ttyflush/0

Module: sio

ttyflush/0 — forces all buffered output to the screen

FORMS

ttyflush

DESCRIPTION

ttyflush/0 is used to make sure that output from tty I/O predicates appears on the screen when you want it to. Screen output is normally flushed whenever Prolog is waiting on some I/O operation. Typically, the I/O operation that is waited on is some form of tty read. In this case, you don’t have to use ttyflush.

A long computation, while causing you to wait for a response, does not qualify as an I/O operation, so any output predicates that were run before the CPU hog started, might not show their output on the screen until the long computation is finished. To combat this problem, ttyflush/0 can be used before entering into the long computation section of your program.

EXAMPLES

If we define the following useless predicate:

infiniteLoop :- infiniteLoop.

and then try to write something to the screen before running it:

?- printf("AprilFool'sDay"), infiniteLoop.
AprilFool's Day 
BreakHandler
------------------------
a-Abort Computation
b-Break shell
c-Continue
d-Debug
e-Exit Prolog
f-Fail
p-Return to Previous Break Level
s-Show goal broken at
t-Stack trace
?- This message
Break(1)> a
Warning: Aborting from Control-C or Control-Break.
Error: Execution aborted.

we find that the output doesn’t appear until the Control-C is pressed. We can avoid this problem by putting a call to ttyflush/0 after the message printing predicate.

The following example shows the result:

?- printf("AprilFool'sDay"), ttyflush, infiniteLoop.
AprilFool's Day
BreakHandler
------------------------
a-Abort Computation
b-Break shell
c-Continue
d-Debug
e-Exit Prolog
f-Fail
p-Return to Previous Break Level
s-Show goal broken at
t-Stack trace
?- This message
Break(1)> a
Warning: Aborting from Control-C or Control-Break.
Error: Execution aborted.

NOTES

The ISO Standard mandates that flush_output/[0,1] is preferred over ttyflush/0.