The
funopen();
function associates a stream with up to four
“I/O functions”.
Either
readfn
or
writefn
must be specified;
the others may be given as
NULL
pointers.
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 exceptions that they are passed the
cookie
argument specified to
funopen();
in place of the traditional file descriptor argument and that
the seek function takes an
fpos_t
argument and not an
off_t
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).