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NAME

       fopencookie - opening a custom stream

SYNOPSIS

       #define _GNU_SOURCE         /* See feature_test_macros(7) */
       #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
              function 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\n");
                   break;
               }

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

           exit(EXIT_SUCCESS);
       }

SEE ALSO

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

COLOPHON

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       description of the project, and information about reporting  bugs,  can
       be found at http://man7.org/linux/man-pages/.