FUNOPEN(3)

NAME
funopen, fropen, fwopen - open a stream

SYNOPSIS
#include <stdio.h>
FILE *
funopen(const void *cookie, int (*readfn)(void *, char *, int), int (*writefn)(void *, const char *, int), fpos_t (*seekfn)(void *, fpos_t, int), int (*closefn)(void *));
FILE *
fropen(const void *cookie, int (*readfn)(void *, char *, int));
FILE *
fwopen(const void *cookie, int (*writefn)(void *, const char *, int));

DESCRIPTION
The funopen() function associates a stream with up to four I/O functions. Either readfn or writefn must be specified; the others may be given as NULL pointers. 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 close function is omitted, closing the stream will flush any buffered output and then succeed.

The calling conventions of readfn, writefn, seekfn, and closefn must match those, respectively, of read(2), write(2), lseek(2), and close(2) with the exceptions that they are passed the cookie argument specified to funopen() in place of the traditional file descriptor argument and that the seek function takes an fpos_t argument and not an off_t argument.

Read and write I/O functions are allowed to change the underlying buffer on fully buffered or line buffered streams by calling setvbuf(3). They are also not required to completely fill or empty the buffer. They are not, however, allowed to change streams from unbuffered to buffered or to change the state of the line buffering flag. They must also be prepared to have read or write calls occur on buffers other than the one most recently specified.

All user I/O functions can report an error by returning -1. Additionally, all of the functions should set the external variable errno appropriately if an error occurs.

An error on closefn() does not keep the stream open.

As a convenience, the include file <stdio.h> defines the macros fropen() and fwopen() as calls to funopen() with only a read or write function specified.

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

ERRORS

[EINVAL]
    The funopen() function was called without either a read or write function. The funopen() function may also fail and set errno for any of the errors specified for the routine malloc(3).

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

HISTORY
The funopen() functions first appeared in 4.4BSD.

BUGS
The funopen() function may not be portable to systems other than BSD.