funopen(3)

NAME

funopen, fropen, fwopen - open a stream

LIBRARY

Standard C Library (libc, -lc)

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(void *cookie, int (*readfn)(void *, char *, int));
FILE *
fwopen(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 can
be given as an appropriately-typed NULL pointer. 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 single exception that they are passed the cookie argu

ment specified
to funopen() in place of the traditional file descriptor ar

gument.

Read and write I/O functions are allowed to change the un

derlying 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 #include <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 point

er. Otherwise,
NULL is returned and the global variable errno is set to in

dicate 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), setbuf(3)

HISTORY

The funopen() functions first appeared in 4.4BSD.

BUGS

The funopen() function may not be portable to systems other

than BSD.

The funopen() interface erroneously assumes that fpos_t is

an integral
type; see fseek(3) for a discussion of this issue.

BSD March 19, 2004

BSD