FUNOPEN(3) BSD Library Functions Manual FUNOPEN(3)
funopen, fropen, fwopen -- open a stream
Standard C Library (libc, -lc)
funopen(void *cookie, int (*readfn)(void *, char *, int),
int (*writefn)(void *, const char *, int), off_t (*seekfn)(void *, off_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, respec-
tively, of read(2), write(2), lseek(2), and close(2); except 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 func-
tions 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.
[EINVAL] The funopen() function was called without either a read or write func-
tion. 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)
The funopen() functions first appeared in 4.4BSD.
All three functions are specific to NetBSD and thus unportable.
BSD January 22, 2012 BSD