However, while the old Prolog I/O system is based on the C standard I/O library, the new Prolog I/O system is not. If an application performs mixed I/O operation in Prolog and foreign code on the three default Prolog streams, it might not work appropriately under the new I/O due to incompatibilities between the buffering mechanism in the C standard I/O stream and the Quintus Prolog stream.
Let's look at an example of a mixed output operation
on a Prolog session under UNIX. Both the C standard output stream
and the Prolog user_output stream write output to the same file
descriptor, 1, which is a tty.
| ?- set_output(user_output), write(first), c_printf('FIRST'),
write(second), c_printf('SECOND'), nl, c_nl.
The predicate c_printf/1 and c_nl/0 calls the following C functions:
void c_printf(atom)
char *atom;
{ printf("%s", atom); }
void c_nl() { putchar('\n'); }
The query yields the following output as expected prior to Quintus Prolog release 3.
firstFIRSTsecondSECOND
However it yields the following output on Quintus Prolog release 3 since each stream has its own buffer and no characters are actually written to the file descriptor 1 until new line operation is called.
firstsecond
FIRSTSECOND
This problem can be solved by supplying a different embedding QU_initio()
function at the time of the installation of Quintus Prolog (or at the time
of creating a statically linked Prolog system)
to create the three default streams based on C standard I/O streams.
How to create an unbuffered Prolog stream based on a C standard
I/O stream has already been shown
in the third example of creating customized Prolog streams
(see fli-ios-uds-sst)