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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.


     For  an  explanation  of  the  terms  used   in   this   section,   see
     |Interface     | Attribute     | Value   |
     |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 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' /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;
             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;
         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 *stream;
         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;
         stream = fopencookie(&mycookie,"w+", memfile_func);
         if (stream == NULL) {
         /* Write command-line arguments to our file */
         for (j = 1; j < argc; j++)
             if (fputs(argv[j], stream) == EOF) {
         /* Read two bytes out of every five, until EOF */
         for (p = 0; ; p += 5) {
             if (fseek(stream, p, SEEK_SET) == -1) {
             nread = fread(buf, 1, 2, stream);
             if (nread == -1) {
             if (nread == 0) {
                 printf("Reached end of file\n");
             printf("/%.*s/\n", nread, buf);
         exit(EXIT_SUCCESS); }


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


     This  page  is  part of release 4.16 of the Linux man-pages project.  A
     description of the project, information about reporting bugs,  and  the
     latest     version     of     this    page,    can    be    found    at

Linux 2017-09-15 FOPENCOOKIE(3)

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