take a string and return an input port that delivers characters | |
859 | from the string. The port can be closed by | |
860 | @code{close-input-port}, though its storage will be reclaimed | |
861 | by the garbage collector if it becomes inaccessible. | |
862 | @end deffn | |
863 | ||
864 | @deffn {Scheme Procedure} open-output-string | |
865 | @deffnx {C Function} scm_open_output_string () | |
866 | Return an output port that will accumulate characters for | |
867 | retrieval by @code{get-output-string}. The port can be closed | |
868 | by the procedure @code{close-output-port}, though its storage | |
869 | will be reclaimed by the garbage collector if it becomes | |
870 | inaccessible. | |
871 | @end deffn | |
872 | ||
873 | @deffn {Scheme Procedure} get-output-string port | |
874 | @deffnx {C Function} scm_get_output_string (port) | |
875 | Given an output port created by @code{open-output-string}, | |
876 | return a string consisting of the characters that have been | |
877 | output to the port so far. | |
878 | ||
879 | @code{get-output-string} must be used before closing @var{port}, once | |
880 | closed the string cannot be obtained. | |
881 | @end deffn | |
882 | ||
883 | A string port can be used in many procedures which accept a port | |
884 | but which are not dependent on implementation details of fports. | |
885 | E.g., seeking and truncating will work on a string port, | |
886 | but trying to extract the file descriptor number will fail. | |
887 | ||
888 | ||
889 | @node Soft Ports | |
890 | @subsubsection Soft Ports | |
891 | ||
892 | A @dfn{soft-port} is a port based on a vector of procedures capable of | |
893 | accepting or delivering characters. It allows emulation of I/O ports. | |
894 | ||
895 | @deffn {Scheme Procedure} make-soft-port pv modes | |
896 | @deffnx {C Function} scm_make_soft_port (pv, modes) | |
897 | Return a port capable of receiving or delivering characters as | |
898 | specified by the @var{modes} string (@pxref{File Ports, | |
899 | open-file}). @var{pv} must be a vector of length 5 or 6. Its | |
900 | components are as follows: | |
901 | ||
902 | @enumerate 0 | |
903 | @item | |
904 | procedure accepting one character for output | |
905 | @item | |
906 | procedure accepting a string for output | |
907 | @item | |
908 | thunk for flushing output | |
909 | @item | |
910 | thunk for getting one character | |
911 | @item | |
912 | thunk for closing port (not by garbage collection) | |
913 | @item | |
914 | (if present and not @code{#f}) thunk for computing the number of | |
915 | characters that can be read from the port without blocking. | |
916 | @end enumerate | |
917 | ||
918 | For an output-only port only elements 0, 1, 2, and 4 need be | |
919 | procedures. For an input-only port only elements 3 and 4 need | |
920 | be procedures. Thunks 2 and 4 can instead be @code{#f} if | |
921 | there is no useful operation for them to perform. | |
922 | ||
923 | If thunk 3 returns @code{#f} or an @code{eof-object} | |
924 | (@pxref{Input, eof-object?, ,r5rs, The Revised^5 Report on | |
925 | Scheme}) it indicates that the port has reached end-of-file. | |
926 | For example: | |
927 | ||
928 | @lisp | |
929 | (define stdout (current-output-port)) | |
930 | (define p (make-soft-port | |
931 | (vector | |
932 | (lambda (c) (write c stdout)) | |
933 | (lambda (s) (display s stdout)) | |
934 | (lambda () (display "." stdout)) | |
935 | (lambda () (char-upcase (read-char))) | |
936 | (lambda () (display "@@" stdout))) | |
937 | "rw")) | |
938 | ||
939 | (write p p) @result{} #<input-output: soft 8081e20> | |
940 | @end lisp | |
941 | @end deffn | |
942 | ||
943 | ||
944 | @node Void Ports | |
945 | @subsubsection Void Ports | |
946 | ||
947 | This kind of port causes any data to be discarded when written to, and | |
948 | always returns the end-of-file object when read from. | |
949 | ||
950 | @deffn {Scheme Procedure} %make-void-port mode | |
951 | @deffnx {C Function} scm_sys_make_void_port (mode) | |
952 | Create and return a new void port. A void port acts like | |
953 | @file{/dev/null}. The @var{mode} argument | |
954 | specifies the input/output modes for this port: see the | |
955 | documentation for @code{open-file} in @ref{File Ports}. | |
956 | @end deffn | |
957 | ||
958 | ||
959 | @node I/O Extensions | |
960 | @subsection Using and Extending Ports in C | |
961 | ||
962 | @menu | |
963 | * C Port Interface:: Using ports from C. | |
964 | * Port Implementation:: How to implement a new port type in C. | |
965 | @end menu | |
966 | ||
967 | ||
968 | @node C Port Interface | |
969 | @subsubsection C Port Interface | |
970 | ||
971 | This section describes how to use Scheme ports from C. | |
972 | ||
973 | @subsubheading Port basics | |
974 | ||
975 | There are two main data structures. A port type object (ptob) is of | |
976 | type @code{scm_ptob_descriptor}. A port instance is of type | |
977 | @code{scm_port}. Given an @code{SCM} variable which points to a port, | |
978 | the corresponding C port object can be obtained using the | |
979 | @code{SCM_PTAB_ENTRY} macro. The ptob can be obtained by using | |
980 | @code{SCM_PTOBNUM} to give an index into the @code{scm_ptobs} | |
981 | global array. | |
982 | ||
983 | @subsubheading Port buffers | |
984 | ||
985 | An input port always has a read buffer and an output port always has a | |
986 | write buffer. However the size of these buffers is not guaranteed to be | |
987 | more than one byte (e.g., the @code{shortbuf} field in @code{scm_port} | |
988 | which is used when no other buffer is allocated). The way in which the | |
989 | buffers are allocated depends on the implementation of the ptob. For | |
990 | example in the case of an fport, buffers may be allocated with malloc | |
991 | when the port is created, but in the case of an strport the underlying | |
992 | string is used as the buffer. | |
993 | ||
994 | @subsubheading The @code{rw_random} flag | |
995 | ||
996 | Special treatment is required for ports which can be seeked at random. | |
997 | Before various operations, such as seeking the port or changing from | |
998 | input to output on a bidirectional port or vice versa, the port | |
999 | implementation must be given a chance to update its state. The write | |
1000 | buffer is updated by calling the @code{flush} ptob procedure and the | |
1001 | input buffer is updated by calling the @code{end_input} ptob procedure. | |
1002 | In the case of an fport, @code{flush} causes buffered output to be | |
1003 | written to the file descriptor, while @code{end_input} causes the | |
1004 | descriptor position to be adjusted to account for buffered input which | |
1005 | was never read. | |
1006 | ||
1007 | The special treatment must be performed if the @code{rw_random} flag in | |
1008 | the port is non-zero. | |
1009 | ||
1010 | @subsubheading The @code{rw_active} variable | |
1011 | ||
1012 | The @code{rw_active} variable in the port is only used if | |
1013 | @code{rw_random} is set. It's defined as an enum with the following | |
1014 | values: | |
1015 | ||
1016 | @table @code | |
1017 | @item SCM_PORT_READ | |
1018 | the read buffer may have unread data. | |
1019 | ||
1020 | @item SCM_PORT_WRITE | |
1021 | the write buffer may have unwritten data. | |
1022 | ||
1023 | @item SCM_PORT_NEITHER | |
1024 | neither the write nor the read buffer has data. | |
1025 | @end table | |
1026 | ||
1027 | @subsubheading Reading from a port. | |
1028 | ||
1029 | To read from a port, it's possible to either call existing libguile | |
1030 | procedures such as @code{scm_getc} and @code{scm_read_line} or to read | |
1031 | data from the read buffer directly. Reading from the buffer involves | |
1032 | the following steps: | |
1033 | ||
1034 | @enumerate | |
1035 | @item | |
1036 | Flush output on the port, if @code{rw_active} is @code{SCM_PORT_WRITE}. | |
1037 | ||
1038 | @item | |
1039 | Fill the read buffer, if it's empty, using @code{scm_fill_input}. | |
1040 | ||
1041 | @item Read the data from the buffer and update the read position in | |
1042 | the buffer. Steps 2) and 3) may be repeated as many times as required. | |
1043 | ||
1044 | @item Set rw_active to @code{SCM_PORT_READ} if @code{rw_random} is set. | |
1045 | ||
1046 | @item update the port's line and column counts. | |
1047 | @end enumerate | |
1048 | ||
1049 | @subsubheading Writing to a port. | |
1050 | ||
1051 | To write data to a port, calling @code{scm_lfwrite} should be sufficient for | |
1052 | most purposes. This takes care of the following steps: | |
1053 | ||
1054 | @enumerate | |
1055 | @item | |
1056 | End input on the port, if @code{rw_active} is @code{SCM_PORT_READ}. | |
1057 | ||
1058 | @item | |
1059 | Pass the data to the ptob implementation using the @code{write} ptob | |
1060 | procedure. The advantage of using the ptob @code{write} instead of | |
1061 | manipulating the write buffer directly is that it allows the data to be | |
1062 | written in one operation even if the port is using the single-byte | |
1063 | @code{shortbuf}. | |
1064 | ||
1065 | @item | |
1066 | Set @code{rw_active} to @code{SCM_PORT_WRITE} if @code{rw_random} | |
1067 | is set. | |
1068 | @end enumerate | |
1069 | ||
1070 | ||
1071 | @node Port Implementation | |
1072 | @subsubsection Port Implementation | |
1073 | ||
1074 | This section describes how to implement a new port type in C. | |
1075 | ||
1076 | As described in the previous section, a port type object (ptob) is | |
1077 | a structure of type @code{scm_ptob_descriptor}. A ptob is created by | |
1078 | calling @code{scm_make_port_type}. | |
1079 | ||
1080 | All of the elements of the ptob, apart from @code{name}, are procedures | |
1081 | which collectively implement the port behaviour. Creating a new port | |
1082 | type mostly involves writing these procedures. | |
1083 | ||
1084 | @code{scm_make_port_type} initializes three elements of the structure | |
1085 | (@code{name}, @code{fill_input} and @code{write}) from its arguments. | |
1086 | The remaining elements are initialized with default values and can be | |
1087 | set later if required. | |
1088 | ||
1089 | @table @code | |
1090 | @item name | |
1091 | A pointer to a NUL terminated string: the name of the port type. This | |
1092 | is the only element of @code{scm_ptob_descriptor} which is not | |
1093 | a procedure. Set via the first argument to @code{scm_make_port_type}. | |
1094 | ||
1095 | @item mark | |
1096 | Called during garbage collection to mark any SCM objects that a port | |
1097 | object may contain. It doesn't need to be set unless the port has | |
1098 | @code{SCM} components. Set using @code{scm_set_port_mark}. | |
1099 | ||
1100 | @item free | |
1101 | Called when the port is collected during gc. It | |
1102 | should free any resources used by the port. | |
1103 | Set using @code{scm_set_port_free}. | |
1104 | ||
1105 | @item print | |
1106 | Called when @code{write} is called on the port object, to print a | |
1107 | port description. e.g., for an fport it may produce something like: | |
1108 | @code{#<input: /etc/passwd 3>}. Set using @code{scm_set_port_print}. | |
1109 | ||
1110 | @item equalp | |
1111 | Not used at present. Set using @code{scm_set_port_equalp}. | |
1112 | ||
1113 | @item close | |
1114 | Called when the port is closed, unless it was collected during gc. It | |
1115 | should free any resources used by the port. | |
1116 | Set using @code{scm_set_port_close}. | |
1117 | ||
1118 | @item write | |
1119 | Accept data which is to be written using the port. The port implementation | |
1120 | may choose to buffer the data instead of processing it directly. | |
1121 | Set via the third argument to @code{scm_make_port_type}. | |
1122 | ||
1123 | @item flush | |
1124 | Complete the processing of buffered output data. Reset the value of | |
1125 | @code{rw_active} to @code{SCM_PORT_NEITHER}. | |
1126 | Set using @code{scm_set_port_flush}. | |
1127 | ||
1128 | @item end_input | |
1129 | Perform any synchronization required when switching from input to output | |
1130 | on the port. Reset the value of @code{rw_active} to @code{SCM_PORT_NEITHER}. | |
1131 | Set using @code{scm_set_port_end_input}. | |
1132 | ||
1133 | @item fill_input | |
1134 | Read new data into the read buffer and return the first character. It | |
1135 | can be assumed that the read buffer is empty when this procedure is called. | |
1136 | Set via the second argument to @code{scm_make_port_type}. | |
1137 | ||
1138 | @item input_waiting | |
1139 | Return a lower bound on the number of bytes that could be read from the | |
1140 | port without blocking. It can be assumed that the current state of | |
1141 | @code{rw_active} is @code{SCM_PORT_NEITHER}. | |
1142 | Set using @code{scm_set_port_input_waiting}. | |
1143 | ||
1144 | @item seek | |
1145 | Set the current position of the port. The procedure can not make | |
1146 | any assumptions about the value of @code{rw_active} when it's | |
1147 | called. It can reset the buffers first if desired by using something | |
1148 | like: | |
1149 | ||
1150 | @example | |
1151 | if (pt->rw_active == SCM_PORT_READ) | |
1152 | scm_end_input (object); | |
1153 | else if (pt->rw_active == SCM_PORT_WRITE) | |
1154 | ptob->flush (object); | |
1155 | @end example | |
1156 | ||
1157 | However note that this will have the side effect of discarding any data | |
1158 | in the unread-char buffer, in addition to any side effects from the | |
1159 | @code{end_input} and @code{flush} ptob procedures. This is undesirable | |
1160 | when seek is called to measure the current position of the port, i.e., | |
1161 | @code{(seek p 0 SEEK_CUR)}. The libguile fport and string port | |
1162 | implementations take care to avoid this problem. | |
1163 | ||
1164 | The procedure is set using @code{scm_set_port_seek}. | |
1165 | ||
1166 | @item truncate | |
1167 | Truncate the port data to be specified length. It can be assumed that the | |
1168 | current state of @code{rw_active} is @code{SCM_PORT_NEITHER}. | |
1169 | Set using @code{scm_set_port_truncate}. | |
1170 | ||
1171 | @end table | |
1172 | ||
1173 | ||
1174 | @c Local Variables: | |
1175 | @c TeX-master: "guile.texi" | |
1176 | @c End: |