FUNOPEN(3) Library Routines FUNOPEN(3)
funopen, fropen, fwopen - open a stream
FILE *funopen (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 *, 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), 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 argu-
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. Addition-
ally, 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 spec-
GNO IMPLEMENTATION NOTE
Since the underlying (BSD) implementation makes assumptions about the
newline character, the returned stream is always in text (newline
translation) mode. If you need to have a stream open in binary mode,
you must call fsetbinary(3) on the returned file pointer; this state
information is not carried over from the previous file pointer.
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 specified for the routine
fcntl(2), open(2), fclose(3), fopen(3), fseek(3), fsetbinary(3), set-
The funopen functions first appeared in 4.4BSD.
The funopen function may not be portable to systems other than BSD.
GNO 28 April 1998 FUNOPEN(3)
Man(1) output converted with