From:	Hans-Bernhard Broeker <broeker AT acp3bf DOT physik DOT rwth-aachen DOT de>
Newsgroups:	comp.os.msdos.djgpp
Subject:	Re: How Do I catch a "printf interrupt" ?
Date:	27 Jan 2000 14:09:04 GMT
Organization:	Aachen University of Technology (RWTH)
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arcadepreserv AT hotmail DOT com wrote:
> I think it is possible to trap a interrupt that accurs when a call to
> printf is made, and then read the string a pointer is pointing to.

You cannot trap that particular interrupt, just like so, because it is
not an interrup of its own. 

To begin with, printf() is not implemented by any interrupt at
all. It's a library function built into the program's code. At the
very heart of its operation, there will of course be a DOS interrupt
call (the 'write something to file' one), but even that you cannot
easily trap from the parent process --- it occurs in the *child*, so
if at all, the child would have to trap it. But that'd mean you'ld no
longer be building a frontend for the other code, but instead modify
that existing code iteself. Probably not what you wanted.

If it's really plain printf() output you want to grab, you can spare
yourself the work and use DJGPP's library function 'popen()', instead
of 'system()' or similar, to call that child. It can automatically
catch all its standard output (asynchronously, i.e. you'll get it
after child process termination) and present it to you as a pseudo
file.

If that other program is a more 'typical' DOS full-screen application,
though, the whole method won't work at all: all of them usually use
direct screen writes, which you cannot catch at all, unless your
frontend program 'owns' the whole system, like Windows does for its
DOS boxes. Technically, you have to be at protection 'ring 0', to do
that, and it's quite hard to get done, even then.
-- 
Hans-Bernhard Broeker (broeker AT physik DOT rwth-aachen DOT de)
Even if all the snow were burnt, ashes would remain.

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