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

       FILE *fopencookie(void *cookie, const char *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 follow-
       ing modes are supported: r, w, a, r+, w+, and  a+.   See  fopen(3)  for

       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;

              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 dis-

       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:

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

              SEEK_CUR  *offset should be added to the current stream offset.

              SEEK_END  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);

       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 pro-
       gram 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;

           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;
           FILE *fp;
           struct memfile_cookie mycookie;
           ssize_t nread;
           long p;
           int j;
           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;

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

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

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

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

           for (p = 0; ; p += 5) {
               if (fseek(fp, p, SEEK_SET) == -1) {
               nread = fread(buf, 1, 2, fp);
               if (nread == -1) {
               if (nread == 0) {
                   printf("Reached end of file\n");

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

Linux                             2013-03-17                    FOPENCOOKIE(3)
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