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Linux 2.6 - man page for fopencookie (linux section 3)

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 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 pack-
	  age  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 sup-
       ported: 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

       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 discarded.

       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 fol-
	      lowing three values is supplied in whence:

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

	      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

	      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 fopen-

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

	      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.

       This function is a nonstandard GNU extension.

       The program below implements a custom stream whose functionality is similar (but not iden-
       tical) 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 *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);


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

       This  page  is  part of release 3.55 of the Linux man-pages project.  A description of the
       project,    and	  information	 about	  reporting    bugs,	can    be    found     at

Linux					    2013-03-17				   FOPENCOOKIE(3)

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