funopen, fropen, fwopen - open a stream

#include <stdio.h>

FILE *funopen(cookie, readfn, writefn, seekfn, closefn)
void *cookie;
int (*readfn)(/* void *, char *,
int (*writefn)(/* void *, const char
fpos_t (*seekfn)(/* void *, fpos_t, int
int (*closefn)(/* void * */);

FILE *fropen(cookie, readfn)
void *cookie
int (*readfn)(/* void *, char *, int */)

FILE *fwopen(cookie, writefn)
void *cookie
int (*writefn)(/* void *, char *, int */)

The funopen function associates a stream with up to four "I/O
functions". Either readfn or writefn must be specified; the
others can be given as an appropriately-typed NULL pointer. These
I/O functions will be used to read, write, seek and close the new

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), seek(2), and
close(2) with the single exception that they are passed the cookie
argument specified to funopen in place of the traditional file
descriptor 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.

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

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).

fcntl(2), open(2), fclose(3), fopen(3), fseek(3), setbuf(3)

These functions are currently under development.

The funopen function may not be portable to systems other than BSD