NAME

     funopen, fropen, fwopen — open a stream

LIBRARY

     Utility functions from BSD systems (libbsd, -lbsd)

SYNOPSIS

     #include <stdio.h>
     (See libbsd(7) for include usage.)

     FILE *
     funopen(const void *cookie, int (*readfn)(void *, char *, int),
         int (*writefn)(void *, const char *, int), off_t (*seekfn)(void *, off_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 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
     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 specified.

RETURN VALUES

     Upon successful completion, funopen() returns a FILE pointer.  Otherwise, NULL is returned
     and the global variable errno is set to indicate 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, glibc- and musl-based
     ones (as the libbsd implementation is only provided when the system has fopencookie()
     available).

     On FreeBSD, OpenBSD and DragonFly the funopen() interface erroneously assumes that fpos_t is
     an integral type, and uses it in the seekfn hook; but because code using a seekfn hook will
     fail to build on systems where fpos_t is a struct, and it will need to be slightly fixed
     anyway, the implementation provided by libbsd (in the same way as NetBSD) uses the correct
     off_t types.