1FOPENCOOKIE(3) Linux Programmer's Manual FOPENCOOKIE(3)
2
6 fopencookie - opening a custom stream
7
9 #define _GNU_SOURCE /* See feature_test_macros(7) */
10 #include <stdio.h>
11 FILE *fopencookie(void *restrict cookie, const char *restrict mode,
13 cookie_io_functions_t io_funcs);
14
16 The fopencookie() function allows the programmer to create a custom im‐
17 plementation for a standard I/O stream. This implementation can store
18 the stream's data at a location of its own choosing; for example,
19 fopencookie() is used to implement fmemopen(3), which provides a stream
20 interface to data that is stored in a buffer in memory.
21 In order to create a custom stream the programmer must:
23 * Implement four "hook" functions that are used internally by the
25 standard I/O library when performing I/O on the stream.
26 * Define a "cookie" data type, a structure that provides bookkeeping
28 information (e.g., where to store data) used by the aforementioned
29 hook functions. The standard I/O package knows nothing about the
30 contents of this cookie (thus it is typed as void * when passed to
31 fopencookie()), but automatically supplies the cookie as the first
32 argument when calling the hook functions.
33 * Call fopencookie() to open a new stream and associate the cookie and
35 hook functions with that stream.
36 The fopencookie() function serves a purpose similar to fopen(3): it
38 opens a new stream and returns a pointer to a FILE object that is used
39 to operate on that stream.
40 The cookie argument is a pointer to the caller's cookie structure that
42 is to be associated with the new stream. This pointer is supplied as
43 the first argument when the standard I/O library invokes any of the
44 hook functions described below.
45 The mode argument serves the same purpose as for fopen(3). The follow‐
47 ing modes are supported: r, w, a, r+, w+, and a+. See fopen(3) for de‐
48 tails.
49 The io_funcs argument is a structure that contains four fields pointing
51 to the programmer-defined hook functions that are used to implement
52 this stream. The structure is defined as follows
53 typedef struct {
55 cookie_read_function_t *read;
56 cookie_write_function_t *write;
57 cookie_seek_function_t *seek;
58 cookie_close_function_t *close;
59 } cookie_io_functions_t;
60 The four fields are as follows:
62 cookie_read_function_t *read
64 This function implements read operations for the stream. When
65 called, it receives three arguments:
66 ssize_t read(void *cookie, char *buf, size_t size);
68 The buf and size arguments are, respectively, a buffer into
70 which input data can be placed and the size of that buffer. As
71 its function result, the read function should return the number
72 of bytes copied into buf, 0 on end of file, or -1 on error. The
73 read function should update the stream offset appropriately.
74 If *read is a null pointer, then reads from the custom stream
76 always return end of file.
77 cookie_write_function_t *write
79 This function implements write operations for the stream. When
80 called, it receives three arguments:
81 ssize_t write(void *cookie, const char *buf, size_t size);
83 The buf and size arguments are, respectively, a buffer of data
85 to be output to the stream and the size of that buffer. As its
86 function result, the write function should return the number of
87 bytes copied from buf, or 0 on error. (The function must not
88 return a negative value.) The write function should update the
89 stream offset appropriately.
90 If *write is a null pointer, then output to the stream is dis‐
92 carded.
93 cookie_seek_function_t *seek
95 This function implements seek operations on the stream. When
96 called, it receives three arguments:
97 int seek(void *cookie, off64_t *offset, int whence);
99 The *offset argument specifies the new file offset depending on
101 which of the following three values is supplied in whence:
102 SEEK_SET
104 The stream offset should be set *offset bytes from the
105 start of the stream.
106 SEEK_CUR
108 *offset should be added to the current stream offset.
109 SEEK_END
111 The stream offset should be set to the size of the stream
112 plus *offset.
113 Before returning, the seek function should update *offset to in‐
115 dicate the new stream offset.
116 As its function result, the seek function should return 0 on
118 success, and -1 on error.
119 If *seek is a null pointer, then it is not possible to perform
121 seek operations on the stream.
122 cookie_close_function_t *close
124 This function closes the stream. The hook function can do
125 things such as freeing buffers allocated for the stream. When
126 called, it receives one argument:
127 int close(void *cookie);
129 The cookie argument is the cookie that the programmer supplied
131 when calling fopencookie().
132 As its function result, the close function should return 0 on
134 success, and EOF on error.
135 If *close is NULL, then no special action is performed when the
137 stream is closed.
138
140 On success fopencookie() returns a pointer to the new stream. On er‐
141 ror, NULL is returned.
142
144 For an explanation of the terms used in this section, see at‐
145 tributes(7).
146 ┌────────────────────────────────────────────┬───────────────┬─────────┐
148 │Interface │ Attribute │ Value │
149 ├────────────────────────────────────────────┼───────────────┼─────────┤
150 │fopencookie() │ Thread safety │ MT-Safe │
151 └────────────────────────────────────────────┴───────────────┴─────────┘
152
154 This function is a nonstandard GNU extension.
155
157 The program below implements a custom stream whose functionality is
158 similar (but not identical) to that available via fmemopen(3). It im‐
159 plements a stream whose data is stored in a memory buffer. The program
160 writes its command-line arguments to the stream, and then seeks through
161 the stream reading two out of every five characters and writing them to
162 standard output. The following shell session demonstrates the use of
163 the program:
164 $ ./a.out 'hello world'
166 /he/
167 / w/
168 /d/
169 Reached end of file
170 Note that a more general version of the program below could be improved
172 to more robustly handle various error situations (e.g., opening a
173 stream with a cookie that already has an open stream; closing a stream
174 that has already been closed).
175 Program source
177 #define _GNU_SOURCE
179 #include <sys/types.h>
180 #include <stdio.h>
181 #include <stdlib.h>
182 #include <unistd.h>
183 #include <string.h>
184 #define INIT_BUF_SIZE 4
186 struct memfile_cookie {
188 char *buf; /* Dynamically sized buffer for data */
189 size_t allocated; /* Size of buf */
190 size_t endpos; /* Number of characters in buf */
191 off_t offset; /* Current file offset in buf */
192 };
193 ssize_t
195 memfile_write(void *c, const char *buf, size_t size)
196 {
197 char *new_buff;
198 struct memfile_cookie *cookie = c;
199 /* Buffer too small? Keep doubling size until big enough. */
201 while (size + cookie->offset > cookie->allocated) {
203 new_buff = realloc(cookie->buf, cookie->allocated * 2);
204 if (new_buff == NULL) {
205 return -1;
206 } else {
207 cookie->allocated *= 2;
208 cookie->buf = new_buff;
209 }
210 }
211 memcpy(cookie->buf + cookie->offset, buf, size);
213 cookie->offset += size;
215 if (cookie->offset > cookie->endpos)
216 cookie->endpos = cookie->offset;
217 return size;
219 }
220 ssize_t
222 memfile_read(void *c, char *buf, size_t size)
223 {
224 ssize_t xbytes;
225 struct memfile_cookie *cookie = c;
226 /* Fetch minimum of bytes requested and bytes available. */
228 xbytes = size;
230 if (cookie->offset + size > cookie->endpos)
231 xbytes = cookie->endpos - cookie->offset;
232 if (xbytes < 0) /* offset may be past endpos */
233 xbytes = 0;
234 memcpy(buf, cookie->buf + cookie->offset, xbytes);
236 cookie->offset += xbytes;
238 return xbytes;
239 }
240 int
242 memfile_seek(void *c, off64_t *offset, int whence)
243 {
244 off64_t new_offset;
245 struct memfile_cookie *cookie = c;
246 if (whence == SEEK_SET)
248 new_offset = *offset;
249 else if (whence == SEEK_END)
250 new_offset = cookie->endpos + *offset;
251 else if (whence == SEEK_CUR)
252 new_offset = cookie->offset + *offset;
253 else
254 return -1;
255 if (new_offset < 0)
257 return -1;
258 cookie->offset = new_offset;
260 *offset = new_offset;
261 return 0;
262 }
263 int
265 memfile_close(void *c)
266 {
267 struct memfile_cookie *cookie = c;
268 free(cookie->buf);
270 cookie->allocated = 0;
271 cookie->buf = NULL;
272 return 0;
274 }
275 int
277 main(int argc, char *argv[])
278 {
279 cookie_io_functions_t memfile_func = {
280 .read = memfile_read,
281 .write = memfile_write,
282 .seek = memfile_seek,
283 .close = memfile_close
284 };
285 FILE *stream;
286 struct memfile_cookie mycookie;
287 size_t nread;
288 char buf[1000];
289 /* Set up the cookie before calling fopencookie(). */
291 mycookie.buf = malloc(INIT_BUF_SIZE);
293 if (mycookie.buf == NULL) {
294 perror("malloc");
295 exit(EXIT_FAILURE);
296 }
297 mycookie.allocated = INIT_BUF_SIZE;
299 mycookie.offset = 0;
300 mycookie.endpos = 0;
301 stream = fopencookie(&mycookie, "w+", memfile_func);
303 if (stream == NULL) {
304 perror("fopencookie");
305 exit(EXIT_FAILURE);
306 }
307 /* Write command-line arguments to our file. */
309 for (int j = 1; j < argc; j++)
311 if (fputs(argv[j], stream) == EOF) {
312 perror("fputs");
313 exit(EXIT_FAILURE);
314 }
315 /* Read two bytes out of every five, until EOF. */
317 for (long p = 0; ; p += 5) {
319 if (fseek(stream, p, SEEK_SET) == -1) {
320 perror("fseek");
321 exit(EXIT_FAILURE);
322 }
323 nread = fread(buf, 1, 2, stream);
324 if (nread == 0) {
325 if (ferror(stream) != 0) {
326 fprintf(stderr, "fread failed\n");
327 exit(EXIT_FAILURE);
328 }
329 printf("Reached end of file\n");
330 break;
331 }
332 printf("/%.*s/\n", (int) nread, buf);
334 }
335 exit(EXIT_SUCCESS);
337 }
338
340 fclose(3), fmemopen(3), fopen(3), fseek(3)
341
343 This page is part of release 5.12 of the Linux man-pages project. A
344 description of the project, information about reporting bugs, and the
345 latest version of this page, can be found at
346 https://www.kernel.org/doc/man-pages/.
347Linux 2021-03-22 FOPENCOOKIE(3)