Commit | Line | Data |
---|---|---|
07d83abe MV |
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}. | |
cdf1ad3b MV |
111 | |
112 | @code{char-ready?} exists to make it possible for a | |
07d83abe MV |
113 | program to accept characters from interactive ports without |
114 | getting stuck waiting for input. Any input editors associated | |
115 | with such ports must make sure that characters whose existence | |
116 | has been asserted by @code{char-ready?} cannot be rubbed out. | |
117 | If @code{char-ready?} were to return @code{#f} at end of file, | |
118 | a port at end of file would be indistinguishable from an | |
cdf1ad3b | 119 | interactive port that has no ready characters. |
07d83abe MV |
120 | @end deffn |
121 | ||
122 | @rnindex read-char | |
123 | @deffn {Scheme Procedure} read-char [port] | |
124 | @deffnx {C Function} scm_read_char (port) | |
125 | Return the next character available from @var{port}, updating | |
126 | @var{port} to point to the following character. If no more | |
127 | characters are available, the end-of-file object is returned. | |
128 | @end deffn | |
129 | ||
130 | @deftypefn {C Function} size_t scm_c_read (SCM port, void *buffer, size_t size) | |
131 | Read up to @var{size} bytes from @var{port} and store them in | |
132 | @var{buffer}. The return value is the number of bytes actually read, | |
133 | which can be less than @var{size} if end-of-file has been reached. | |
134 | ||
135 | Note that this function does not update @code{port-line} and | |
136 | @code{port-column} below. | |
137 | @end deftypefn | |
138 | ||
139 | @rnindex peek-char | |
140 | @deffn {Scheme Procedure} peek-char [port] | |
141 | @deffnx {C Function} scm_peek_char (port) | |
142 | Return the next character available from @var{port}, | |
143 | @emph{without} updating @var{port} to point to the following | |
144 | character. If no more characters are available, the | |
cdf1ad3b MV |
145 | end-of-file object is returned. |
146 | ||
147 | The value returned by | |
07d83abe MV |
148 | a call to @code{peek-char} is the same as the value that would |
149 | have been returned by a call to @code{read-char} on the same | |
150 | port. The only difference is that the very next call to | |
151 | @code{read-char} or @code{peek-char} on that @var{port} will | |
152 | return the value returned by the preceding call to | |
153 | @code{peek-char}. In particular, a call to @code{peek-char} on | |
154 | an interactive port will hang waiting for input whenever a call | |
cdf1ad3b | 155 | to @code{read-char} would have hung. |
07d83abe MV |
156 | @end deffn |
157 | ||
158 | @deffn {Scheme Procedure} unread-char cobj [port] | |
159 | @deffnx {C Function} scm_unread_char (cobj, port) | |
160 | Place @var{char} in @var{port} so that it will be read by the | |
161 | next read operation. If called multiple times, the unread characters | |
162 | will be read again in last-in first-out order. If @var{port} is | |
163 | not supplied, the current input port is used. | |
164 | @end deffn | |
165 | ||
166 | @deffn {Scheme Procedure} unread-string str port | |
167 | @deffnx {C Function} scm_unread_string (str, port) | |
168 | Place the string @var{str} in @var{port} so that its characters will | |
169 | be read from left-to-right as the next characters from @var{port} | |
170 | during subsequent read operations. If called multiple times, the | |
171 | unread characters will be read again in last-in first-out order. If | |
9782da8a | 172 | @var{port} is not supplied, the @code{current-input-port} is used. |
07d83abe MV |
173 | @end deffn |
174 | ||
175 | @deffn {Scheme Procedure} drain-input port | |
176 | @deffnx {C Function} scm_drain_input (port) | |
177 | This procedure clears a port's input buffers, similar | |
178 | to the way that force-output clears the output buffer. The | |
179 | contents of the buffers are returned as a single string, e.g., | |
180 | ||
181 | @lisp | |
182 | (define p (open-input-file ...)) | |
183 | (drain-input p) => empty string, nothing buffered yet. | |
184 | (unread-char (read-char p) p) | |
185 | (drain-input p) => initial chars from p, up to the buffer size. | |
186 | @end lisp | |
187 | ||
188 | Draining the buffers may be useful for cleanly finishing | |
189 | buffered I/O so that the file descriptor can be used directly | |
190 | for further input. | |
191 | @end deffn | |
192 | ||
193 | @deffn {Scheme Procedure} port-column port | |
194 | @deffnx {Scheme Procedure} port-line port | |
195 | @deffnx {C Function} scm_port_column (port) | |
196 | @deffnx {C Function} scm_port_line (port) | |
197 | Return the current column number or line number of @var{port}. | |
198 | If the number is | |
199 | unknown, the result is #f. Otherwise, the result is a 0-origin integer | |
200 | - i.e.@: the first character of the first line is line 0, column 0. | |
201 | (However, when you display a file position, for example in an error | |
202 | message, we recommend you add 1 to get 1-origin integers. This is | |
203 | because lines and column numbers traditionally start with 1, and that is | |
204 | what non-programmers will find most natural.) | |
205 | @end deffn | |
206 | ||
207 | @deffn {Scheme Procedure} set-port-column! port column | |
208 | @deffnx {Scheme Procedure} set-port-line! port line | |
209 | @deffnx {C Function} scm_set_port_column_x (port, column) | |
210 | @deffnx {C Function} scm_set_port_line_x (port, line) | |
211 | Set the current column or line number of @var{port}. | |
212 | @end deffn | |
213 | ||
214 | @node Writing | |
215 | @subsection Writing | |
216 | ||
217 | [Generic procedures for writing to ports.] | |
218 | ||
219 | @deffn {Scheme Procedure} get-print-state port | |
220 | @deffnx {C Function} scm_get_print_state (port) | |
221 | Return the print state of the port @var{port}. If @var{port} | |
222 | has no associated print state, @code{#f} is returned. | |
223 | @end deffn | |
224 | ||
225 | @rnindex write | |
226 | @deffn {Scheme Procedure} write obj [port] | |
227 | Send a representation of @var{obj} to @var{port} or to the current | |
228 | output port if not given. | |
229 | ||
230 | The output is designed to be machine readable, and can be read back | |
231 | with @code{read} (@pxref{Reading}). Strings are printed in | |
232 | doublequotes, with escapes if necessary, and characters are printed in | |
233 | @samp{#\} notation. | |
234 | @end deffn | |
235 | ||
236 | @rnindex display | |
237 | @deffn {Scheme Procedure} display obj [port] | |
238 | Send a representation of @var{obj} to @var{port} or to the current | |
239 | output port if not given. | |
240 | ||
241 | The output is designed for human readability, it differs from | |
242 | @code{write} in that strings are printed without doublequotes and | |
243 | escapes, and characters are printed as per @code{write-char}, not in | |
244 | @samp{#\} form. | |
245 | @end deffn | |
246 | ||
247 | @rnindex newline | |
248 | @deffn {Scheme Procedure} newline [port] | |
249 | @deffnx {C Function} scm_newline (port) | |
250 | Send a newline to @var{port}. | |
251 | If @var{port} is omitted, send to the current output port. | |
252 | @end deffn | |
253 | ||
cdf1ad3b | 254 | @deffn {Scheme Procedure} port-with-print-state port [pstate] |
07d83abe MV |
255 | @deffnx {C Function} scm_port_with_print_state (port, pstate) |
256 | Create a new port which behaves like @var{port}, but with an | |
cdf1ad3b MV |
257 | included print state @var{pstate}. @var{pstate} is optional. |
258 | If @var{pstate} isn't supplied and @var{port} already has | |
259 | a print state, the old print state is reused. | |
07d83abe MV |
260 | @end deffn |
261 | ||
262 | @deffn {Scheme Procedure} print-options-interface [setting] | |
263 | @deffnx {C Function} scm_print_options (setting) | |
264 | Option interface for the print options. Instead of using | |
265 | this procedure directly, use the procedures | |
266 | @code{print-enable}, @code{print-disable}, @code{print-set!} | |
267 | and @code{print-options}. | |
268 | @end deffn | |
269 | ||
270 | @deffn {Scheme Procedure} simple-format destination message . args | |
271 | @deffnx {C Function} scm_simple_format (destination, message, args) | |
272 | Write @var{message} to @var{destination}, defaulting to | |
273 | the current output port. | |
274 | @var{message} can contain @code{~A} (was @code{%s}) and | |
275 | @code{~S} (was @code{%S}) escapes. When printed, | |
276 | the escapes are replaced with corresponding members of | |
277 | @var{ARGS}: | |
278 | @code{~A} formats using @code{display} and @code{~S} formats | |
279 | using @code{write}. | |
280 | If @var{destination} is @code{#t}, then use the current output | |
281 | port, if @var{destination} is @code{#f}, then return a string | |
282 | containing the formatted text. Does not add a trailing newline. | |
283 | @end deffn | |
284 | ||
285 | @rnindex write-char | |
286 | @deffn {Scheme Procedure} write-char chr [port] | |
287 | @deffnx {C Function} scm_write_char (chr, port) | |
288 | Send character @var{chr} to @var{port}. | |
289 | @end deffn | |
290 | ||
291 | @deftypefn {C Function} void scm_c_write (SCM port, const void *buffer, size_t size) | |
292 | Write @var{size} bytes at @var{buffer} to @var{port}. | |
293 | ||
294 | Note that this function does not update @code{port-line} and | |
295 | @code{port-column} (@pxref{Reading}). | |
296 | @end deftypefn | |
297 | ||
298 | @findex fflush | |
299 | @deffn {Scheme Procedure} force-output [port] | |
300 | @deffnx {C Function} scm_force_output (port) | |
301 | Flush the specified output port, or the current output port if @var{port} | |
302 | is omitted. The current output buffer contents are passed to the | |
303 | underlying port implementation (e.g., in the case of fports, the | |
304 | data will be written to the file and the output buffer will be cleared.) | |
305 | It has no effect on an unbuffered port. | |
306 | ||
307 | The return value is unspecified. | |
308 | @end deffn | |
309 | ||
310 | @deffn {Scheme Procedure} flush-all-ports | |
311 | @deffnx {C Function} scm_flush_all_ports () | |
312 | Equivalent to calling @code{force-output} on | |
313 | all open output ports. The return value is unspecified. | |
314 | @end deffn | |
315 | ||
316 | ||
317 | @node Closing | |
318 | @subsection Closing | |
319 | ||
320 | @deffn {Scheme Procedure} close-port port | |
321 | @deffnx {C Function} scm_close_port (port) | |
322 | Close the specified port object. Return @code{#t} if it | |
323 | successfully closes a port or @code{#f} if it was already | |
324 | closed. An exception may be raised if an error occurs, for | |
325 | example when flushing buffered output. See also @ref{Ports and | |
326 | File Descriptors, close}, for a procedure which can close file | |
327 | descriptors. | |
328 | @end deffn | |
329 | ||
330 | @deffn {Scheme Procedure} close-input-port port | |
331 | @deffnx {Scheme Procedure} close-output-port port | |
332 | @deffnx {C Function} scm_close_input_port (port) | |
333 | @deffnx {C Function} scm_close_output_port (port) | |
334 | @rnindex close-input-port | |
335 | @rnindex close-output-port | |
336 | Close the specified input or output @var{port}. An exception may be | |
337 | raised if an error occurs while closing. If @var{port} is already | |
338 | closed, nothing is done. The return value is unspecified. | |
339 | ||
340 | See also @ref{Ports and File Descriptors, close}, for a procedure | |
341 | which can close file descriptors. | |
342 | @end deffn | |
343 | ||
344 | @deffn {Scheme Procedure} port-closed? port | |
345 | @deffnx {C Function} scm_port_closed_p (port) | |
346 | Return @code{#t} if @var{port} is closed or @code{#f} if it is | |
347 | open. | |
348 | @end deffn | |
349 | ||
350 | ||
351 | @node Random Access | |
352 | @subsection Random Access | |
353 | ||
354 | @deffn {Scheme Procedure} seek fd_port offset whence | |
355 | @deffnx {C Function} scm_seek (fd_port, offset, whence) | |
356 | Sets the current position of @var{fd/port} to the integer | |
357 | @var{offset}, which is interpreted according to the value of | |
358 | @var{whence}. | |
359 | ||
360 | One of the following variables should be supplied for | |
361 | @var{whence}: | |
362 | @defvar SEEK_SET | |
363 | Seek from the beginning of the file. | |
364 | @end defvar | |
365 | @defvar SEEK_CUR | |
366 | Seek from the current position. | |
367 | @end defvar | |
368 | @defvar SEEK_END | |
369 | Seek from the end of the file. | |
370 | @end defvar | |
371 | If @var{fd/port} is a file descriptor, the underlying system | |
372 | call is @code{lseek}. @var{port} may be a string port. | |
373 | ||
374 | The value returned is the new position in the file. This means | |
375 | that the current position of a port can be obtained using: | |
376 | @lisp | |
377 | (seek port 0 SEEK_CUR) | |
378 | @end lisp | |
379 | @end deffn | |
380 | ||
381 | @deffn {Scheme Procedure} ftell fd_port | |
382 | @deffnx {C Function} scm_ftell (fd_port) | |
383 | Return an integer representing the current position of | |
384 | @var{fd/port}, measured from the beginning. Equivalent to: | |
385 | ||
386 | @lisp | |
387 | (seek port 0 SEEK_CUR) | |
388 | @end lisp | |
389 | @end deffn | |
390 | ||
391 | @findex truncate | |
392 | @findex ftruncate | |
393 | @deffn {Scheme Procedure} truncate-file object [length] | |
394 | @deffnx {C Function} scm_truncate_file (object, length) | |
395 | Truncates the object referred to by @var{object} to at most | |
396 | @var{length} bytes. @var{object} can be a string containing a | |
397 | file name or an integer file descriptor or a port. | |
398 | @var{length} may be omitted if @var{object} is not a file name, | |
cdf1ad3b | 399 | in which case the truncation occurs at the current port |
07d83abe MV |
400 | position. The return value is unspecified. |
401 | @end deffn | |
402 | ||
403 | @node Line/Delimited | |
404 | @subsection Line Oriented and Delimited Text | |
405 | ||
406 | The delimited-I/O module can be accessed with: | |
407 | ||
408 | @smalllisp | |
409 | (use-modules (ice-9 rdelim)) | |
410 | @end smalllisp | |
411 | ||
412 | It can be used to read or write lines of text, or read text delimited by | |
413 | a specified set of characters. It's similar to the @code{(scsh rdelim)} | |
414 | module from guile-scsh, but does not use multiple values or character | |
415 | sets and has an extra procedure @code{write-line}. | |
416 | ||
417 | @c begin (scm-doc-string "rdelim.scm" "read-line") | |
418 | @deffn {Scheme Procedure} read-line [port] [handle-delim] | |
419 | Return a line of text from @var{port} if specified, otherwise from the | |
420 | value returned by @code{(current-input-port)}. Under Unix, a line of text | |
421 | is terminated by the first end-of-line character or by end-of-file. | |
422 | ||
423 | If @var{handle-delim} is specified, it should be one of the following | |
424 | symbols: | |
425 | @table @code | |
426 | @item trim | |
427 | Discard the terminating delimiter. This is the default, but it will | |
428 | be impossible to tell whether the read terminated with a delimiter or | |
429 | end-of-file. | |
430 | @item concat | |
431 | Append the terminating delimiter (if any) to the returned string. | |
432 | @item peek | |
433 | Push the terminating delimiter (if any) back on to the port. | |
434 | @item split | |
435 | Return a pair containing the string read from the port and the | |
436 | terminating delimiter or end-of-file object. | |
437 | @end table | |
438 | @end deffn | |
439 | ||
440 | @c begin (scm-doc-string "rdelim.scm" "read-line!") | |
441 | @deffn {Scheme Procedure} read-line! buf [port] | |
442 | Read a line of text into the supplied string @var{buf} and return the | |
443 | number of characters added to @var{buf}. If @var{buf} is filled, then | |
444 | @code{#f} is returned. | |
445 | Read from @var{port} if | |
446 | specified, otherwise from the value returned by @code{(current-input-port)}. | |
447 | @end deffn | |
448 | ||
449 | @c begin (scm-doc-string "rdelim.scm" "read-delimited") | |
450 | @deffn {Scheme Procedure} read-delimited delims [port] [handle-delim] | |
451 | Read text until one of the characters in the string @var{delims} is found | |
452 | or end-of-file is reached. Read from @var{port} if supplied, otherwise | |
453 | from the value returned by @code{(current-input-port)}. | |
454 | @var{handle-delim} takes the same values as described for @code{read-line}. | |
455 | @end deffn | |
456 | ||
457 | @c begin (scm-doc-string "rdelim.scm" "read-delimited!") | |
458 | @deffn {Scheme Procedure} read-delimited! delims buf [port] [handle-delim] [start] [end] | |
459 | Read text into the supplied string @var{buf} and return the number of | |
460 | characters added to @var{buf} (subject to @var{handle-delim}, which takes | |
461 | the same values specified for @code{read-line}. If @var{buf} is filled, | |
462 | @code{#f} is returned for both the number of characters read and the | |
463 | delimiter. Also terminates if one of the characters in the string | |
464 | @var{delims} is found | |
465 | or end-of-file is reached. Read from @var{port} if supplied, otherwise | |
466 | from the value returned by @code{(current-input-port)}. | |
467 | @end deffn | |
468 | ||
469 | @deffn {Scheme Procedure} write-line obj [port] | |
470 | @deffnx {C Function} scm_write_line (obj, port) | |
471 | Display @var{obj} and a newline character to @var{port}. If | |
472 | @var{port} is not specified, @code{(current-output-port)} is | |
473 | used. This function is equivalent to: | |
474 | @lisp | |
475 | (display obj [port]) | |
476 | (newline [port]) | |
477 | @end lisp | |
478 | @end deffn | |
479 | ||
480 | Some of the abovementioned I/O functions rely on the following C | |
481 | primitives. These will mainly be of interest to people hacking Guile | |
482 | internals. | |
483 | ||
484 | @deffn {Scheme Procedure} %read-delimited! delims str gobble [port [start [end]]] | |
485 | @deffnx {C Function} scm_read_delimited_x (delims, str, gobble, port, start, end) | |
486 | Read characters from @var{port} into @var{str} until one of the | |
487 | characters in the @var{delims} string is encountered. If | |
488 | @var{gobble} is true, discard the delimiter character; | |
489 | otherwise, leave it in the input stream for the next read. If | |
490 | @var{port} is not specified, use the value of | |
491 | @code{(current-input-port)}. If @var{start} or @var{end} are | |
492 | specified, store data only into the substring of @var{str} | |
493 | bounded by @var{start} and @var{end} (which default to the | |
494 | beginning and end of the string, respectively). | |
495 | ||
496 | Return a pair consisting of the delimiter that terminated the | |
497 | string and the number of characters read. If reading stopped | |
498 | at the end of file, the delimiter returned is the | |
499 | @var{eof-object}; if the string was filled without encountering | |
500 | a delimiter, this value is @code{#f}. | |
501 | @end deffn | |
502 | ||
503 | @deffn {Scheme Procedure} %read-line [port] | |
504 | @deffnx {C Function} scm_read_line (port) | |
505 | Read a newline-terminated line from @var{port}, allocating storage as | |
506 | necessary. The newline terminator (if any) is removed from the string, | |
507 | and a pair consisting of the line and its delimiter is returned. The | |
508 | delimiter may be either a newline or the @var{eof-object}; if | |
509 | @code{%read-line} is called at the end of file, it returns the pair | |
510 | @code{(#<eof> . #<eof>)}. | |
511 | @end deffn | |
512 | ||
513 | @node Block Reading and Writing | |
514 | @subsection Block reading and writing | |
515 | ||
516 | The Block-string-I/O module can be accessed with: | |
517 | ||
518 | @smalllisp | |
519 | (use-modules (ice-9 rw)) | |
520 | @end smalllisp | |
521 | ||
522 | It currently contains procedures that help to implement the | |
523 | @code{(scsh rw)} module in guile-scsh. | |
524 | ||
525 | @deffn {Scheme Procedure} read-string!/partial str [port_or_fdes [start [end]]] | |
526 | @deffnx {C Function} scm_read_string_x_partial (str, port_or_fdes, start, end) | |
527 | Read characters from a port or file descriptor into a | |
528 | string @var{str}. A port must have an underlying file | |
529 | descriptor --- a so-called fport. This procedure is | |
530 | scsh-compatible and can efficiently read large strings. | |
531 | It will: | |
532 | ||
533 | @itemize | |
534 | @item | |
535 | attempt to fill the entire string, unless the @var{start} | |
536 | and/or @var{end} arguments are supplied. i.e., @var{start} | |
537 | defaults to 0 and @var{end} defaults to | |
538 | @code{(string-length str)} | |
539 | @item | |
540 | use the current input port if @var{port_or_fdes} is not | |
541 | supplied. | |
542 | @item | |
543 | return fewer than the requested number of characters in some | |
544 | cases, e.g., on end of file, if interrupted by a signal, or if | |
545 | not all the characters are immediately available. | |
546 | @item | |
547 | wait indefinitely for some input if no characters are | |
548 | currently available, | |
549 | unless the port is in non-blocking mode. | |
550 | @item | |
551 | read characters from the port's input buffers if available, | |
552 | instead from the underlying file descriptor. | |
553 | @item | |
554 | return @code{#f} if end-of-file is encountered before reading | |
555 | any characters, otherwise return the number of characters | |
556 | read. | |
557 | @item | |
558 | return 0 if the port is in non-blocking mode and no characters | |
559 | are immediately available. | |
560 | @item | |
561 | return 0 if the request is for 0 bytes, with no | |
562 | end-of-file check. | |
563 | @end itemize | |
564 | @end deffn | |
565 | ||
566 | @deffn {Scheme Procedure} write-string/partial str [port_or_fdes [start [end]]] | |
567 | @deffnx {C Function} scm_write_string_partial (str, port_or_fdes, start, end) | |
568 | Write characters from a string @var{str} to a port or file | |
569 | descriptor. A port must have an underlying file descriptor | |
570 | --- a so-called fport. This procedure is | |
571 | scsh-compatible and can efficiently write large strings. | |
572 | It will: | |
573 | ||
574 | @itemize | |
575 | @item | |
576 | attempt to write the entire string, unless the @var{start} | |
577 | and/or @var{end} arguments are supplied. i.e., @var{start} | |
578 | defaults to 0 and @var{end} defaults to | |
579 | @code{(string-length str)} | |
580 | @item | |
581 | use the current output port if @var{port_of_fdes} is not | |
582 | supplied. | |
583 | @item | |
584 | in the case of a buffered port, store the characters in the | |
585 | port's output buffer, if all will fit. If they will not fit | |
586 | then any existing buffered characters will be flushed | |
587 | before attempting | |
588 | to write the new characters directly to the underlying file | |
589 | descriptor. If the port is in non-blocking mode and | |
590 | buffered characters can not be flushed immediately, then an | |
591 | @code{EAGAIN} system-error exception will be raised (Note: | |
592 | scsh does not support the use of non-blocking buffered ports.) | |
593 | @item | |
594 | write fewer than the requested number of | |
595 | characters in some cases, e.g., if interrupted by a signal or | |
596 | if not all of the output can be accepted immediately. | |
597 | @item | |
598 | wait indefinitely for at least one character | |
599 | from @var{str} to be accepted by the port, unless the port is | |
600 | in non-blocking mode. | |
601 | @item | |
602 | return the number of characters accepted by the port. | |
603 | @item | |
604 | return 0 if the port is in non-blocking mode and can not accept | |
605 | at least one character from @var{str} immediately | |
606 | @item | |
607 | return 0 immediately if the request size is 0 bytes. | |
608 | @end itemize | |
609 | @end deffn | |
610 | ||
611 | @node Default Ports | |
612 | @subsection Default Ports for Input, Output and Errors | |
613 | ||
614 | @rnindex current-input-port | |
615 | @deffn {Scheme Procedure} current-input-port | |
616 | @deffnx {C Function} scm_current_input_port () | |
34846414 | 617 | @cindex standard input |
07d83abe | 618 | Return the current input port. This is the default port used |
3fa0a042 KR |
619 | by many input procedures. |
620 | ||
621 | Initially this is the @dfn{standard input} in Unix and C terminology. | |
622 | When the standard input is a tty the port is unbuffered, otherwise | |
623 | it's fully buffered. | |
624 | ||
625 | Unbuffered input is good if an application runs an interactive | |
626 | subprocess, since any type-ahead input won't go into Guile's buffer | |
9782da8a | 627 | and be unavailable to the subprocess. |
3fa0a042 KR |
628 | |
629 | Note that Guile buffering is completely separate from the tty ``line | |
9782da8a KR |
630 | discipline''. In the usual cooked mode on a tty Guile only sees a |
631 | line of input once the user presses @key{Return}. | |
07d83abe MV |
632 | @end deffn |
633 | ||
634 | @rnindex current-output-port | |
635 | @deffn {Scheme Procedure} current-output-port | |
636 | @deffnx {C Function} scm_current_output_port () | |
34846414 | 637 | @cindex standard output |
07d83abe | 638 | Return the current output port. This is the default port used |
3fa0a042 KR |
639 | by many output procedures. |
640 | ||
641 | Initially this is the @dfn{standard output} in Unix and C terminology. | |
642 | When the standard output is a tty this port is unbuffered, otherwise | |
643 | it's fully buffered. | |
644 | ||
645 | Unbuffered output to a tty is good for ensuring progress output or a | |
646 | prompt is seen. But an application which always prints whole lines | |
647 | could change to line buffered, or an application with a lot of output | |
648 | could go fully buffered and perhaps make explicit @code{force-output} | |
649 | calls (@pxref{Writing}) at selected points. | |
07d83abe MV |
650 | @end deffn |
651 | ||
652 | @deffn {Scheme Procedure} current-error-port | |
653 | @deffnx {C Function} scm_current_error_port () | |
34846414 | 654 | @cindex standard error output |
3fa0a042 KR |
655 | Return the port to which errors and warnings should be sent. |
656 | ||
657 | Initially this is the @dfn{standard error} in Unix and C terminology. | |
658 | When the standard error is a tty this port is unbuffered, otherwise | |
659 | it's fully buffered. | |
07d83abe MV |
660 | @end deffn |
661 | ||
662 | @deffn {Scheme Procedure} set-current-input-port port | |
663 | @deffnx {Scheme Procedure} set-current-output-port port | |
664 | @deffnx {Scheme Procedure} set-current-error-port port | |
665 | @deffnx {C Function} scm_set_current_input_port (port) | |
666 | @deffnx {C Function} scm_set_current_output_port (port) | |
667 | @deffnx {C Function} scm_set_current_error_port (port) | |
668 | Change the ports returned by @code{current-input-port}, | |
669 | @code{current-output-port} and @code{current-error-port}, respectively, | |
670 | so that they use the supplied @var{port} for input or output. | |
671 | @end deffn | |
672 | ||
661ae7ab MV |
673 | @deftypefn {C Function} void scm_dynwind_current_input_port (SCM port) |
674 | @deftypefnx {C Function} void scm_dynwind_current_output_port (SCM port) | |
675 | @deftypefnx {C Function} void scm_dynwind_current_error_port (SCM port) | |
07d83abe | 676 | These functions must be used inside a pair of calls to |
661ae7ab MV |
677 | @code{scm_dynwind_begin} and @code{scm_dynwind_end} (@pxref{Dynamic |
678 | Wind}). During the dynwind context, the indicated port is set to | |
07d83abe MV |
679 | @var{port}. |
680 | ||
681 | More precisely, the current port is swapped with a `backup' value | |
661ae7ab | 682 | whenever the dynwind context is entered or left. The backup value is |
07d83abe MV |
683 | initialized with the @var{port} argument. |
684 | @end deftypefn | |
685 | ||
686 | @node Port Types | |
687 | @subsection Types of Port | |
688 | ||
689 | [Types of port; how to make them.] | |
690 | ||
691 | @menu | |
692 | * File Ports:: Ports on an operating system file. | |
693 | * String Ports:: Ports on a Scheme string. | |
694 | * Soft Ports:: Ports on arbitrary Scheme procedures. | |
695 | * Void Ports:: Ports on nothing at all. | |
696 | @end menu | |
697 | ||
698 | ||
699 | @node File Ports | |
700 | @subsubsection File Ports | |
701 | ||
702 | The following procedures are used to open file ports. | |
703 | See also @ref{Ports and File Descriptors, open}, for an interface | |
704 | to the Unix @code{open} system call. | |
705 | ||
706 | Most systems have limits on how many files can be open, so it's | |
707 | strongly recommended that file ports be closed explicitly when no | |
708 | longer required (@pxref{Ports}). | |
709 | ||
710 | @deffn {Scheme Procedure} open-file filename mode | |
711 | @deffnx {C Function} scm_open_file (filename, mode) | |
712 | Open the file whose name is @var{filename}, and return a port | |
713 | representing that file. The attributes of the port are | |
714 | determined by the @var{mode} string. The way in which this is | |
715 | interpreted is similar to C stdio. The first character must be | |
716 | one of the following: | |
c755b861 | 717 | |
07d83abe MV |
718 | @table @samp |
719 | @item r | |
720 | Open an existing file for input. | |
721 | @item w | |
722 | Open a file for output, creating it if it doesn't already exist | |
723 | or removing its contents if it does. | |
724 | @item a | |
725 | Open a file for output, creating it if it doesn't already | |
726 | exist. All writes to the port will go to the end of the file. | |
727 | The "append mode" can be turned off while the port is in use | |
728 | @pxref{Ports and File Descriptors, fcntl} | |
729 | @end table | |
c755b861 | 730 | |
07d83abe | 731 | The following additional characters can be appended: |
c755b861 | 732 | |
07d83abe MV |
733 | @table @samp |
734 | @item + | |
735 | Open the port for both input and output. E.g., @code{r+}: open | |
736 | an existing file for both input and output. | |
737 | @item 0 | |
738 | Create an "unbuffered" port. In this case input and output | |
739 | operations are passed directly to the underlying port | |
740 | implementation without additional buffering. This is likely to | |
741 | slow down I/O operations. The buffering mode can be changed | |
742 | while a port is in use @pxref{Ports and File Descriptors, | |
743 | setvbuf} | |
744 | @item l | |
745 | Add line-buffering to the port. The port output buffer will be | |
746 | automatically flushed whenever a newline character is written. | |
c755b861 KR |
747 | @item b |
748 | Use binary mode. On DOS systems the default text mode converts CR+LF | |
749 | in the file to newline for the program, whereas binary mode reads and | |
750 | writes all bytes unchanged. On Unix-like systems there is no such | |
751 | distinction, text files already contain just newlines and no | |
752 | conversion is ever made. The @code{b} flag is accepted on all | |
753 | systems, but has no effect on Unix-like systems. | |
754 | ||
755 | (For reference, Guile leaves text versus binary up to the C library, | |
756 | @code{b} here just adds @code{O_BINARY} to the underlying @code{open} | |
757 | call, when that flag is available.) | |
07d83abe | 758 | @end table |
c755b861 | 759 | |
092bdcc4 KR |
760 | If a file cannot be opened with the access |
761 | requested, @code{open-file} throws an exception. | |
762 | ||
07d83abe MV |
763 | In theory we could create read/write ports which were buffered |
764 | in one direction only. However this isn't included in the | |
092bdcc4 | 765 | current interfaces. |
07d83abe MV |
766 | @end deffn |
767 | ||
768 | @rnindex open-input-file | |
769 | @deffn {Scheme Procedure} open-input-file filename | |
770 | Open @var{filename} for input. Equivalent to | |
771 | @smalllisp | |
772 | (open-file @var{filename} "r") | |
773 | @end smalllisp | |
774 | @end deffn | |
775 | ||
776 | @rnindex open-output-file | |
777 | @deffn {Scheme Procedure} open-output-file filename | |
778 | Open @var{filename} for output. Equivalent to | |
779 | @smalllisp | |
780 | (open-file @var{filename} "w") | |
781 | @end smalllisp | |
782 | @end deffn | |
783 | ||
784 | @deffn {Scheme Procedure} call-with-input-file filename proc | |
785 | @deffnx {Scheme Procedure} call-with-output-file filename proc | |
786 | @rnindex call-with-input-file | |
787 | @rnindex call-with-output-file | |
788 | Open @var{filename} for input or output, and call @code{(@var{proc} | |
789 | port)} with the resulting port. Return the value returned by | |
790 | @var{proc}. @var{filename} is opened as per @code{open-input-file} or | |
791 | @code{open-output-file} respectively, and an error is signalled if it | |
792 | cannot be opened. | |
793 | ||
794 | When @var{proc} returns, the port is closed. If @var{proc} does not | |
795 | return (eg.@: if it throws an error), then the port might not be | |
796 | closed automatically, though it will be garbage collected in the usual | |
797 | way if not otherwise referenced. | |
798 | @end deffn | |
799 | ||
800 | @deffn {Scheme Procedure} with-input-from-file filename thunk | |
801 | @deffnx {Scheme Procedure} with-output-to-file filename thunk | |
802 | @deffnx {Scheme Procedure} with-error-to-file filename thunk | |
803 | @rnindex with-input-from-file | |
804 | @rnindex with-output-to-file | |
805 | Open @var{filename} and call @code{(@var{thunk})} with the new port | |
806 | setup as respectively the @code{current-input-port}, | |
807 | @code{current-output-port}, or @code{current-error-port}. Return the | |
808 | value returned by @var{thunk}. @var{filename} is opened as per | |
809 | @code{open-input-file} or @code{open-output-file} respectively, and an | |
810 | error is signalled if it cannot be opened. | |
811 | ||
812 | When @var{thunk} returns, the port is closed and the previous setting | |
813 | of the respective current port is restored. | |
814 | ||
815 | The current port setting is managed with @code{dynamic-wind}, so the | |
816 | previous value is restored no matter how @var{thunk} exits (eg.@: an | |
817 | exception), and if @var{thunk} is re-entered (via a captured | |
818 | continuation) then it's set again to the @var{FILENAME} port. | |
819 | ||
820 | The port is closed when @var{thunk} returns normally, but not when | |
821 | exited via an exception or new continuation. This ensures it's still | |
822 | ready for use if @var{thunk} is re-entered by a captured continuation. | |
823 | Of course the port is always garbage collected and closed in the usual | |
824 | way when no longer referenced anywhere. | |
825 | @end deffn | |
826 | ||
827 | @deffn {Scheme Procedure} port-mode port | |
828 | @deffnx {C Function} scm_port_mode (port) | |
829 | Return the port modes associated with the open port @var{port}. | |
830 | These will not necessarily be identical to the modes used when | |
831 | the port was opened, since modes such as "append" which are | |
832 | used only during port creation are not retained. | |
833 | @end deffn | |
834 | ||
835 | @deffn {Scheme Procedure} port-filename port | |
836 | @deffnx {C Function} scm_port_filename (port) | |
837 | Return the filename associated with @var{port}. This function returns | |
838 | the strings "standard input", "standard output" and "standard error" | |
839 | when called on the current input, output and error ports respectively. | |
e55abf41 KR |
840 | |
841 | @var{port} must be open, @code{port-filename} cannot be used once the | |
842 | port is closed. | |
07d83abe MV |
843 | @end deffn |
844 | ||
845 | @deffn {Scheme Procedure} set-port-filename! port filename | |
846 | @deffnx {C Function} scm_set_port_filename_x (port, filename) | |
847 | Change the filename associated with @var{port}, using the current input | |
848 | port if none is specified. Note that this does not change the port's | |
849 | source of data, but only the value that is returned by | |
850 | @code{port-filename} and reported in diagnostic output. | |
851 | @end deffn | |
852 | ||
853 | @deffn {Scheme Procedure} file-port? obj | |
854 | @deffnx {C Function} scm_file_port_p (obj) | |
855 | Determine whether @var{obj} is a port that is related to a file. | |
856 | @end deffn | |
857 | ||
858 | ||
859 | @node String Ports | |
860 | @subsubsection String Ports | |
861 | ||
862 | The following allow string ports to be opened by analogy to R4R* | |
863 | file port facilities: | |
864 | ||
865 | @deffn {Scheme Procedure} call-with-output-string proc | |
866 | @deffnx {C Function} scm_call_with_output_string (proc) | |
867 | Calls the one-argument procedure @var{proc} with a newly created output | |
868 | port. When the function returns, the string composed of the characters | |
869 | written into the port is returned. @var{proc} should not close the port. | |
870 | @end deffn | |
871 | ||
872 | @deffn {Scheme Procedure} call-with-input-string string proc | |
873 | @deffnx {C Function} scm_call_with_input_string (string, proc) | |
874 | Calls the one-argument procedure @var{proc} with a newly | |
875 | created input port from which @var{string}'s contents may be | |
876 | read. The value yielded by the @var{proc} is returned. | |
877 | @end deffn | |
878 | ||
879 | @deffn {Scheme Procedure} with-output-to-string thunk | |
880 | Calls the zero-argument procedure @var{thunk} with the current output | |
881 | port set temporarily to a new string port. It returns a string | |
882 | composed of the characters written to the current output. | |
883 | @end deffn | |
884 | ||
885 | @deffn {Scheme Procedure} with-input-from-string string thunk | |
886 | Calls the zero-argument procedure @var{thunk} with the current input | |
887 | port set temporarily to a string port opened on the specified | |
888 | @var{string}. The value yielded by @var{thunk} is returned. | |
889 | @end deffn | |
890 | ||
891 | @deffn {Scheme Procedure} open-input-string str | |
892 | @deffnx {C Function} scm_open_input_string (str) | |
893 | Take a string and return an input port that delivers characters | |
894 | from the string. The port can be closed by | |
895 | @code{close-input-port}, though its storage will be reclaimed | |
896 | by the garbage collector if it becomes inaccessible. | |
897 | @end deffn | |
898 | ||
899 | @deffn {Scheme Procedure} open-output-string | |
900 | @deffnx {C Function} scm_open_output_string () | |
901 | Return an output port that will accumulate characters for | |
902 | retrieval by @code{get-output-string}. The port can be closed | |
903 | by the procedure @code{close-output-port}, though its storage | |
904 | will be reclaimed by the garbage collector if it becomes | |
905 | inaccessible. | |
906 | @end deffn | |
907 | ||
908 | @deffn {Scheme Procedure} get-output-string port | |
909 | @deffnx {C Function} scm_get_output_string (port) | |
910 | Given an output port created by @code{open-output-string}, | |
911 | return a string consisting of the characters that have been | |
912 | output to the port so far. | |
913 | ||
914 | @code{get-output-string} must be used before closing @var{port}, once | |
915 | closed the string cannot be obtained. | |
916 | @end deffn | |
917 | ||
918 | A string port can be used in many procedures which accept a port | |
919 | but which are not dependent on implementation details of fports. | |
920 | E.g., seeking and truncating will work on a string port, | |
921 | but trying to extract the file descriptor number will fail. | |
922 | ||
923 | ||
924 | @node Soft Ports | |
925 | @subsubsection Soft Ports | |
926 | ||
927 | A @dfn{soft-port} is a port based on a vector of procedures capable of | |
928 | accepting or delivering characters. It allows emulation of I/O ports. | |
929 | ||
930 | @deffn {Scheme Procedure} make-soft-port pv modes | |
931 | @deffnx {C Function} scm_make_soft_port (pv, modes) | |
932 | Return a port capable of receiving or delivering characters as | |
933 | specified by the @var{modes} string (@pxref{File Ports, | |
934 | open-file}). @var{pv} must be a vector of length 5 or 6. Its | |
935 | components are as follows: | |
936 | ||
937 | @enumerate 0 | |
938 | @item | |
939 | procedure accepting one character for output | |
940 | @item | |
941 | procedure accepting a string for output | |
942 | @item | |
943 | thunk for flushing output | |
944 | @item | |
945 | thunk for getting one character | |
946 | @item | |
947 | thunk for closing port (not by garbage collection) | |
948 | @item | |
949 | (if present and not @code{#f}) thunk for computing the number of | |
950 | characters that can be read from the port without blocking. | |
951 | @end enumerate | |
952 | ||
953 | For an output-only port only elements 0, 1, 2, and 4 need be | |
954 | procedures. For an input-only port only elements 3 and 4 need | |
955 | be procedures. Thunks 2 and 4 can instead be @code{#f} if | |
956 | there is no useful operation for them to perform. | |
957 | ||
958 | If thunk 3 returns @code{#f} or an @code{eof-object} | |
959 | (@pxref{Input, eof-object?, ,r5rs, The Revised^5 Report on | |
960 | Scheme}) it indicates that the port has reached end-of-file. | |
961 | For example: | |
962 | ||
963 | @lisp | |
964 | (define stdout (current-output-port)) | |
965 | (define p (make-soft-port | |
966 | (vector | |
967 | (lambda (c) (write c stdout)) | |
968 | (lambda (s) (display s stdout)) | |
969 | (lambda () (display "." stdout)) | |
970 | (lambda () (char-upcase (read-char))) | |
971 | (lambda () (display "@@" stdout))) | |
972 | "rw")) | |
973 | ||
974 | (write p p) @result{} #<input-output: soft 8081e20> | |
975 | @end lisp | |
976 | @end deffn | |
977 | ||
978 | ||
979 | @node Void Ports | |
980 | @subsubsection Void Ports | |
981 | ||
982 | This kind of port causes any data to be discarded when written to, and | |
983 | always returns the end-of-file object when read from. | |
984 | ||
985 | @deffn {Scheme Procedure} %make-void-port mode | |
986 | @deffnx {C Function} scm_sys_make_void_port (mode) | |
987 | Create and return a new void port. A void port acts like | |
988 | @file{/dev/null}. The @var{mode} argument | |
989 | specifies the input/output modes for this port: see the | |
990 | documentation for @code{open-file} in @ref{File Ports}. | |
991 | @end deffn | |
992 | ||
993 | ||
994 | @node I/O Extensions | |
995 | @subsection Using and Extending Ports in C | |
996 | ||
997 | @menu | |
998 | * C Port Interface:: Using ports from C. | |
999 | * Port Implementation:: How to implement a new port type in C. | |
1000 | @end menu | |
1001 | ||
1002 | ||
1003 | @node C Port Interface | |
1004 | @subsubsection C Port Interface | |
1005 | ||
1006 | This section describes how to use Scheme ports from C. | |
1007 | ||
1008 | @subsubheading Port basics | |
1009 | ||
3081aee1 KR |
1010 | @cindex ptob |
1011 | @tindex scm_ptob_descriptor | |
1012 | @tindex scm_port | |
1013 | @findex SCM_PTAB_ENTRY | |
1014 | @findex SCM_PTOBNUM | |
1015 | @vindex scm_ptobs | |
07d83abe MV |
1016 | There are two main data structures. A port type object (ptob) is of |
1017 | type @code{scm_ptob_descriptor}. A port instance is of type | |
1018 | @code{scm_port}. Given an @code{SCM} variable which points to a port, | |
1019 | the corresponding C port object can be obtained using the | |
1020 | @code{SCM_PTAB_ENTRY} macro. The ptob can be obtained by using | |
1021 | @code{SCM_PTOBNUM} to give an index into the @code{scm_ptobs} | |
1022 | global array. | |
1023 | ||
1024 | @subsubheading Port buffers | |
1025 | ||
1026 | An input port always has a read buffer and an output port always has a | |
1027 | write buffer. However the size of these buffers is not guaranteed to be | |
1028 | more than one byte (e.g., the @code{shortbuf} field in @code{scm_port} | |
1029 | which is used when no other buffer is allocated). The way in which the | |
1030 | buffers are allocated depends on the implementation of the ptob. For | |
1031 | example in the case of an fport, buffers may be allocated with malloc | |
1032 | when the port is created, but in the case of an strport the underlying | |
1033 | string is used as the buffer. | |
1034 | ||
1035 | @subsubheading The @code{rw_random} flag | |
1036 | ||
1037 | Special treatment is required for ports which can be seeked at random. | |
1038 | Before various operations, such as seeking the port or changing from | |
1039 | input to output on a bidirectional port or vice versa, the port | |
1040 | implementation must be given a chance to update its state. The write | |
1041 | buffer is updated by calling the @code{flush} ptob procedure and the | |
1042 | input buffer is updated by calling the @code{end_input} ptob procedure. | |
1043 | In the case of an fport, @code{flush} causes buffered output to be | |
1044 | written to the file descriptor, while @code{end_input} causes the | |
1045 | descriptor position to be adjusted to account for buffered input which | |
1046 | was never read. | |
1047 | ||
1048 | The special treatment must be performed if the @code{rw_random} flag in | |
1049 | the port is non-zero. | |
1050 | ||
1051 | @subsubheading The @code{rw_active} variable | |
1052 | ||
1053 | The @code{rw_active} variable in the port is only used if | |
1054 | @code{rw_random} is set. It's defined as an enum with the following | |
1055 | values: | |
1056 | ||
1057 | @table @code | |
1058 | @item SCM_PORT_READ | |
1059 | the read buffer may have unread data. | |
1060 | ||
1061 | @item SCM_PORT_WRITE | |
1062 | the write buffer may have unwritten data. | |
1063 | ||
1064 | @item SCM_PORT_NEITHER | |
1065 | neither the write nor the read buffer has data. | |
1066 | @end table | |
1067 | ||
1068 | @subsubheading Reading from a port. | |
1069 | ||
1070 | To read from a port, it's possible to either call existing libguile | |
1071 | procedures such as @code{scm_getc} and @code{scm_read_line} or to read | |
1072 | data from the read buffer directly. Reading from the buffer involves | |
1073 | the following steps: | |
1074 | ||
1075 | @enumerate | |
1076 | @item | |
1077 | Flush output on the port, if @code{rw_active} is @code{SCM_PORT_WRITE}. | |
1078 | ||
1079 | @item | |
1080 | Fill the read buffer, if it's empty, using @code{scm_fill_input}. | |
1081 | ||
1082 | @item Read the data from the buffer and update the read position in | |
1083 | the buffer. Steps 2) and 3) may be repeated as many times as required. | |
1084 | ||
1085 | @item Set rw_active to @code{SCM_PORT_READ} if @code{rw_random} is set. | |
1086 | ||
1087 | @item update the port's line and column counts. | |
1088 | @end enumerate | |
1089 | ||
1090 | @subsubheading Writing to a port. | |
1091 | ||
1092 | To write data to a port, calling @code{scm_lfwrite} should be sufficient for | |
1093 | most purposes. This takes care of the following steps: | |
1094 | ||
1095 | @enumerate | |
1096 | @item | |
1097 | End input on the port, if @code{rw_active} is @code{SCM_PORT_READ}. | |
1098 | ||
1099 | @item | |
1100 | Pass the data to the ptob implementation using the @code{write} ptob | |
1101 | procedure. The advantage of using the ptob @code{write} instead of | |
1102 | manipulating the write buffer directly is that it allows the data to be | |
1103 | written in one operation even if the port is using the single-byte | |
1104 | @code{shortbuf}. | |
1105 | ||
1106 | @item | |
1107 | Set @code{rw_active} to @code{SCM_PORT_WRITE} if @code{rw_random} | |
1108 | is set. | |
1109 | @end enumerate | |
1110 | ||
1111 | ||
1112 | @node Port Implementation | |
1113 | @subsubsection Port Implementation | |
1114 | ||
1115 | This section describes how to implement a new port type in C. | |
1116 | ||
1117 | As described in the previous section, a port type object (ptob) is | |
1118 | a structure of type @code{scm_ptob_descriptor}. A ptob is created by | |
1119 | calling @code{scm_make_port_type}. | |
1120 | ||
1121 | All of the elements of the ptob, apart from @code{name}, are procedures | |
1122 | which collectively implement the port behaviour. Creating a new port | |
1123 | type mostly involves writing these procedures. | |
1124 | ||
1125 | @code{scm_make_port_type} initializes three elements of the structure | |
1126 | (@code{name}, @code{fill_input} and @code{write}) from its arguments. | |
1127 | The remaining elements are initialized with default values and can be | |
1128 | set later if required. | |
1129 | ||
1130 | @table @code | |
1131 | @item name | |
1132 | A pointer to a NUL terminated string: the name of the port type. This | |
1133 | is the only element of @code{scm_ptob_descriptor} which is not | |
1134 | a procedure. Set via the first argument to @code{scm_make_port_type}. | |
1135 | ||
1136 | @item mark | |
1137 | Called during garbage collection to mark any SCM objects that a port | |
1138 | object may contain. It doesn't need to be set unless the port has | |
1139 | @code{SCM} components. Set using @code{scm_set_port_mark}. | |
1140 | ||
1141 | @item free | |
1142 | Called when the port is collected during gc. It | |
1143 | should free any resources used by the port. | |
1144 | Set using @code{scm_set_port_free}. | |
1145 | ||
1146 | @item print | |
1147 | Called when @code{write} is called on the port object, to print a | |
1148 | port description. e.g., for an fport it may produce something like: | |
1149 | @code{#<input: /etc/passwd 3>}. Set using @code{scm_set_port_print}. | |
1150 | ||
1151 | @item equalp | |
1152 | Not used at present. Set using @code{scm_set_port_equalp}. | |
1153 | ||
1154 | @item close | |
1155 | Called when the port is closed, unless it was collected during gc. It | |
1156 | should free any resources used by the port. | |
1157 | Set using @code{scm_set_port_close}. | |
1158 | ||
1159 | @item write | |
1160 | Accept data which is to be written using the port. The port implementation | |
1161 | may choose to buffer the data instead of processing it directly. | |
1162 | Set via the third argument to @code{scm_make_port_type}. | |
1163 | ||
1164 | @item flush | |
1165 | Complete the processing of buffered output data. Reset the value of | |
1166 | @code{rw_active} to @code{SCM_PORT_NEITHER}. | |
1167 | Set using @code{scm_set_port_flush}. | |
1168 | ||
1169 | @item end_input | |
1170 | Perform any synchronization required when switching from input to output | |
1171 | on the port. Reset the value of @code{rw_active} to @code{SCM_PORT_NEITHER}. | |
1172 | Set using @code{scm_set_port_end_input}. | |
1173 | ||
1174 | @item fill_input | |
1175 | Read new data into the read buffer and return the first character. It | |
1176 | can be assumed that the read buffer is empty when this procedure is called. | |
1177 | Set via the second argument to @code{scm_make_port_type}. | |
1178 | ||
1179 | @item input_waiting | |
1180 | Return a lower bound on the number of bytes that could be read from the | |
1181 | port without blocking. It can be assumed that the current state of | |
1182 | @code{rw_active} is @code{SCM_PORT_NEITHER}. | |
1183 | Set using @code{scm_set_port_input_waiting}. | |
1184 | ||
1185 | @item seek | |
1186 | Set the current position of the port. The procedure can not make | |
1187 | any assumptions about the value of @code{rw_active} when it's | |
1188 | called. It can reset the buffers first if desired by using something | |
1189 | like: | |
1190 | ||
1191 | @example | |
1192 | if (pt->rw_active == SCM_PORT_READ) | |
1193 | scm_end_input (object); | |
1194 | else if (pt->rw_active == SCM_PORT_WRITE) | |
1195 | ptob->flush (object); | |
1196 | @end example | |
1197 | ||
1198 | However note that this will have the side effect of discarding any data | |
1199 | in the unread-char buffer, in addition to any side effects from the | |
1200 | @code{end_input} and @code{flush} ptob procedures. This is undesirable | |
1201 | when seek is called to measure the current position of the port, i.e., | |
1202 | @code{(seek p 0 SEEK_CUR)}. The libguile fport and string port | |
1203 | implementations take care to avoid this problem. | |
1204 | ||
1205 | The procedure is set using @code{scm_set_port_seek}. | |
1206 | ||
1207 | @item truncate | |
1208 | Truncate the port data to be specified length. It can be assumed that the | |
1209 | current state of @code{rw_active} is @code{SCM_PORT_NEITHER}. | |
1210 | Set using @code{scm_set_port_truncate}. | |
1211 | ||
1212 | @end table | |
1213 | ||
1214 | ||
1215 | @c Local Variables: | |
1216 | @c TeX-master: "guile.texi" | |
1217 | @c End: |