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 *cookie, const char *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 └──────────────┴───────────────┴─────────┘
153 This function is a nonstandard GNU extension.
154
156 The program below implements a custom stream whose functionality is
157 similar (but not identical) to that available via fmemopen(3). It im‐
158 plements a stream whose data is stored in a memory buffer. The program
159 writes its command-line arguments to the stream, and then seeks through
160 the stream reading two out of every five characters and writing them to
161 standard output. The following shell session demonstrates the use of
162 the program:
163 $ ./a.out 'hello world'
165 /he/
166 / w/
167 /d/
168 Reached end of file
169 Note that a more general version of the program below could be improved
171 to more robustly handle various error situations (e.g., opening a
172 stream with a cookie that already has an open stream; closing a stream
173 that has already been closed).
174 Program source
176 #define _GNU_SOURCE
178 #include <sys/types.h>
179 #include <stdio.h>
180 #include <stdlib.h>
181 #include <unistd.h>
182 #include <string.h>
183 #define INIT_BUF_SIZE 4
185 struct memfile_cookie {
187 char *buf; /* Dynamically sized buffer for data */
188 size_t allocated; /* Size of buf */
189 size_t endpos; /* Number of characters in buf */
190 off_t offset; /* Current file offset in buf */
191 };
192 ssize_t
194 memfile_write(void *c, const char *buf, size_t size)
195 {
196 char *new_buff;
197 struct memfile_cookie *cookie = c;
198 /* Buffer too small? Keep doubling size until big enough */
200 while (size + cookie->offset > cookie->allocated) {
202 new_buff = realloc(cookie->buf, cookie->allocated * 2);
203 if (new_buff == NULL) {
204 return -1;
205 } else {
206 cookie->allocated *= 2;
207 cookie->buf = new_buff;
208 }
209 }
210 memcpy(cookie->buf + cookie->offset, buf, size);
212 cookie->offset += size;
214 if (cookie->offset > cookie->endpos)
215 cookie->endpos = cookie->offset;
216 return size;
218 }
219 ssize_t
221 memfile_read(void *c, char *buf, size_t size)
222 {
223 ssize_t xbytes;
224 struct memfile_cookie *cookie = c;
225 /* Fetch minimum of bytes requested and bytes available */
227 xbytes = size;
229 if (cookie->offset + size > cookie->endpos)
230 xbytes = cookie->endpos - cookie->offset;
231 if (xbytes < 0) /* offset may be past endpos */
232 xbytes = 0;
233 memcpy(buf, cookie->buf + cookie->offset, xbytes);
235 cookie->offset += xbytes;
237 return xbytes;
238 }
239 int
241 memfile_seek(void *c, off64_t *offset, int whence)
242 {
243 off64_t new_offset;
244 struct memfile_cookie *cookie = c;
245 if (whence == SEEK_SET)
247 new_offset = *offset;
248 else if (whence == SEEK_END)
249 new_offset = cookie->endpos + *offset;
250 else if (whence == SEEK_CUR)
251 new_offset = cookie->offset + *offset;
252 else
253 return -1;
254 if (new_offset < 0)
256 return -1;
257 cookie->offset = new_offset;
259 *offset = new_offset;
260 return 0;
261 }
262 int
264 memfile_close(void *c)
265 {
266 struct memfile_cookie *cookie = c;
267 free(cookie->buf);
269 cookie->allocated = 0;
270 cookie->buf = NULL;
271 return 0;
273 }
274 int
276 main(int argc, char *argv[])
277 {
278 cookie_io_functions_t memfile_func = {
279 .read = memfile_read,
280 .write = memfile_write,
281 .seek = memfile_seek,
282 .close = memfile_close
283 };
284 FILE *stream;
285 struct memfile_cookie mycookie;
286 size_t nread;
287 char buf[1000];
288 /* Set up the cookie before calling fopencookie() */
290 mycookie.buf = malloc(INIT_BUF_SIZE);
292 if (mycookie.buf == NULL) {
293 perror("malloc");
294 exit(EXIT_FAILURE);
295 }
296 mycookie.allocated = INIT_BUF_SIZE;
298 mycookie.offset = 0;
299 mycookie.endpos = 0;
300 stream = fopencookie(&mycookie,"w+", memfile_func);
302 if (stream == NULL) {
303 perror("fopencookie");
304 exit(EXIT_FAILURE);
305 }
306 /* Write command-line arguments to our file */
308 for (int j = 1; j < argc; j++)
310 if (fputs(argv[j], stream) == EOF) {
311 perror("fputs");
312 exit(EXIT_FAILURE);
313 }
314 /* Read two bytes out of every five, until EOF */
316 for (long p = 0; ; p += 5) {
318 if (fseek(stream, p, SEEK_SET) == -1) {
319 perror("fseek");
320 exit(EXIT_FAILURE);
321 }
322 nread = fread(buf, 1, 2, stream);
323 if (nread == 0) {
324 if (ferror(stream) != 0) {
325 fprintf(stderr, "fread failed\n");
326 exit(EXIT_FAILURE);
327 }
328 printf("Reached end of file\n");
329 break;
330 }
331 printf("/%.*s/\n", (int) nread, buf);
333 }
334 exit(EXIT_SUCCESS);
336 }
337
339 fclose(3), fmemopen(3), fopen(3), fseek(3)
340
342 This page is part of release 5.10 of the Linux man-pages project. A
343 description of the project, information about reporting bugs, and the
344 latest version of this page, can be found at
345 https://www.kernel.org/doc/man-pages/.
346Linux 2020-11-01 FOPENCOOKIE(3)