1FOPENCOOKIE(3)             Linux Programmer's Manual            FOPENCOOKIE(3)
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NAME

6       fopencookie - opening a custom stream
7

SYNOPSIS

9       #define _GNU_SOURCE         /* See feature_test_macros(7) */
10       #include <stdio.h>
11
12       FILE *fopencookie(void *cookie, const char *mode,
13                         cookie_io_functions_t io_funcs);
14

DESCRIPTION

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
22       In order to create a custom stream the programmer must:
23
24       *  Implement  four  "hook"  functions  that  are used internally by the
25          standard I/O library when performing I/O on the stream.
26
27       *  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
34       *  Call fopencookie() to open a new stream and associate the cookie and
35          hook functions with that stream.
36
37       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
41       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
46       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
50       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
54           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
61       The four fields are as follows:
62
63       cookie_read_function_t *read
64              This  function  implements read operations for the stream.  When
65              called, it receives three arguments:
66
67                  ssize_t read(void *cookie, char *buf, size_t size);
68
69              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
75              If *read is a null pointer, then reads from  the  custom  stream
76              always return end of file.
77
78       cookie_write_function_t *write
79              This  function implements write operations for the stream.  When
80              called, it receives three arguments:
81
82                  ssize_t write(void *cookie, const char *buf, size_t size);
83
84              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
91              If  *write  is a null pointer, then output to the stream is dis‐
92              carded.
93
94       cookie_seek_function_t *seek
95              This function implements seek operations on  the  stream.   When
96              called, it receives three arguments:
97
98                  int seek(void *cookie, off64_t *offset, int whence);
99
100              The  *offset argument specifies the new file offset depending on
101              which of the following three values is supplied in whence:
102
103              SEEK_SET
104                     The stream offset should be set *offset  bytes  from  the
105                     start of the stream.
106
107              SEEK_CUR
108                     *offset should be added to the current stream offset.
109
110              SEEK_END
111                     The stream offset should be set to the size of the stream
112                     plus *offset.
113
114              Before returning, the seek function should update *offset to in‐
115              dicate the new stream offset.
116
117              As  its  function  result,  the seek function should return 0 on
118              success, and -1 on error.
119
120              If *seek is a null pointer, then it is not possible  to  perform
121              seek operations on the stream.
122
123       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
128                  int close(void *cookie);
129
130              The  cookie  argument is the cookie that the programmer supplied
131              when calling fopencookie().
132
133              As its function result, the close function should  return  0  on
134              success, and EOF on error.
135
136              If  *close is NULL, then no special action is performed when the
137              stream is closed.
138

RETURN VALUE

140       On success fopencookie() returns a pointer to the new stream.   On  er‐
141       ror, NULL is returned.
142

ATTRIBUTES

144       For  an  explanation  of  the  terms  used  in  this  section,  see at‐
145       tributes(7).
146
147       ┌──────────────┬───────────────┬─────────┐
148Interface     Attribute     Value   
149       ├──────────────┼───────────────┼─────────┤
150fopencookie() │ Thread safety │ MT-Safe │
151       └──────────────┴───────────────┴─────────┘

CONFORMING TO

153       This function is a nonstandard GNU extension.
154

EXAMPLES

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
164           $ ./a.out 'hello world'
165           /he/
166           / w/
167           /d/
168           Reached end of file
169
170       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
175   Program source
176
177       #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
184       #define INIT_BUF_SIZE 4
185
186       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
193       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
199           /* Buffer too small? Keep doubling size until big enough */
200
201           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
211           memcpy(cookie->buf + cookie->offset, buf, size);
212
213           cookie->offset += size;
214           if (cookie->offset > cookie->endpos)
215               cookie->endpos = cookie->offset;
216
217           return size;
218       }
219
220       ssize_t
221       memfile_read(void *c, char *buf, size_t size)
222       {
223           ssize_t xbytes;
224           struct memfile_cookie *cookie = c;
225
226           /* Fetch minimum of bytes requested and bytes available */
227
228           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
234           memcpy(buf, cookie->buf + cookie->offset, xbytes);
235
236           cookie->offset += xbytes;
237           return xbytes;
238       }
239
240       int
241       memfile_seek(void *c, off64_t *offset, int whence)
242       {
243           off64_t new_offset;
244           struct memfile_cookie *cookie = c;
245
246           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
255           if (new_offset < 0)
256               return -1;
257
258           cookie->offset = new_offset;
259           *offset = new_offset;
260           return 0;
261       }
262
263       int
264       memfile_close(void *c)
265       {
266           struct memfile_cookie *cookie = c;
267
268           free(cookie->buf);
269           cookie->allocated = 0;
270           cookie->buf = NULL;
271
272           return 0;
273       }
274
275       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
289           /* Set up the cookie before calling fopencookie() */
290
291           mycookie.buf = malloc(INIT_BUF_SIZE);
292           if (mycookie.buf == NULL) {
293               perror("malloc");
294               exit(EXIT_FAILURE);
295           }
296
297           mycookie.allocated = INIT_BUF_SIZE;
298           mycookie.offset = 0;
299           mycookie.endpos = 0;
300
301           stream = fopencookie(&mycookie,"w+", memfile_func);
302           if (stream == NULL) {
303               perror("fopencookie");
304               exit(EXIT_FAILURE);
305           }
306
307           /* Write command-line arguments to our file */
308
309           for (int j = 1; j < argc; j++)
310               if (fputs(argv[j], stream) == EOF) {
311                   perror("fputs");
312                   exit(EXIT_FAILURE);
313               }
314
315           /* Read two bytes out of every five, until EOF */
316
317           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
332               printf("/%.*s/\n", (int) nread, buf);
333           }
334
335           exit(EXIT_SUCCESS);
336       }
337

SEE ALSO

339       fclose(3), fmemopen(3), fopen(3), fseek(3)
340

COLOPHON

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/.
346
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349Linux                             2020-11-01                    FOPENCOOKIE(3)
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