FUNOPEN(3) BSD Programmer's Manual FUNOPEN(3)
funopen, fropen, fwopen - open a stream
funopen(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 *, 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 as-
sociated operation on the resulting stream will fail. If the close func-
tion is omitted, closing the stream will flush any buffered output and
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 re-
All user I/O functions can report an error by returning -1. Additional-
ly, all of the functions should set the external variable errno appropri-
ately 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 er-
[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
fcntl(2), open(2), fclose(3), fopen(3), fseek(3), setbuf(3)
The funopen() functions first appeared in 4.4BSD.
The funopen() function may not be portable to systems other than BSD.
4.4BSD June 9, 1993 2