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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 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;

       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
	      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.

       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;
	       } 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 3.65 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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