FUNOPEN(3) BSD Library Functions Manual FUNOPEN(3)NAME
funopen, fropen, fwopen — open a stream
Standard C Library (libc, -lc)
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 *));
fropen(void *cookie, int (*readfn)(void *, char *, int));
fwopen(void *cookie, int (*writefn)(void *, const 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 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 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
All user I/O functions can report an error by returning -1. Addition‐
ally, all of the functions should set the external variable errno appro‐
priately 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
Upon successful completion, funopen() returns a FILE pointer. Otherwise,
NULL is returned and the global variable errno is set to indicate the
[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 speci‐
fied for the routine malloc(3).
SEE ALSOfcntl(2), open(2), fclose(3), fopen(3), fseek(3), setbuf(3)HISTORY
The funopen() functions first appeared in 4.4BSD.
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