Re: Flushing the output buffer after writing to stdout?

On Sunday 16 April 2006 16:22, Stuart Brorson wrote:
> I'd like to talk to gnucap via a pipe opened from another process.  In
> particular, I am using the "popen2" facility from Python.  So
> far, my efforts in this direction have failed.  I suspect that one of
> my problems is that the IO buffers aren't being flushed
> after gnucap outputs a bunch of text to the stdout.

The buffers are automatically flushed on every \n, \r, \b.

> I have looked through the gnucap code to try to explicitly stick calls
> to flush at the critical places where IO occurs.  However, I am
> completely flummoxed by how gnucap does IO.  I have discovered the IO
> class as well as the OMSTREAM class, but can't figure out how these
> inherit from cout, which is the only thing I know about C++ IO.  I
> also don't know the difference between IO and OMSTREAM, or what they
> are trying to accomplish.

The use of OMSTREAM is sort of like any stream, but it is a multiple stream.  
Think of it as a set of streams.  Writing once writes to all of them.

This part of the code was written many years ago, before C++ I/O was stable, 
so it is built around the old C "FILE*".  Even now, it does some things that 
the usual C++ streams don't, such as floating point numbers with letters 
representing multipliers, better control of formatting, including support for 
devices and files with limited line length.  The files included in a single 
OMSTREAM are not necessarily formatted the same.  Still, writing once does 
them all.

Basically, you open a C style FILE*, then attach it.

If you look inside, you will see that the C++ streams do this too.

>
> Questions:
>
> 1.  Within gnucap, which statement actually writes something to the
> stdout?

Usually "fputc", but please don't use it directly.

> 2.  How do I explicitly tell stdout to flush?  Has this fcn been built
> into the relevant classes yet?

Writing \n, \r, or \b.  Doing it as part of a string has the desired effect.  
If you want to flush in the middle of a line, write a blank then backspace.

Since it uses the standard C I/O, which links stdin and stdout, reading from 
stdin flushes stdout.

> 3.  Ummm, can somebody please give me the 30 second overview of what
> IO and OMSTREAM classes are for, and how one should use them?

ALL output uses OMSTREAM.

The IO class should really be a namespace.  It is a class only because there 
was no such thing as namespace when I wrote that part.  It exists for the 
purpose of minimizing clutter in the global namespace.  Actually, it probably 
should be more like a regular class, a container for all IO stuff.  That way 
there could be more than one.