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NetBSD 6.1.5 - man page for fwopen (netbsd section 3)

FUNOPEN(3)			   BSD Library Functions Manual 		       FUNOPEN(3)

     funopen, fropen, fwopen -- open a stream

     Standard C Library (libc, -lc)

     #include <stdio.h>

     FILE *
     funopen(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));

     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

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