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); except that they are passed the
cookie argument specified to
funopen() in place of the traditional file descriptor argument, and
seekfn uses
fpos_t instead of
off_t.
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.