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 *cookie, const char *mode,
                         cookie_io_functions_t io_funcs);

       The fopencookie() function allows the programmer to create a custom im-
       plementation 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 de-

       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 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 in-
              dicate 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  er-
       ror, NULL is returned.

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

       |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 im-
       plements 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;
           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;

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

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

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

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

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

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


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

       This  page  is  part of release 5.05 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                             2019-03-06                    FOPENCOOKIE(3)
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