Path: utzoo!utgpu!jarvis.csri.toronto.edu!rutgers!cmcl2!adm!xadmx!keith@fstohp.crd.ge.com
From: keith@fstohp.crd.ge.com (Keith D Gregory)
Newsgroups: comp.unix.wizards
Subject: Apparent bug in fclose()-exec()-freopen() sequence
Message-ID: <20032@adm.BRL.MIL>
Date: 16 Jun 89 18:03:57 GMT
Sender: news@adm.BRL.MIL
Lines: 70


In short, if one attempts to fclose() files, exec() another program,
and freopen() the same files, the freopen() appears to fail.  In the
specific case, we ran the following programs on an HP-9000 (HP-UX 6.2)
and a Mac-2 (A/UX 1.1 beta).  The second program failed on the second
call to fprintf() (the first completed), leaving beind a core dump.
The _iob for StdErr appeared to be in pretty bad shape (as I recall,
the character pointers were trashed).
 
As a guess, the startup code in P2 is initalizing its _iob array,
and the call to fclose() (as part of freopen()) is thinking that
the structure refers to a valid stream.

Is this what is happening?  Is there any reason (documented) that
we should not close a file before exec()ing a program?  The obvious
workaround (leave them open) is in place, but this application is
to run in the background (after a foreground launch), and I don't
particularly want any ill-behave programs to start sending data to
an unclosed StdFile.

Please mail any responses; you will need to explicitly use CRDGW1
as a gateway.

Thanks

   Keith D Gregory
   keith%fstohp@crdgw1.ge.com


# Program 1 Start, Cut here ###################################################
#include <stdio.h>

void main()
{
    fclose( stdin );
    fclose( stdout );
    fclose( stderr );

    execl( "p2", "p2", NULL );
}
# Program 2 Start, Cut here ###################################################
#include <stdio.h>

void main()
{
    if (freopen( "/dev/tty", "w", stderr) == NULL)
        exit( -1 );
    else
        fprintf( stderr, "Reopened StdErr, Flags = %d, FileNo = %d\n",
                         stderr->_flag, stderr->_file );

    fflush( stderr );

    if (freopen( "/etc/passwd", "r", stdin) == NULL)
        perror( "Unable to freopen StdIn" );
    else
        fprintf( stderr, "Reopened StdIn, Flags = %d, FileNo = %d\n",
                         stdin->_flag, stdin->_file );

    fflush( stderr );
    
    if (freopen( "/tmp/ix", "w", stdout) == NULL)
        perror( "Unable to freopen StdOut" );
    else
        fprintf( stderr, "Reopened StdOut, Flags = %d, FileNo = %d\n",
                         stdout->_flag, stdout->_file );

    fflush( stderr );
}
# Program 2 End, Cut here #####################################################