FOPENCOOKIE(3)

NAME
fopencookie - open a stream

LIBRARY
libc

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, off64_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.