I've been puzzled by the interaction between GNU APL and aplwrap, in which ⎕ and ⍞ output doesn't appear until APL prompts for input. This behavior is bad for the case where the program tries to emit periodic progress messages during long computations.
It turns out that the C++ std lib is responsible for this behavior. While cerr is unbuffered, cout holds onto buffered content for as long as possible. (Apparently it only does this when stdout is not a terminal, as running the same test in a shell window does not exhibit the aggressive buffering behavior.)
I learned that cout's buffering behavior can be be changed using the call:
std::cout.setf(std::ios::unitbuf);
This can be called sometime before running the main APL loop, and will cause cout's buffer to be flushed at each <<, the same as cerr.
The attached patch (GNU APL SVN 441) runs the 'unitbuf' code only in the case that stdin is not a tty.
--
"The secret to creativity is knowing how to hide your sources."
Albert Einstein
lamkins_cout_pipe_unbuffer.patch
outtest.apl