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From: trier@cwlim.CWRU.EDU (Stephen C. Trier)
Newsgroups: comp.sys.ibm.pc.programmer,alt.msdos.programmer
Subject: Re: Redirecting to/from spawnxx()ed program?
Summary: Step-by-step instructions
Keywords: redirect stdin/stdout
Message-ID: <1990Jul7.040141.23830@usenet.ins.cwru.edu>
Date: 7 Jul 90 04:01:41 GMT
References: <49978@iuvax.cs.indiana.edu> <1990Jul7.035730.23379@usenet.ins.cwru.edu>
Sender: news@usenet.ins.cwru.edu
Reply-To: sct%seldon@scl.cwru.edu
Followup-To: alt.msdos.programmer
Organization: Case Western Reserve Univ. Cleveland, Ohio, (USA)
Lines: 77

To understand what the sample program someone has already posted does,
examine the MS-DOS dup(), dup2(), and spawn() calls.

The key to making redirection work is that the spawn() call passes the
currently open stdin, stdout, and stderr handles to its child.  Therefore,
all that a redirection must do is to change those three handles to point
to the proper alternate devices, do the spawn, and change the handles
back.

The first step in doing this is to save the current values of the standard
handles somewhere safe.  This is where dup() comes in.  For example,

    #define STDIN	0
    old_stdin = dup(STDIN);

This code creates a new handle with exactly the same attributes as the
current stdin and stores its number in the variable old_stdin.  Note that
since we're dealing with low-level I/O instead of with C's high-level
functions, all of these calls deal with numerical handles instead of FILE *'s.

    new_stdin = open(filename, O_RDONLY);

This line is straightforward enoguh; note that I'm using open, not C's
freopen, since freopen changes the C stdin file pointer.  It is _not_
guaranteed to change the underlying MS-DOS stdin handle.

    dup2(new_stdin, STDIN);

This is the critical line, the one that actually does the redirection.
It forces handle number STDIN to point to the same file as new_stdin.

    spawnlp("program", ...);

Use whatever version of spawn is needed; they all treat redirection in
the same manner.

    dup2(old_stdin, STDIN);

This dup2 line "undoes" the redirection, returning the MS-DOS stdin handle
to whatever it originally was.

    close(new_stdin);

Don't forget to close the redirected file; otherwise you'll quickly face
the MS-DOS 20-file limit!  :-)

And that's all it takes to do input redirection.  If you want to add output
or error redirection, just add a line with "stdout" or "stderr" for every
line of "stdin" above.  The description seems long, but it's only 7 lines
of C.

There's also a library function to do some I/O redirection, borrowed from
Unix.  It's called popen(), for "pipe open".  popen() allows you to use
the C I/O library (fgets, fputs, and so on) to read or write to a pipe to
a child program.  I found two implementations of popen() in the
comp.sources.misc archives.  I also wrote a simplified version for smail/PC.
I'd be happy to mail copies of the functions; just ask.

#include <stdio.h>

main()
{
    FILE *fp;

    fp = popen("smail sct", "w");
    fprintf(fp, "Hello, world!\n");
    pclose(fp);
}

And presto!  The first program in K&R moves into the e-mail era.

-- 
Stephen Trier                              Case Western Reserve University
Home: sct%seldon@scl.cwru.edu              Information Network Services
Work: trier@cwlim.ins.cwru.edu
                 I do _not_ speak for the University.