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FOPENCOOKIE(3)        DragonFly Library Functions Manual        FOPENCOOKIE(3)

NAME
     fopencookie - open a stream

LIBRARY
     Standard C Library (libc, -lc)

SYNOPSIS
     #include <stdio.h>

     typedef ssize_t
     (*cookie_read_function_t)(void *cookie, char *buf, size_t size);

     typedef ssize_t
     (*cookie_write_function_t)(void *cookie, const char *buf, size_t size);

     typedef int
     (*cookie_seek_function_t)(void *cookie, off_t *offset, int whence);

     typedef int
     (*cookie_close_function_t)(void *cookie);

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

DESCRIPTION
     The fopencookie function associates a stream with up to four "I/O
     functions".  These I/O functions will be used to read, write, seek and
     close the new stream.

     In general, omitting a function means that any attempt to perform the
     associated operation on the resulting stream will fail.  If the write
     function is omitted, data written to the stream is discarded.  If the
     close function is omitted, closing the stream will flush any buffered
     output and then succeed.

     The calling conventions of read, write, and close must match those,
     respectively, of read(2), write(2), and close(2) with the single
     exception that they are passed the cookie argument specified to
     fopencookie in place of the traditional file descriptor argument.  The
     seek function updates the current stream offset using *offset and whence.
     If *offset is non-NULL, it updates *offset with the current stream
     offset.

     fopencookie is implemented as a thin shim around the funopen(3)
     interface.  Limitations, possibilities, and requirements of that
     interface apply to fopencookie.

RETURN VALUES
     Upon successful completion, fopencookie returns a FILE pointer.
     Otherwise, NULL is returned and the global variable errno is set to
     indicate the error.

ERRORS
     [EINVAL]           A bogus mode was provided to fopencookie.

     [ENOMEM]           The fopencookie function may fail and set errno for
                        any of the errors specified for the malloc(3) routine.

SEE ALSO
     fcntl(2), open(2), fclose(3), fopen(3), fseek(3), funopen(3)

HISTORY
     The funopen() functions first appeared in 4.4BSD.  The fopencookie
     function first appeared in FreeBSD 11.

BUGS
     The fopencookie function is a nonstandard glibc extension and may not be
     portable to systems other than FreeBSD and Linux.

DragonFly 6.5-DEVELOPMENT      February 4, 2023      DragonFly 6.5-DEVELOPMENT