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           #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;
                  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);


           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) {
           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;
               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);

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