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

NAME

       fopencookie - opening a custom stream

SYNOPSIS

       #define _GNU_SOURCE
       #include <stdio.h>

       FILE *fopencookie(void *cookie, const char *mode,
			 cookie_io_functions_t io_funcs);

DESCRIPTION

       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

	   struct cookie_io_functions_t {
	       cookie_read_function_t  *read;
	       cookie_write_function_t *write;
	       cookie_seek_function_t  *seek;
	       cookie_close_function_t *close;
	   };

       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 func-
	      tion result, the write function should return the number of bytes copied from buf, or -1 on error.  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 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.

RETURN VALUE

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

CONFORMING TO

       This function is a nonstandard GNU extension.

EXAMPLE

       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'
	   /he/
	   / w/
	   /d/
	   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 */
       };

       ssize_t
       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;
       }

       ssize_t
       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;
       }

       int
       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;
	   else
	       return -1;

	   if (new_offset < 0)
	       return -1;

	   cookie->offset = new_offset;
	   *offset = new_offset;
	   return 0;
       }

       int
       memfile_close(void *c)
       {
	   struct memfile_cookie *cookie = c;

	   free(cookie->buf);
	   cookie->allocated = 0;
	   cookie->buf = NULL;

	   return 0;
       }

       int
       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) {
	       perror("malloc");
	       exit(EXIT_FAILURE);
	   }

	   mycookie.allocated = INIT_BUF_SIZE;
	   mycookie.offset = 0;
	   mycookie.endpos = 0;

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

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

	   for (j = 1; j < argc; j++)
	       if (fputs(argv[j], fp) == EOF) {
		   perror("fputs");
		   exit(EXIT_FAILURE);
	       }

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

	   for (p = 0; ; p += 5) {
	       if (fseek(fp, p, SEEK_SET) == -1) {
		   perror("fseek");
		   exit(EXIT_FAILURE);
	       }
	       nread = fread(buf, 1, 2, fp);
	       if (nread == -1) {
		   perror("fread");
		   exit(EXIT_FAILURE);
	       }
	       if (nread == 0) {
		   printf("Reached end of file
");
		   break;
	       }

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

	   exit(EXIT_SUCCESS);
       }

SEE ALSO

       fclose(3), fmemopen(3), fopen(3), fseek(3), feature_test_macros(7)

COLOPHON

       This page is part of release 3.25 of the Linux man-pages project.  A description of the project, and information about reporting bugs,  can
       be found at http://www.kernel.org/doc/man-pages/.

Linux								    2008-12-05							    FOPENCOOKIE(3)