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1 | @page |
2 | @node Input and Output | |
3 | @chapter Input and Output | |
4 | ||
5 | @menu | |
6 | * Ports:: The idea of the port abstraction. | |
7 | * Reading:: Procedures for reading from a port. | |
8 | * Writing:: Procedures for writing to a port. | |
9 | * Closing:: Procedures to close a port. | |
10 | * Random Access:: Moving around a random access port. | |
11 | * Line/Delimited:: Read and write lines or delimited text. | |
12 | * Block Reading and Writing:: Reading and writing blocks of text. | |
13 | * Default Ports:: Defaults for input, output and errors. | |
14 | * Port Types:: Types of port and how to make them. | |
9401323e | 15 | * I/O Extensions:: Using and extending ports in C. |
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16 | @end menu |
17 | ||
18 | ||
19 | @node Ports | |
20 | @section Ports | |
21 | ||
a0e07ba4 | 22 | Sequential input/output in Scheme is represented by operations on a |
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23 | @dfn{port}. This chapter explains the operations that Guile provides |
24 | for working with ports. | |
25 | ||
26 | Ports are created by opening, for instance @code{open-file} for a file | |
27 | (@pxref{File Ports}). Characters can be read from an input port and | |
28 | written to an output port, or both on an input/output port. A port | |
29 | can be closed (@pxref{Closing}) when no longer required, after which | |
30 | any attempt to read or write is an error. | |
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31 | |
32 | The formal definition of a port is very generic: an input port is | |
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33 | simply ``an object which can deliver characters on demand,'' and an |
34 | output port is ``an object which can accept characters.'' Because | |
35 | this definition is so loose, it is easy to write functions that | |
36 | simulate ports in software. @dfn{Soft ports} and @dfn{string ports} | |
37 | are two interesting and powerful examples of this technique. | |
38 | (@pxref{Soft Ports}, and @ref{String Ports}.) | |
39 | ||
40 | Ports are garbage collected in the usual way (@pxref{Memory | |
41 | Management}), and will be closed at that time if not already closed. | |
42 | In this case any errors occuring in the close will not be reported. | |
43 | Usually a program will want to explicitly close so as to be sure all | |
44 | its operations have been successful. Of course if a program has | |
45 | abandoned something due to an error or other condition then closing | |
46 | problems are probably not of interest. | |
47 | ||
48 | It is strongly recommended that file ports be closed explicitly when | |
49 | no longer required. Most systems have limits on how many files can be | |
50 | open, both on a per-process and a system-wide basis. A program that | |
51 | uses many files should take care not to hit those limits. The same | |
52 | applies to similar system resources such as pipes and sockets. | |
53 | ||
54 | Note that automatic garbage collection is triggered only by memory | |
55 | consumption, not by file or other resource usage, so a program cannot | |
56 | rely on that to keep it away from system limits. An explicit call to | |
57 | @code{gc} can of course be relied on to pick up unreferenced ports. | |
58 | If program flow makes it hard to be certain when to close then this | |
59 | may be an acceptable way to control resource usage. | |
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60 | |
61 | @rnindex input-port? | |
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62 | @deffn {Scheme Procedure} input-port? x |
63 | @deffnx {C Function} scm_input_port_p (x) | |
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64 | Return @code{#t} if @var{x} is an input port, otherwise return |
65 | @code{#f}. Any object satisfying this predicate also satisfies | |
66 | @code{port?}. | |
67 | @end deffn | |
68 | ||
69 | @rnindex output-port? | |
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70 | @deffn {Scheme Procedure} output-port? x |
71 | @deffnx {C Function} scm_output_port_p (x) | |
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72 | Return @code{#t} if @var{x} is an output port, otherwise return |
73 | @code{#f}. Any object satisfying this predicate also satisfies | |
74 | @code{port?}. | |
75 | @end deffn | |
76 | ||
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77 | @deffn {Scheme Procedure} port? x |
78 | @deffnx {C Function} scm_port_p (x) | |
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79 | Return a boolean indicating whether @var{x} is a port. |
80 | Equivalent to @code{(or (input-port? @var{x}) (output-port? | |
81 | @var{x}))}. | |
82 | @end deffn | |
83 | ||
84 | ||
85 | @node Reading | |
86 | @section Reading | |
87 | ||
88 | [Generic procedures for reading from ports.] | |
89 | ||
90 | @rnindex eof-object? | |
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91 | @deffn {Scheme Procedure} eof-object? x |
92 | @deffnx {C Function} scm_eof_object_p (x) | |
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93 | Return @code{#t} if @var{x} is an end-of-file object; otherwise |
94 | return @code{#f}. | |
95 | @end deffn | |
96 | ||
97 | @rnindex char-ready? | |
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98 | @deffn {Scheme Procedure} char-ready? [port] |
99 | @deffnx {C Function} scm_char_ready_p (port) | |
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100 | Return @code{#t} if a character is ready on input @var{port} |
101 | and return @code{#f} otherwise. If @code{char-ready?} returns | |
102 | @code{#t} then the next @code{read-char} operation on | |
103 | @var{port} is guaranteed not to hang. If @var{port} is a file | |
104 | port at end of file then @code{char-ready?} returns @code{#t}. | |
105 | @footnote{@code{char-ready?} exists to make it possible for a | |
106 | program to accept characters from interactive ports without | |
107 | getting stuck waiting for input. Any input editors associated | |
108 | with such ports must make sure that characters whose existence | |
109 | has been asserted by @code{char-ready?} cannot be rubbed out. | |
110 | If @code{char-ready?} were to return @code{#f} at end of file, | |
111 | a port at end of file would be indistinguishable from an | |
112 | interactive port that has no ready characters.} | |
113 | @end deffn | |
114 | ||
5a90a263 | 115 | @rnindex read-char |
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116 | @deffn {Scheme Procedure} read-char [port] |
117 | @deffnx {C Function} scm_read_char (port) | |
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118 | Return the next character available from @var{port}, updating |
119 | @var{port} to point to the following character. If no more | |
120 | characters are available, the end-of-file object is returned. | |
121 | @end deffn | |
122 | ||
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123 | @deftypefn {C Function} size_t scm_c_read (SCM port, void *buffer, size_t size) |
124 | Read up to @var{size} bytes from @var{port} and store them in | |
125 | @var{buffer}. The return value is the number of bytes actually read, | |
126 | which can be less than @var{size} if end-of-file has been reached. | |
127 | ||
128 | Note that this function does not update @code{port-line} and | |
129 | @code{port-column} below. | |
130 | @end deftypefn | |
131 | ||
5a90a263 | 132 | @rnindex peek-char |
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133 | @deffn {Scheme Procedure} peek-char [port] |
134 | @deffnx {C Function} scm_peek_char (port) | |
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135 | Return the next character available from @var{port}, |
136 | @emph{without} updating @var{port} to point to the following | |
137 | character. If no more characters are available, the | |
138 | end-of-file object is returned.@footnote{The value returned by | |
139 | a call to @code{peek-char} is the same as the value that would | |
140 | have been returned by a call to @code{read-char} on the same | |
141 | port. The only difference is that the very next call to | |
142 | @code{read-char} or @code{peek-char} on that @var{port} will | |
143 | return the value returned by the preceding call to | |
144 | @code{peek-char}. In particular, a call to @code{peek-char} on | |
145 | an interactive port will hang waiting for input whenever a call | |
146 | to @code{read-char} would have hung.} | |
147 | @end deffn | |
148 | ||
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149 | @deffn {Scheme Procedure} unread-char cobj [port] |
150 | @deffnx {C Function} scm_unread_char (cobj, port) | |
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151 | Place @var{char} in @var{port} so that it will be read by the |
152 | next read operation. If called multiple times, the unread characters | |
153 | will be read again in last-in first-out order. If @var{port} is | |
154 | not supplied, the current input port is used. | |
155 | @end deffn | |
156 | ||
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157 | @deffn {Scheme Procedure} unread-string str port |
158 | @deffnx {C Function} scm_unread_string (str, port) | |
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159 | Place the string @var{str} in @var{port} so that its characters will |
160 | be read from left-to-right as the next characters from @var{port} | |
161 | during subsequent read operations. If called multiple times, the | |
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162 | unread characters will be read again in last-in first-out order. If |
163 | @var{port} is not supplied, the current-input-port is used. | |
164 | @end deffn | |
165 | ||
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166 | @deffn {Scheme Procedure} drain-input port |
167 | @deffnx {C Function} scm_drain_input (port) | |
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168 | This procedure clears a port's input buffers, similar |
169 | to the way that force-output clears the output buffer. The | |
170 | contents of the buffers are returned as a single string, e.g., | |
171 | ||
172 | @lisp | |
173 | (define p (open-input-file ...)) | |
174 | (drain-input p) => empty string, nothing buffered yet. | |
175 | (unread-char (read-char p) p) | |
176 | (drain-input p) => initial chars from p, up to the buffer size. | |
177 | @end lisp | |
178 | ||
179 | Draining the buffers may be useful for cleanly finishing | |
180 | buffered I/O so that the file descriptor can be used directly | |
181 | for further input. | |
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182 | @end deffn |
183 | ||
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184 | @deffn {Scheme Procedure} port-column port |
185 | @deffnx {Scheme Procedure} port-line port | |
186 | @deffnx {C Function} scm_port_column (port) | |
187 | @deffnx {C Function} scm_port_line (port) | |
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188 | Return the current column number or line number of @var{port}. |
189 | If the number is | |
a0e07ba4 | 190 | unknown, the result is #f. Otherwise, the result is a 0-origin integer |
5a90a263 | 191 | - i.e.@: the first character of the first line is line 0, column 0. |
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192 | (However, when you display a file position, for example in an error |
193 | message, we recommend you add 1 to get 1-origin integers. This is | |
194 | because lines and column numbers traditionally start with 1, and that is | |
195 | what non-programmers will find most natural.) | |
196 | @end deffn | |
197 | ||
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198 | @deffn {Scheme Procedure} set-port-column! port column |
199 | @deffnx {Scheme Procedure} set-port-line! port line | |
200 | @deffnx {C Function} scm_set_port_column_x (port, column) | |
201 | @deffnx {C Function} scm_set_port_line_x (port, line) | |
eaa032c3 | 202 | Set the current column or line number of @var{port}. |
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203 | @end deffn |
204 | ||
205 | @node Writing | |
206 | @section Writing | |
207 | ||
208 | [Generic procedures for writing to ports.] | |
209 | ||
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210 | @deffn {Scheme Procedure} get-print-state port |
211 | @deffnx {C Function} scm_get_print_state (port) | |
5a90a263 | 212 | Return the print state of the port @var{port}. If @var{port} |
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213 | has no associated print state, @code{#f} is returned. |
214 | @end deffn | |
215 | ||
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216 | @rnindex write |
217 | @deffn {Scheme Procedure} write obj [port] | |
218 | Send a representation of @var{obj} to @var{port} or to the current | |
219 | output port if not given. | |
220 | ||
221 | The output is designed to be machine readable, and can be read back | |
222 | with @code{read} (@pxref{Reading}). Strings are printed in | |
223 | doublequotes, with escapes if necessary, and characters are printed in | |
224 | @samp{#\} notation. | |
225 | @end deffn | |
226 | ||
9eb96786 | 227 | @rnindex display |
8f85c0c6 | 228 | @deffn {Scheme Procedure} display obj [port] |
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229 | Send a representation of @var{obj} to @var{port} or to the current |
230 | output port if not given. | |
231 | ||
232 | The output is designed for human readability, it differs from | |
233 | @code{write} in that strings are printed without doublequotes and | |
234 | escapes, and characters are printed as per @code{write-char}, not in | |
235 | @samp{#\} form. | |
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236 | @end deffn |
237 | ||
a0e07ba4 | 238 | @rnindex newline |
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239 | @deffn {Scheme Procedure} newline [port] |
240 | @deffnx {C Function} scm_newline (port) | |
241 | Send a newline to @var{port}. | |
242 | If @var{port} is omitted, send to the current output port. | |
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243 | @end deffn |
244 | ||
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245 | @deffn {Scheme Procedure} port-with-print-state port pstate |
246 | @deffnx {C Function} scm_port_with_print_state (port, pstate) | |
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247 | Create a new port which behaves like @var{port}, but with an |
248 | included print state @var{pstate}. | |
249 | @end deffn | |
250 | ||
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251 | @deffn {Scheme Procedure} print-options-interface [setting] |
252 | @deffnx {C Function} scm_print_options (setting) | |
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253 | Option interface for the print options. Instead of using |
254 | this procedure directly, use the procedures | |
255 | @code{print-enable}, @code{print-disable}, @code{print-set!} | |
256 | and @code{print-options}. | |
257 | @end deffn | |
258 | ||
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259 | @deffn {Scheme Procedure} simple-format destination message . args |
260 | @deffnx {C Function} scm_simple_format (destination, message, args) | |
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261 | Write @var{message} to @var{destination}, defaulting to |
262 | the current output port. | |
263 | @var{message} can contain @code{~A} (was @code{%s}) and | |
264 | @code{~S} (was @code{%S}) escapes. When printed, | |
265 | the escapes are replaced with corresponding members of | |
266 | @var{ARGS}: | |
267 | @code{~A} formats using @code{display} and @code{~S} formats | |
268 | using @code{write}. | |
269 | If @var{destination} is @code{#t}, then use the current output | |
270 | port, if @var{destination} is @code{#f}, then return a string | |
271 | containing the formatted text. Does not add a trailing newline. | |
272 | @end deffn | |
273 | ||
274 | @rnindex write-char | |
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275 | @deffn {Scheme Procedure} write-char chr [port] |
276 | @deffnx {C Function} scm_write_char (chr, port) | |
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277 | Send character @var{chr} to @var{port}. |
278 | @end deffn | |
279 | ||
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280 | @deftypefn {C Function} void scm_c_write (SCM port, const void *buffer, size_t size) |
281 | Write @var{size} bytes at @var{buffer} to @var{port}. | |
282 | ||
283 | Note that this function does not update @code{port-line} and | |
284 | @code{port-column} (@pxref{Reading}). | |
285 | @end deftypefn | |
286 | ||
a0e07ba4 | 287 | @findex fflush |
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288 | @deffn {Scheme Procedure} force-output [port] |
289 | @deffnx {C Function} scm_force_output (port) | |
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290 | Flush the specified output port, or the current output port if @var{port} |
291 | is omitted. The current output buffer contents are passed to the | |
292 | underlying port implementation (e.g., in the case of fports, the | |
293 | data will be written to the file and the output buffer will be cleared.) | |
294 | It has no effect on an unbuffered port. | |
295 | ||
296 | The return value is unspecified. | |
297 | @end deffn | |
298 | ||
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299 | @deffn {Scheme Procedure} flush-all-ports |
300 | @deffnx {C Function} scm_flush_all_ports () | |
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301 | Equivalent to calling @code{force-output} on |
302 | all open output ports. The return value is unspecified. | |
303 | @end deffn | |
304 | ||
305 | ||
306 | @node Closing | |
307 | @section Closing | |
308 | ||
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309 | @deffn {Scheme Procedure} close-port port |
310 | @deffnx {C Function} scm_close_port (port) | |
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311 | Close the specified port object. Return @code{#t} if it |
312 | successfully closes a port or @code{#f} if it was already | |
313 | closed. An exception may be raised if an error occurs, for | |
314 | example when flushing buffered output. See also @ref{Ports and | |
315 | File Descriptors, close}, for a procedure which can close file | |
316 | descriptors. | |
317 | @end deffn | |
318 | ||
8f85c0c6 | 319 | @deffn {Scheme Procedure} close-input-port port |
2288712b | 320 | @deffnx {Scheme Procedure} close-output-port port |
8f85c0c6 | 321 | @deffnx {C Function} scm_close_input_port (port) |
8f85c0c6 | 322 | @deffnx {C Function} scm_close_output_port (port) |
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323 | @rnindex close-input-port |
324 | @rnindex close-output-port | |
325 | Close the specified input or output @var{port}. An exception may be | |
326 | raised if an error occurs while closing. If @var{port} is already | |
327 | closed, nothing is done. The return value is unspecified. | |
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328 | |
329 | See also @ref{Ports and File Descriptors, close}, for a procedure | |
330 | which can close file descriptors. | |
331 | @end deffn | |
332 | ||
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333 | @deffn {Scheme Procedure} port-closed? port |
334 | @deffnx {C Function} scm_port_closed_p (port) | |
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335 | Return @code{#t} if @var{port} is closed or @code{#f} if it is |
336 | open. | |
337 | @end deffn | |
338 | ||
339 | ||
340 | @node Random Access | |
341 | @section Random Access | |
342 | ||
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343 | @deffn {Scheme Procedure} seek fd_port offset whence |
344 | @deffnx {C Function} scm_seek (fd_port, offset, whence) | |
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345 | Sets the current position of @var{fd/port} to the integer |
346 | @var{offset}, which is interpreted according to the value of | |
347 | @var{whence}. | |
348 | ||
349 | One of the following variables should be supplied for | |
350 | @var{whence}: | |
351 | @defvar SEEK_SET | |
352 | Seek from the beginning of the file. | |
353 | @end defvar | |
354 | @defvar SEEK_CUR | |
355 | Seek from the current position. | |
356 | @end defvar | |
357 | @defvar SEEK_END | |
358 | Seek from the end of the file. | |
359 | @end defvar | |
360 | If @var{fd/port} is a file descriptor, the underlying system | |
361 | call is @code{lseek}. @var{port} may be a string port. | |
362 | ||
363 | The value returned is the new position in the file. This means | |
364 | that the current position of a port can be obtained using: | |
365 | @lisp | |
366 | (seek port 0 SEEK_CUR) | |
367 | @end lisp | |
368 | @end deffn | |
369 | ||
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370 | @deffn {Scheme Procedure} ftell fd_port |
371 | @deffnx {C Function} scm_ftell (fd_port) | |
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372 | Return an integer representing the current position of |
373 | @var{fd/port}, measured from the beginning. Equivalent to: | |
374 | ||
375 | @lisp | |
376 | (seek port 0 SEEK_CUR) | |
377 | @end lisp | |
378 | @end deffn | |
379 | ||
380 | @findex truncate | |
381 | @findex ftruncate | |
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382 | @deffn {Scheme Procedure} truncate-file object [length] |
383 | @deffnx {C Function} scm_truncate_file (object, length) | |
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384 | Truncates the object referred to by @var{object} to at most |
385 | @var{length} bytes. @var{object} can be a string containing a | |
386 | file name or an integer file descriptor or a port. | |
387 | @var{length} may be omitted if @var{object} is not a file name, | |
388 | in which case the truncation occurs at the current port. | |
389 | position. The return value is unspecified. | |
390 | @end deffn | |
391 | ||
392 | @node Line/Delimited | |
393 | @section Line Oriented and Delimited Text | |
394 | ||
395 | The delimited-I/O module can be accessed with: | |
396 | ||
397 | @smalllisp | |
398 | (use-modules (ice-9 rdelim)) | |
399 | @end smalllisp | |
400 | ||
401 | It can be used to read or write lines of text, or read text delimited by | |
402 | a specified set of characters. It's similar to the @code{(scsh rdelim)} | |
403 | module from guile-scsh, but does not use multiple values or character | |
404 | sets and has an extra procedure @code{write-line}. | |
405 | ||
406 | @c begin (scm-doc-string "rdelim.scm" "read-line") | |
8f85c0c6 | 407 | @deffn {Scheme Procedure} read-line [port] [handle-delim] |
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408 | Return a line of text from @var{port} if specified, otherwise from the |
409 | value returned by @code{(current-input-port)}. Under Unix, a line of text | |
410 | is terminated by the first end-of-line character or by end-of-file. | |
411 | ||
412 | If @var{handle-delim} is specified, it should be one of the following | |
413 | symbols: | |
414 | @table @code | |
415 | @item trim | |
416 | Discard the terminating delimiter. This is the default, but it will | |
417 | be impossible to tell whether the read terminated with a delimiter or | |
418 | end-of-file. | |
419 | @item concat | |
420 | Append the terminating delimiter (if any) to the returned string. | |
421 | @item peek | |
422 | Push the terminating delimiter (if any) back on to the port. | |
423 | @item split | |
424 | Return a pair containing the string read from the port and the | |
425 | terminating delimiter or end-of-file object. | |
426 | @end table | |
427 | @end deffn | |
428 | ||
429 | @c begin (scm-doc-string "rdelim.scm" "read-line!") | |
8f85c0c6 | 430 | @deffn {Scheme Procedure} read-line! buf [port] |
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431 | Read a line of text into the supplied string @var{buf} and return the |
432 | number of characters added to @var{buf}. If @var{buf} is filled, then | |
433 | @code{#f} is returned. | |
434 | Read from @var{port} if | |
435 | specified, otherwise from the value returned by @code{(current-input-port)}. | |
436 | @end deffn | |
437 | ||
438 | @c begin (scm-doc-string "rdelim.scm" "read-delimited") | |
8f85c0c6 | 439 | @deffn {Scheme Procedure} read-delimited delims [port] [handle-delim] |
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440 | Read text until one of the characters in the string @var{delims} is found |
441 | or end-of-file is reached. Read from @var{port} if supplied, otherwise | |
442 | from the value returned by @code{(current-input-port)}. | |
443 | @var{handle-delim} takes the same values as described for @code{read-line}. | |
444 | @end deffn | |
445 | ||
446 | @c begin (scm-doc-string "rdelim.scm" "read-delimited!") | |
8f85c0c6 | 447 | @deffn {Scheme Procedure} read-delimited! delims buf [port] [handle-delim] [start] [end] |
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448 | Read text into the supplied string @var{buf} and return the number of |
449 | characters added to @var{buf} (subject to @var{handle-delim}, which takes | |
450 | the same values specified for @code{read-line}. If @var{buf} is filled, | |
451 | @code{#f} is returned for both the number of characters read and the | |
452 | delimiter. Also terminates if one of the characters in the string | |
453 | @var{delims} is found | |
454 | or end-of-file is reached. Read from @var{port} if supplied, otherwise | |
455 | from the value returned by @code{(current-input-port)}. | |
456 | @end deffn | |
457 | ||
8f85c0c6 NJ |
458 | @deffn {Scheme Procedure} write-line obj [port] |
459 | @deffnx {C Function} scm_write_line (obj, port) | |
a0e07ba4 NJ |
460 | Display @var{obj} and a newline character to @var{port}. If |
461 | @var{port} is not specified, @code{(current-output-port)} is | |
462 | used. This function is equivalent to: | |
463 | @lisp | |
464 | (display obj [port]) | |
465 | (newline [port]) | |
466 | @end lisp | |
467 | @end deffn | |
468 | ||
469 | Some of the abovementioned I/O functions rely on the following C | |
470 | primitives. These will mainly be of interest to people hacking Guile | |
471 | internals. | |
472 | ||
8f85c0c6 NJ |
473 | @deffn {Scheme Procedure} %read-delimited! delims str gobble [port [start [end]]] |
474 | @deffnx {C Function} scm_read_delimited_x (delims, str, gobble, port, start, end) | |
a0e07ba4 NJ |
475 | Read characters from @var{port} into @var{str} until one of the |
476 | characters in the @var{delims} string is encountered. If | |
477 | @var{gobble} is true, discard the delimiter character; | |
478 | otherwise, leave it in the input stream for the next read. If | |
479 | @var{port} is not specified, use the value of | |
480 | @code{(current-input-port)}. If @var{start} or @var{end} are | |
481 | specified, store data only into the substring of @var{str} | |
482 | bounded by @var{start} and @var{end} (which default to the | |
483 | beginning and end of the string, respectively). | |
484 | ||
485 | Return a pair consisting of the delimiter that terminated the | |
486 | string and the number of characters read. If reading stopped | |
487 | at the end of file, the delimiter returned is the | |
488 | @var{eof-object}; if the string was filled without encountering | |
489 | a delimiter, this value is @code{#f}. | |
490 | @end deffn | |
491 | ||
8f85c0c6 NJ |
492 | @deffn {Scheme Procedure} %read-line [port] |
493 | @deffnx {C Function} scm_read_line (port) | |
a0e07ba4 NJ |
494 | Read a newline-terminated line from @var{port}, allocating storage as |
495 | necessary. The newline terminator (if any) is removed from the string, | |
496 | and a pair consisting of the line and its delimiter is returned. The | |
497 | delimiter may be either a newline or the @var{eof-object}; if | |
498 | @code{%read-line} is called at the end of file, it returns the pair | |
499 | @code{(#<eof> . #<eof>)}. | |
500 | @end deffn | |
501 | ||
502 | @node Block Reading and Writing | |
503 | @section Block reading and writing | |
504 | ||
505 | The Block-string-I/O module can be accessed with: | |
506 | ||
507 | @smalllisp | |
508 | (use-modules (ice-9 rw)) | |
509 | @end smalllisp | |
510 | ||
511 | It currently contains procedures that help to implement the | |
512 | @code{(scsh rw)} module in guile-scsh. | |
513 | ||
8f85c0c6 NJ |
514 | @deffn {Scheme Procedure} read-string!/partial str [port_or_fdes [start [end]]] |
515 | @deffnx {C Function} scm_read_string_x_partial (str, port_or_fdes, start, end) | |
a0e07ba4 NJ |
516 | Read characters from a port or file descriptor into a |
517 | string @var{str}. A port must have an underlying file | |
518 | descriptor --- a so-called fport. This procedure is | |
519 | scsh-compatible and can efficiently read large strings. | |
520 | It will: | |
521 | ||
522 | @itemize | |
523 | @item | |
524 | attempt to fill the entire string, unless the @var{start} | |
525 | and/or @var{end} arguments are supplied. i.e., @var{start} | |
526 | defaults to 0 and @var{end} defaults to | |
527 | @code{(string-length str)} | |
528 | @item | |
529 | use the current input port if @var{port_or_fdes} is not | |
530 | supplied. | |
531 | @item | |
532 | return fewer than the requested number of characters in some | |
533 | cases, e.g., on end of file, if interrupted by a signal, or if | |
534 | not all the characters are immediately available. | |
535 | @item | |
536 | wait indefinitely for some input if no characters are | |
537 | currently available, | |
538 | unless the port is in non-blocking mode. | |
539 | @item | |
540 | read characters from the port's input buffers if available, | |
541 | instead from the underlying file descriptor. | |
542 | @item | |
543 | return @code{#f} if end-of-file is encountered before reading | |
544 | any characters, otherwise return the number of characters | |
545 | read. | |
546 | @item | |
547 | return 0 if the port is in non-blocking mode and no characters | |
548 | are immediately available. | |
549 | @item | |
550 | return 0 if the request is for 0 bytes, with no | |
551 | end-of-file check. | |
552 | @end itemize | |
553 | @end deffn | |
554 | ||
8f85c0c6 NJ |
555 | @deffn {Scheme Procedure} write-string/partial str [port_or_fdes [start [end]]] |
556 | @deffnx {C Function} scm_write_string_partial (str, port_or_fdes, start, end) | |
a0e07ba4 NJ |
557 | Write characters from a string @var{str} to a port or file |
558 | descriptor. A port must have an underlying file descriptor | |
559 | --- a so-called fport. This procedure is | |
560 | scsh-compatible and can efficiently write large strings. | |
561 | It will: | |
562 | ||
563 | @itemize | |
564 | @item | |
565 | attempt to write the entire string, unless the @var{start} | |
566 | and/or @var{end} arguments are supplied. i.e., @var{start} | |
567 | defaults to 0 and @var{end} defaults to | |
568 | @code{(string-length str)} | |
569 | @item | |
570 | use the current output port if @var{port_of_fdes} is not | |
571 | supplied. | |
572 | @item | |
573 | in the case of a buffered port, store the characters in the | |
574 | port's output buffer, if all will fit. If they will not fit | |
575 | then any existing buffered characters will be flushed | |
576 | before attempting | |
577 | to write the new characters directly to the underlying file | |
578 | descriptor. If the port is in non-blocking mode and | |
579 | buffered characters can not be flushed immediately, then an | |
580 | @code{EAGAIN} system-error exception will be raised (Note: | |
581 | scsh does not support the use of non-blocking buffered ports.) | |
582 | @item | |
583 | write fewer than the requested number of | |
584 | characters in some cases, e.g., if interrupted by a signal or | |
585 | if not all of the output can be accepted immediately. | |
586 | @item | |
587 | wait indefinitely for at least one character | |
588 | from @var{str} to be accepted by the port, unless the port is | |
589 | in non-blocking mode. | |
590 | @item | |
591 | return the number of characters accepted by the port. | |
592 | @item | |
593 | return 0 if the port is in non-blocking mode and can not accept | |
594 | at least one character from @var{str} immediately | |
595 | @item | |
596 | return 0 immediately if the request size is 0 bytes. | |
597 | @end itemize | |
598 | @end deffn | |
599 | ||
600 | @node Default Ports | |
601 | @section Default Ports for Input, Output and Errors | |
602 | ||
603 | @rnindex current-input-port | |
8f85c0c6 NJ |
604 | @deffn {Scheme Procedure} current-input-port |
605 | @deffnx {C Function} scm_current_input_port () | |
a0e07ba4 NJ |
606 | Return the current input port. This is the default port used |
607 | by many input procedures. Initially, @code{current-input-port} | |
608 | returns the @dfn{standard input} in Unix and C terminology. | |
609 | @end deffn | |
610 | ||
611 | @rnindex current-output-port | |
8f85c0c6 NJ |
612 | @deffn {Scheme Procedure} current-output-port |
613 | @deffnx {C Function} scm_current_output_port () | |
a0e07ba4 NJ |
614 | Return the current output port. This is the default port used |
615 | by many output procedures. Initially, | |
616 | @code{current-output-port} returns the @dfn{standard output} in | |
617 | Unix and C terminology. | |
618 | @end deffn | |
619 | ||
8f85c0c6 NJ |
620 | @deffn {Scheme Procedure} current-error-port |
621 | @deffnx {C Function} scm_current_error_port () | |
a0e07ba4 NJ |
622 | Return the port to which errors and warnings should be sent (the |
623 | @dfn{standard error} in Unix and C terminology). | |
624 | @end deffn | |
625 | ||
8f85c0c6 NJ |
626 | @deffn {Scheme Procedure} set-current-input-port port |
627 | @deffnx {Scheme Procedure} set-current-output-port port | |
628 | @deffnx {Scheme Procedure} set-current-error-port port | |
629 | @deffnx {C Function} scm_set_current_input_port (port) | |
630 | @deffnx {C Function} scm_set_current_output_port (port) | |
631 | @deffnx {C Function} scm_set_current_error_port (port) | |
a0e07ba4 NJ |
632 | Change the ports returned by @code{current-input-port}, |
633 | @code{current-output-port} and @code{current-error-port}, respectively, | |
634 | so that they use the supplied @var{port} for input or output. | |
635 | @end deffn | |
636 | ||
a0e07ba4 NJ |
637 | |
638 | @node Port Types | |
639 | @section Types of Port | |
640 | ||
641 | [Types of port; how to make them.] | |
642 | ||
643 | @menu | |
644 | * File Ports:: Ports on an operating system file. | |
645 | * String Ports:: Ports on a Scheme string. | |
646 | * Soft Ports:: Ports on arbitrary Scheme procedures. | |
647 | * Void Ports:: Ports on nothing at all. | |
648 | @end menu | |
649 | ||
650 | ||
651 | @node File Ports | |
652 | @subsection File Ports | |
653 | ||
654 | The following procedures are used to open file ports. | |
655 | See also @ref{Ports and File Descriptors, open}, for an interface | |
656 | to the Unix @code{open} system call. | |
657 | ||
7d5b2929 KR |
658 | Most systems have limits on how many files can be open, so it's |
659 | strongly recommended that file ports be closed explicitly when no | |
660 | longer required (@pxref{Ports}). | |
661 | ||
8f85c0c6 NJ |
662 | @deffn {Scheme Procedure} open-file filename mode |
663 | @deffnx {C Function} scm_open_file (filename, mode) | |
a0e07ba4 NJ |
664 | Open the file whose name is @var{filename}, and return a port |
665 | representing that file. The attributes of the port are | |
666 | determined by the @var{mode} string. The way in which this is | |
667 | interpreted is similar to C stdio. The first character must be | |
668 | one of the following: | |
669 | @table @samp | |
670 | @item r | |
671 | Open an existing file for input. | |
672 | @item w | |
673 | Open a file for output, creating it if it doesn't already exist | |
674 | or removing its contents if it does. | |
675 | @item a | |
676 | Open a file for output, creating it if it doesn't already | |
677 | exist. All writes to the port will go to the end of the file. | |
678 | The "append mode" can be turned off while the port is in use | |
679 | @pxref{Ports and File Descriptors, fcntl} | |
680 | @end table | |
681 | The following additional characters can be appended: | |
682 | @table @samp | |
683 | @item + | |
684 | Open the port for both input and output. E.g., @code{r+}: open | |
685 | an existing file for both input and output. | |
686 | @item 0 | |
687 | Create an "unbuffered" port. In this case input and output | |
688 | operations are passed directly to the underlying port | |
689 | implementation without additional buffering. This is likely to | |
690 | slow down I/O operations. The buffering mode can be changed | |
691 | while a port is in use @pxref{Ports and File Descriptors, | |
692 | setvbuf} | |
693 | @item l | |
694 | Add line-buffering to the port. The port output buffer will be | |
695 | automatically flushed whenever a newline character is written. | |
696 | @end table | |
697 | In theory we could create read/write ports which were buffered | |
698 | in one direction only. However this isn't included in the | |
699 | current interfaces. If a file cannot be opened with the access | |
700 | requested, @code{open-file} throws an exception. | |
701 | @end deffn | |
702 | ||
703 | @rnindex open-input-file | |
8f85c0c6 | 704 | @deffn {Scheme Procedure} open-input-file filename |
a0e07ba4 NJ |
705 | Open @var{filename} for input. Equivalent to |
706 | @smalllisp | |
707 | (open-file @var{filename} "r") | |
708 | @end smalllisp | |
709 | @end deffn | |
710 | ||
711 | @rnindex open-output-file | |
8f85c0c6 | 712 | @deffn {Scheme Procedure} open-output-file filename |
a0e07ba4 NJ |
713 | Open @var{filename} for output. Equivalent to |
714 | @smalllisp | |
715 | (open-file @var{filename} "w") | |
716 | @end smalllisp | |
717 | @end deffn | |
718 | ||
2288712b KR |
719 | @deffn {Scheme Procedure} call-with-input-file filename proc |
720 | @deffnx {Scheme Procedure} call-with-output-file filename proc | |
a0e07ba4 | 721 | @rnindex call-with-input-file |
a0e07ba4 | 722 | @rnindex call-with-output-file |
2288712b KR |
723 | Open @var{filename} for input or output, and call @code{(@var{proc} |
724 | port)} with the resulting port. Return the value returned by | |
725 | @var{proc}. @var{filename} is opened as per @code{open-input-file} or | |
726 | @code{open-output-file} respectively, and an error is signalled if it | |
727 | cannot be opened. | |
a0e07ba4 | 728 | |
2288712b KR |
729 | When @var{proc} returns, the port is closed. If @var{proc} does not |
730 | return (eg.@: if it throws an error), then the port might not be | |
731 | closed automatically, though it will be garbage collected in the usual | |
732 | way if not otherwise referenced. | |
a0e07ba4 NJ |
733 | @end deffn |
734 | ||
2288712b KR |
735 | @deffn {Scheme Procedure} with-input-from-file filename thunk |
736 | @deffnx {Scheme Procedure} with-output-to-file filename thunk | |
737 | @deffnx {Scheme Procedure} with-error-to-file filename thunk | |
738 | @rnindex with-input-from-file | |
a0e07ba4 | 739 | @rnindex with-output-to-file |
2288712b KR |
740 | Open @var{filename} and call @code{(@var{thunk})} with the new port |
741 | setup as respectively the @code{current-input-port}, | |
742 | @code{current-output-port}, or @code{current-error-port}. Return the | |
743 | value returned by @var{thunk}. @var{filename} is opened as per | |
744 | @code{open-input-file} or @code{open-output-file} respectively, and an | |
745 | error is signalled if it cannot be opened. | |
746 | ||
747 | When @var{thunk} returns, the port is closed and the previous setting | |
748 | of the respective current port is restored. | |
749 | ||
750 | The current port setting is managed with @code{dynamic-wind}, so the | |
751 | previous value is restored no matter how @var{thunk} exits (eg.@: an | |
752 | exception), and if @var{thunk} is re-entered (via a captured | |
753 | continuation) then it's set again to the @var{FILENAME} port. | |
754 | ||
755 | The port is closed when @var{thunk} returns normally, but not when | |
756 | exited via an exception or new continuation. This ensures it's still | |
757 | ready for use if @var{thunk} is re-entered by a captured continuation. | |
758 | Of course the port is always garbage collected and closed in the usual | |
759 | way when no longer referenced anywhere. | |
a0e07ba4 NJ |
760 | @end deffn |
761 | ||
8f85c0c6 NJ |
762 | @deffn {Scheme Procedure} port-mode port |
763 | @deffnx {C Function} scm_port_mode (port) | |
764 | Return the port modes associated with the open port @var{port}. | |
765 | These will not necessarily be identical to the modes used when | |
766 | the port was opened, since modes such as "append" which are | |
767 | used only during port creation are not retained. | |
a0e07ba4 NJ |
768 | @end deffn |
769 | ||
8f85c0c6 NJ |
770 | @deffn {Scheme Procedure} port-filename port |
771 | @deffnx {C Function} scm_port_filename (port) | |
a0e07ba4 NJ |
772 | Return the filename associated with @var{port}. This function returns |
773 | the strings "standard input", "standard output" and "standard error" | |
774 | when called on the current input, output and error ports respectively. | |
775 | @end deffn | |
776 | ||
8f85c0c6 NJ |
777 | @deffn {Scheme Procedure} set-port-filename! port filename |
778 | @deffnx {C Function} scm_set_port_filename_x (port, filename) | |
a0e07ba4 NJ |
779 | Change the filename associated with @var{port}, using the current input |
780 | port if none is specified. Note that this does not change the port's | |
781 | source of data, but only the value that is returned by | |
782 | @code{port-filename} and reported in diagnostic output. | |
783 | @end deffn | |
784 | ||
8f85c0c6 NJ |
785 | @deffn {Scheme Procedure} file-port? obj |
786 | @deffnx {C Function} scm_file_port_p (obj) | |
a0e07ba4 NJ |
787 | Determine whether @var{obj} is a port that is related to a file. |
788 | @end deffn | |
789 | ||
790 | ||
791 | @node String Ports | |
792 | @subsection String Ports | |
793 | ||
794 | The following allow string ports to be opened by analogy to R4R* | |
795 | file port facilities: | |
796 | ||
8f85c0c6 NJ |
797 | @deffn {Scheme Procedure} call-with-output-string proc |
798 | @deffnx {C Function} scm_call_with_output_string (proc) | |
a0e07ba4 NJ |
799 | Calls the one-argument procedure @var{proc} with a newly created output |
800 | port. When the function returns, the string composed of the characters | |
801 | written into the port is returned. | |
802 | @end deffn | |
803 | ||
8f85c0c6 NJ |
804 | @deffn {Scheme Procedure} call-with-input-string string proc |
805 | @deffnx {C Function} scm_call_with_input_string (string, proc) | |
a0e07ba4 NJ |
806 | Calls the one-argument procedure @var{proc} with a newly |
807 | created input port from which @var{string}'s contents may be | |
808 | read. The value yielded by the @var{proc} is returned. | |
809 | @end deffn | |
810 | ||
8f85c0c6 | 811 | @deffn {Scheme Procedure} with-output-to-string thunk |
a0e07ba4 NJ |
812 | Calls the zero-argument procedure @var{thunk} with the current output |
813 | port set temporarily to a new string port. It returns a string | |
814 | composed of the characters written to the current output. | |
815 | @end deffn | |
816 | ||
8f85c0c6 | 817 | @deffn {Scheme Procedure} with-input-from-string string thunk |
a0e07ba4 NJ |
818 | Calls the zero-argument procedure @var{thunk} with the current input |
819 | port set temporarily to a string port opened on the specified | |
820 | @var{string}. The value yielded by @var{thunk} is returned. | |
821 | @end deffn | |
822 | ||
8f85c0c6 NJ |
823 | @deffn {Scheme Procedure} open-input-string str |
824 | @deffnx {C Function} scm_open_input_string (str) | |
a0e07ba4 NJ |
825 | Take a string and return an input port that delivers characters |
826 | from the string. The port can be closed by | |
827 | @code{close-input-port}, though its storage will be reclaimed | |
828 | by the garbage collector if it becomes inaccessible. | |
829 | @end deffn | |
830 | ||
8f85c0c6 NJ |
831 | @deffn {Scheme Procedure} open-output-string |
832 | @deffnx {C Function} scm_open_output_string () | |
a0e07ba4 NJ |
833 | Return an output port that will accumulate characters for |
834 | retrieval by @code{get-output-string}. The port can be closed | |
835 | by the procedure @code{close-output-port}, though its storage | |
836 | will be reclaimed by the garbage collector if it becomes | |
837 | inaccessible. | |
838 | @end deffn | |
839 | ||
8f85c0c6 NJ |
840 | @deffn {Scheme Procedure} get-output-string port |
841 | @deffnx {C Function} scm_get_output_string (port) | |
a0e07ba4 NJ |
842 | Given an output port created by @code{open-output-string}, |
843 | return a string consisting of the characters that have been | |
844 | output to the port so far. | |
845 | @end deffn | |
846 | ||
847 | A string port can be used in many procedures which accept a port | |
848 | but which are not dependent on implementation details of fports. | |
849 | E.g., seeking and truncating will work on a string port, | |
850 | but trying to extract the file descriptor number will fail. | |
851 | ||
852 | ||
853 | @node Soft Ports | |
854 | @subsection Soft Ports | |
855 | ||
856 | A @dfn{soft-port} is a port based on a vector of procedures capable of | |
857 | accepting or delivering characters. It allows emulation of I/O ports. | |
858 | ||
8f85c0c6 NJ |
859 | @deffn {Scheme Procedure} make-soft-port pv modes |
860 | @deffnx {C Function} scm_make_soft_port (pv, modes) | |
a0e07ba4 NJ |
861 | Return a port capable of receiving or delivering characters as |
862 | specified by the @var{modes} string (@pxref{File Ports, | |
0a50eeaa | 863 | open-file}). @var{pv} must be a vector of length 5 or 6. Its |
a0e07ba4 NJ |
864 | components are as follows: |
865 | ||
866 | @enumerate 0 | |
867 | @item | |
868 | procedure accepting one character for output | |
869 | @item | |
870 | procedure accepting a string for output | |
871 | @item | |
872 | thunk for flushing output | |
873 | @item | |
874 | thunk for getting one character | |
875 | @item | |
876 | thunk for closing port (not by garbage collection) | |
0a50eeaa NJ |
877 | @item |
878 | (if present and not @code{#f}) thunk for computing the number of | |
879 | characters that can be read from the port without blocking. | |
a0e07ba4 NJ |
880 | @end enumerate |
881 | ||
882 | For an output-only port only elements 0, 1, 2, and 4 need be | |
883 | procedures. For an input-only port only elements 3 and 4 need | |
884 | be procedures. Thunks 2 and 4 can instead be @code{#f} if | |
885 | there is no useful operation for them to perform. | |
886 | ||
887 | If thunk 3 returns @code{#f} or an @code{eof-object} | |
888 | (@pxref{Input, eof-object?, ,r5rs, The Revised^5 Report on | |
889 | Scheme}) it indicates that the port has reached end-of-file. | |
890 | For example: | |
891 | ||
892 | @lisp | |
893 | (define stdout (current-output-port)) | |
894 | (define p (make-soft-port | |
895 | (vector | |
896 | (lambda (c) (write c stdout)) | |
897 | (lambda (s) (display s stdout)) | |
898 | (lambda () (display "." stdout)) | |
899 | (lambda () (char-upcase (read-char))) | |
900 | (lambda () (display "@@" stdout))) | |
901 | "rw")) | |
902 | ||
903 | (write p p) @result{} #<input-output: soft 8081e20> | |
904 | @end lisp | |
905 | @end deffn | |
906 | ||
907 | ||
908 | @node Void Ports | |
909 | @subsection Void Ports | |
910 | ||
911 | This kind of port causes any data to be discarded when written to, and | |
912 | always returns the end-of-file object when read from. | |
913 | ||
8f85c0c6 NJ |
914 | @deffn {Scheme Procedure} %make-void-port mode |
915 | @deffnx {C Function} scm_sys_make_void_port (mode) | |
a0e07ba4 | 916 | Create and return a new void port. A void port acts like |
198586ed | 917 | @file{/dev/null}. The @var{mode} argument |
8f85c0c6 NJ |
918 | specifies the input/output modes for this port: see the |
919 | documentation for @code{open-file} in @ref{File Ports}. | |
a0e07ba4 NJ |
920 | @end deffn |
921 | ||
922 | ||
9401323e NJ |
923 | @node I/O Extensions |
924 | @section Using and Extending Ports in C | |
925 | ||
926 | @menu | |
927 | * C Port Interface:: Using ports from C. | |
928 | * Port Implementation:: How to implement a new port type in C. | |
929 | @end menu | |
930 | ||
931 | ||
932 | @node C Port Interface | |
933 | @subsection C Port Interface | |
934 | ||
935 | This section describes how to use Scheme ports from C. | |
936 | ||
937 | @subsubsection Port basics | |
938 | ||
939 | There are two main data structures. A port type object (ptob) is of | |
940 | type @code{scm_ptob_descriptor}. A port instance is of type | |
941 | @code{scm_port}. Given an @code{SCM} variable which points to a port, | |
942 | the corresponding C port object can be obtained using the | |
943 | @code{SCM_PTAB_ENTRY} macro. The ptob can be obtained by using | |
944 | @code{SCM_PTOBNUM} to give an index into the @code{scm_ptobs} | |
945 | global array. | |
946 | ||
947 | @subsubsection Port buffers | |
948 | ||
949 | An input port always has a read buffer and an output port always has a | |
950 | write buffer. However the size of these buffers is not guaranteed to be | |
951 | more than one byte (e.g., the @code{shortbuf} field in @code{scm_port} | |
952 | which is used when no other buffer is allocated). The way in which the | |
953 | buffers are allocated depends on the implementation of the ptob. For | |
954 | example in the case of an fport, buffers may be allocated with malloc | |
955 | when the port is created, but in the case of an strport the underlying | |
956 | string is used as the buffer. | |
957 | ||
958 | @subsubsection The @code{rw_random} flag | |
959 | ||
960 | Special treatment is required for ports which can be seeked at random. | |
961 | Before various operations, such as seeking the port or changing from | |
962 | input to output on a bidirectional port or vice versa, the port | |
85a9b4ed | 963 | implementation must be given a chance to update its state. The write |
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964 | buffer is updated by calling the @code{flush} ptob procedure and the |
965 | input buffer is updated by calling the @code{end_input} ptob procedure. | |
966 | In the case of an fport, @code{flush} causes buffered output to be | |
967 | written to the file descriptor, while @code{end_input} causes the | |
968 | descriptor position to be adjusted to account for buffered input which | |
969 | was never read. | |
970 | ||
971 | The special treatment must be performed if the @code{rw_random} flag in | |
972 | the port is non-zero. | |
973 | ||
974 | @subsubsection The @code{rw_active} variable | |
975 | ||
976 | The @code{rw_active} variable in the port is only used if | |
977 | @code{rw_random} is set. It's defined as an enum with the following | |
978 | values: | |
979 | ||
980 | @table @code | |
981 | @item SCM_PORT_READ | |
982 | the read buffer may have unread data. | |
983 | ||
984 | @item SCM_PORT_WRITE | |
985 | the write buffer may have unwritten data. | |
986 | ||
987 | @item SCM_PORT_NEITHER | |
988 | neither the write nor the read buffer has data. | |
989 | @end table | |
990 | ||
991 | @subsubsection Reading from a port. | |
992 | ||
993 | To read from a port, it's possible to either call existing libguile | |
994 | procedures such as @code{scm_getc} and @code{scm_read_line} or to read | |
995 | data from the read buffer directly. Reading from the buffer involves | |
996 | the following steps: | |
997 | ||
998 | @enumerate | |
999 | @item | |
1000 | Flush output on the port, if @code{rw_active} is @code{SCM_PORT_WRITE}. | |
1001 | ||
1002 | @item | |
1003 | Fill the read buffer, if it's empty, using @code{scm_fill_input}. | |
1004 | ||
1005 | @item Read the data from the buffer and update the read position in | |
1006 | the buffer. Steps 2) and 3) may be repeated as many times as required. | |
1007 | ||
1008 | @item Set rw_active to @code{SCM_PORT_READ} if @code{rw_random} is set. | |
1009 | ||
1010 | @item update the port's line and column counts. | |
1011 | @end enumerate | |
1012 | ||
1013 | @subsubsection Writing to a port. | |
1014 | ||
1015 | To write data to a port, calling @code{scm_lfwrite} should be sufficient for | |
1016 | most purposes. This takes care of the following steps: | |
1017 | ||
1018 | @enumerate | |
1019 | @item | |
1020 | End input on the port, if @code{rw_active} is @code{SCM_PORT_READ}. | |
1021 | ||
1022 | @item | |
1023 | Pass the data to the ptob implementation using the @code{write} ptob | |
1024 | procedure. The advantage of using the ptob @code{write} instead of | |
1025 | manipulating the write buffer directly is that it allows the data to be | |
1026 | written in one operation even if the port is using the single-byte | |
1027 | @code{shortbuf}. | |
1028 | ||
1029 | @item | |
1030 | Set @code{rw_active} to @code{SCM_PORT_WRITE} if @code{rw_random} | |
1031 | is set. | |
1032 | @end enumerate | |
1033 | ||
1034 | ||
1035 | @node Port Implementation | |
1036 | @subsection Port Implementation | |
1037 | ||
1038 | This section describes how to implement a new port type in C. | |
1039 | ||
1040 | As described in the previous section, a port type object (ptob) is | |
1041 | a structure of type @code{scm_ptob_descriptor}. A ptob is created by | |
1042 | calling @code{scm_make_port_type}. | |
1043 | ||
1044 | All of the elements of the ptob, apart from @code{name}, are procedures | |
1045 | which collectively implement the port behaviour. Creating a new port | |
1046 | type mostly involves writing these procedures. | |
1047 | ||
1048 | @code{scm_make_port_type} initializes three elements of the structure | |
1049 | (@code{name}, @code{fill_input} and @code{write}) from its arguments. | |
1050 | The remaining elements are initialized with default values and can be | |
1051 | set later if required. | |
1052 | ||
1053 | @table @code | |
1054 | @item name | |
1055 | A pointer to a NUL terminated string: the name of the port type. This | |
1056 | is the only element of @code{scm_ptob_descriptor} which is not | |
1057 | a procedure. Set via the first argument to @code{scm_make_port_type}. | |
1058 | ||
1059 | @item mark | |
1060 | Called during garbage collection to mark any SCM objects that a port | |
1061 | object may contain. It doesn't need to be set unless the port has | |
1062 | @code{SCM} components. Set using @code{scm_set_port_mark}. | |
1063 | ||
1064 | @item free | |
1065 | Called when the port is collected during gc. It | |
1066 | should free any resources used by the port. | |
1067 | Set using @code{scm_set_port_free}. | |
1068 | ||
1069 | @item print | |
1070 | Called when @code{write} is called on the port object, to print a | |
1071 | port description. e.g., for an fport it may produce something like: | |
1072 | @code{#<input: /etc/passwd 3>}. Set using @code{scm_set_port_print}. | |
1073 | ||
1074 | @item equalp | |
1075 | Not used at present. Set using @code{scm_set_port_equalp}. | |
1076 | ||
1077 | @item close | |
1078 | Called when the port is closed, unless it was collected during gc. It | |
1079 | should free any resources used by the port. | |
1080 | Set using @code{scm_set_port_close}. | |
1081 | ||
1082 | @item write | |
1083 | Accept data which is to be written using the port. The port implementation | |
1084 | may choose to buffer the data instead of processing it directly. | |
1085 | Set via the third argument to @code{scm_make_port_type}. | |
1086 | ||
1087 | @item flush | |
1088 | Complete the processing of buffered output data. Reset the value of | |
1089 | @code{rw_active} to @code{SCM_PORT_NEITHER}. | |
1090 | Set using @code{scm_set_port_flush}. | |
1091 | ||
1092 | @item end_input | |
85a9b4ed | 1093 | Perform any synchronization required when switching from input to output |
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1094 | on the port. Reset the value of @code{rw_active} to @code{SCM_PORT_NEITHER}. |
1095 | Set using @code{scm_set_port_end_input}. | |
1096 | ||
1097 | @item fill_input | |
1098 | Read new data into the read buffer and return the first character. It | |
1099 | can be assumed that the read buffer is empty when this procedure is called. | |
1100 | Set via the second argument to @code{scm_make_port_type}. | |
1101 | ||
1102 | @item input_waiting | |
1103 | Return a lower bound on the number of bytes that could be read from the | |
1104 | port without blocking. It can be assumed that the current state of | |
1105 | @code{rw_active} is @code{SCM_PORT_NEITHER}. | |
1106 | Set using @code{scm_set_port_input_waiting}. | |
1107 | ||
1108 | @item seek | |
1109 | Set the current position of the port. The procedure can not make | |
1110 | any assumptions about the value of @code{rw_active} when it's | |
1111 | called. It can reset the buffers first if desired by using something | |
1112 | like: | |
1113 | ||
1114 | @example | |
1115 | if (pt->rw_active == SCM_PORT_READ) | |
1116 | scm_end_input (object); | |
1117 | else if (pt->rw_active == SCM_PORT_WRITE) | |
1118 | ptob->flush (object); | |
1119 | @end example | |
1120 | ||
1121 | However note that this will have the side effect of discarding any data | |
1122 | in the unread-char buffer, in addition to any side effects from the | |
1123 | @code{end_input} and @code{flush} ptob procedures. This is undesirable | |
1124 | when seek is called to measure the current position of the port, i.e., | |
1125 | @code{(seek p 0 SEEK_CUR)}. The libguile fport and string port | |
1126 | implementations take care to avoid this problem. | |
1127 | ||
1128 | The procedure is set using @code{scm_set_port_seek}. | |
1129 | ||
1130 | @item truncate | |
1131 | Truncate the port data to be specified length. It can be assumed that the | |
1132 | current state of @code{rw_active} is @code{SCM_PORT_NEITHER}. | |
1133 | Set using @code{scm_set_port_truncate}. | |
1134 | ||
1135 | @end table | |
1136 | ||
1137 | ||
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1138 | @c Local Variables: |
1139 | @c TeX-master: "guile.texi" | |
1140 | @c End: |