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       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 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

           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
              following three values is supplied in whence:

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

                     *offset should be added to the current stream offset.

                     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

              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.


       For an explanation of the terms used in this section, see attributes(7).

       │InterfaceAttributeValue   │
       │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 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 *stream;
           struct memfile_cookie mycookie;
           size_t nread;
           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 (int j = 1; j < argc; j++)
               if (fputs(argv[j], stream) == EOF) {

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

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

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



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


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