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From: maart@cs.vu.nl (Maarten Litmaath)
Newsgroups: comp.unix.wizards
Subject: Re: sh/csh and wait()
Message-ID: <7127@star.cs.vu.nl>
Date: 17 Jul 90 02:04:59 GMT
References: <ANSELMO-ED.90Jul16174646@bigbird.cs.yale.edu>
Sender: news@cs.vu.nl
Reply-To: maart@cs.vu.nl (Maarten Litmaath)
Organization: VU Dept. of Computer Science, Amsterdam, The Netherlands
Lines: 66

In article <ANSELMO-ED.90Jul16174646@bigbird.cs.yale.edu>,
Anselmo-Ed@cs.yale.edu (Ed Anselmo) shows a program containing various bugs.
)...
)    for (i = 2;  i < argc;  i++)  {         /* Create chain of processes */
)	pipe (fd);
)	if ((pid = fork()) < 0)
)	    errorExit(-1);
)	else if (pid == 0) {                    /* Child process */
)	    close (fd[0]);                      /* stdout to parent */
)	    if (fd[1] != 1)  {
)		dup2 (fd[1], 1);
)		close (fd[1]);
)	    }

`fd[1]' must ALWAYS be closed, even if fd[1] == 1.

)	    execlp (argv[i-1], argv[i-1],0);    /* Overlay by (i-1)st filter */
)	    errorExit(-1);
)	}
)	else {                                  /* Parent process */
)	    table[i-1].pid = pid;               /* Save child pid */
)	    close (fd[1]);
)	    if (fd[0] != 0)  {                  /* stdin from child */
)		dup2 (fd[0], 0);
)		close (fd[0]);
)	    }

`fd[0]' must always be closed as well.

)	}
)    }
)
)    if ((pid = fork()) < 0)                 /* Create last process in chain */
)	errorExit(-1);
)    else if (pid == 0) {                        /* Child process */
)	execlp(argv[argc-1],argv[argc-1],0);    /* Overlay by last filter */
)	errorExit(-1);
)    }

Add (in parent):

     close(0);

)    table[argc-1].pid = pid;                /* Save child pid */
)
)    for (i = 1;  i < argc;  i++)  {         /* Wait for all children to die */
)	pid = wait(&status);
)	for (j = 1;  table[j].pid != pid;  j++)
)	    ;
)	table[j].status = status;

`pid' is NOT guaranteed to be in the table!  (Question: what could have
happened?)  So change the code to something like this:

	for (j = 1; j < argc && table[j].pid != pid; j++)
	    ;
	if (j == argc)
	    i--;	/* hack */
	else
	    table[j].status = status;

)    }
)...
--
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   be a new user! quick kill it before it multiplies!"      (Loren J. Miller)