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

       fopencookie - opening a custom stream

       #define _GNU_SOURCE         /* See feature_test_macros(7) */
       #include <stdio.h>

       FILE *fopencookie(void *restrict cookie, const char *restrict mode,
                         cookie_io_functions_t io_funcs);

       The fopencookie() function allows the programmer to create a custom
       implementation for a standard I/O stream.  This implementation can store
       the stream's data at a location of its own choosing; for example,
       fopencookie() is used to implement fmemopen(3), which provides a stream
       interface to data that is stored in a buffer in memory.

       In order to create a custom stream the programmer must:

       *  Implement four "hook" functions that are used internally by the
          standard I/O library when performing I/O on the stream.

       *  Define a "cookie" data type, a structure that provides bookkeeping
          information (e.g., where to store data) used by the aforementioned
          hook functions.  The standard I/O package knows nothing about the
          contents of this cookie (thus it is typed as void * when passed to
          fopencookie()), but automatically supplies the cookie as the first
          argument when calling the hook functions.

       *  Call fopencookie() to open a new stream and associate the cookie and
          hook functions with that stream.

       The fopencookie() function serves a purpose similar to fopen(3): it opens
       a new stream and returns a pointer to a FILE object that is used to
       operate on that stream.

       The cookie argument is a pointer to the caller's cookie structure that is
       to be associated with the new stream.  This pointer is supplied as the
       first argument when the standard I/O library invokes any of the hook
       functions described below.

       The mode argument serves the same purpose as for fopen(3).  The following
       modes are supported: r, w, a, r+, w+, and a+.  See fopen(3) for details.

       The io_funcs argument is a structure that contains four fields pointing
       to the programmer-defined hook functions that are used to implement this
       stream.  The structure is defined as follows

           typedef struct {
               cookie_read_function_t  *read;
               cookie_write_function_t *write;
               cookie_seek_function_t  *seek;
               cookie_close_function_t *close;
           } cookie_io_functions_t;

       The four fields are as follows:

       cookie_read_function_t *read
              This function implements read operations for the stream.  When
              called, it receives three arguments:

                  ssize_t read(void *cookie, char *buf, size_t size);

              The buf and size arguments are, respectively, a buffer into which
              input data can be placed and the size of that buffer.  As its
              function result, the read function should return the number of
              bytes copied into buf, 0 on end of file, or -1 on error.  The read
              function should update the stream offset appropriately.

              If *read is a null pointer, then reads from the custom stream
              always return end of file.

       cookie_write_function_t *write
              This function implements write operations for the stream.  When
              called, it receives three arguments:

                  ssize_t write(void *cookie, const char *buf, size_t size);

              The buf and size arguments are, respectively, a buffer of data to
              be output to the stream and the size of that buffer.  As its
              function result, the write function should return the number of
              bytes copied from buf, or 0 on error.  (The function must not
              return a negative value.)  The write function should update the
              stream offset appropriately.

              If *write is a null pointer, then output to the stream is

       cookie_seek_function_t *seek
              This function implements seek operations on the stream.  When
              called, it receives three arguments:

                  int seek(void *cookie, off64_t *offset, int whence);

              The *offset argument specifies the new file offset depending on
              which of the following three values is supplied in whence:

                     The stream offset should be set *offset bytes from the
                     start of the stream.

                     *offset should be added to the current stream offset.

                     The stream offset should be set to the size of the stream
                     plus *offset.

              Before returning, the seek function should update *offset to
              indicate the new stream offset.

              As its function result, the seek function should return 0 on
              success, and -1 on error.

              If *seek is a null pointer, then it is not possible to perform
              seek operations on the stream.

       cookie_close_function_t *close
              This function closes the stream.  The hook function can do things
              such as freeing buffers allocated for the stream.  When called, it
              receives one argument:

                  int close(void *cookie);

              The cookie argument is the cookie that the programmer supplied
              when calling fopencookie().

              As its function result, the close function should return 0 on
              success, and EOF on error.

              If *close is NULL, then no special action is performed when the
              stream is closed.

       On success fopencookie() returns a pointer to the new stream.  On error,
       NULL is returned.

       For an explanation of the terms used in this section, see attributes(7).

       │Interface                                     Attribute     Value   │
       │fopencookie()                                 │ Thread safety │ MT-Safe │

       This function is a nonstandard GNU extension.

       The program below implements a custom stream whose functionality is
       similar (but not identical) to that available via fmemopen(3).  It
       implements a stream whose data is stored in a memory buffer.  The program
       writes its command-line arguments to the stream, and then seeks through
       the stream reading two out of every five characters and writing them to
       standard output.  The following shell session demonstrates the use of the

           $ ./a.out 'hello world'
           / w/
           Reached end of file

       Note that a more general version of the program below could be improved
       to more robustly handle various error situations (e.g., opening a stream
       with a cookie that already has an open stream; closing a stream that has
       already been closed).

   Program source

       #define _GNU_SOURCE
       #include <sys/types.h>
       #include <stdio.h>
       #include <stdlib.h>
       #include <unistd.h>
       #include <string.h>

       #define INIT_BUF_SIZE 4

       struct memfile_cookie {
           char   *buf;        /* Dynamically sized buffer for data */
           size_t  allocated;  /* Size of buf */
           size_t  endpos;     /* Number of characters in buf */
           off_t   offset;     /* Current file offset in buf */

       memfile_write(void *c, const char *buf, size_t size)
           char *new_buff;
           struct memfile_cookie *cookie = c;

           /* Buffer too small? Keep doubling size until big enough. */

           while (size + cookie->offset > cookie->allocated) {
               new_buff = realloc(cookie->buf, cookie->allocated * 2);
               if (new_buff == NULL) {
                   return -1;
               } else {
                   cookie->allocated *= 2;
                   cookie->buf = new_buff;

           memcpy(cookie->buf + cookie->offset, buf, size);

           cookie->offset += size;
           if (cookie->offset > cookie->endpos)
               cookie->endpos = cookie->offset;

           return size;

       memfile_read(void *c, char *buf, size_t size)
           ssize_t xbytes;
           struct memfile_cookie *cookie = c;

           /* Fetch minimum of bytes requested and bytes available. */

           xbytes = size;
           if (cookie->offset + size > cookie->endpos)
               xbytes = cookie->endpos - cookie->offset;
           if (xbytes < 0)     /* offset may be past endpos */
              xbytes = 0;

           memcpy(buf, cookie->buf + cookie->offset, xbytes);

           cookie->offset += xbytes;
           return xbytes;

       memfile_seek(void *c, off64_t *offset, int whence)
           off64_t new_offset;
           struct memfile_cookie *cookie = c;

           if (whence == SEEK_SET)
               new_offset = *offset;
           else if (whence == SEEK_END)
               new_offset = cookie->endpos + *offset;
           else if (whence == SEEK_CUR)
               new_offset = cookie->offset + *offset;
               return -1;

           if (new_offset < 0)
               return -1;

           cookie->offset = new_offset;
           *offset = new_offset;
           return 0;

       memfile_close(void *c)
           struct memfile_cookie *cookie = c;

           cookie->allocated = 0;
           cookie->buf = NULL;

           return 0;

       main(int argc, char *argv[])
           cookie_io_functions_t  memfile_func = {
               .read  = memfile_read,
               .write = memfile_write,
               .seek  = memfile_seek,
               .close = memfile_close
           FILE *stream;
           struct memfile_cookie mycookie;
           size_t nread;
           char buf[1000];

           /* Set up the cookie before calling fopencookie(). */

           mycookie.buf = malloc(INIT_BUF_SIZE);
           if (mycookie.buf == NULL) {

           mycookie.allocated = INIT_BUF_SIZE;
           mycookie.offset = 0;
           mycookie.endpos = 0;

           stream = fopencookie(&mycookie, "w+", memfile_func);
           if (stream == NULL) {

           /* Write command-line arguments to our file. */

           for (int j = 1; j < argc; j++)
               if (fputs(argv[j], stream) == EOF) {

           /* Read two bytes out of every five, until EOF. */

           for (long p = 0; ; p += 5) {
               if (fseek(stream, p, SEEK_SET) == -1) {
               nread = fread(buf, 1, 2, stream);
               if (nread == 0) {
                   if (ferror(stream) != 0) {
                       fprintf(stderr, "fread failed\n");
                   printf("Reached end of file\n");

               printf("/%.*s/\n", (int) nread, buf);


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

       This page is part of release 5.13 of the Linux man-pages project.  A
       description of the project, information about reporting bugs, and the
       latest version of this page, can be found at

Linux                              2021-03-22                     FOPENCOOKIE(3)