fopencookie(3) — Linux manual page


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

NAME         top

       fopencookie - opening a custom stream

SYNOPSIS         top

       #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);

DESCRIPTION         top

       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

              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

              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

                     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.

RETURN VALUE         top

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

ATTRIBUTES         top

       For an explanation of the terms used in this section, see

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

CONFORMING TO         top

       This function is a nonstandard GNU extension.

EXAMPLES         top

       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 program:

           $ ./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);


SEE ALSO         top

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

COLOPHON         top

       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)

Pages that refer to this page: fmemopen(3)fopen(3)