Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!iuvax!cica!ctrsol!ginosko!uunet!apctrc!zjmw0a
From: zjmw0a@apctrc.trc.amoco.com (Joe M. Wade)
Newsgroups: comp.windows.x
Subject: fork() and popen() in an X environment
Message-ID: <1015@apctrc.UUCP>
Date: 7 Sep 89 22:01:23 GMT
Reply-To: zjmw0a@apctrc.trc.amoco.com (Joe M. Wade)
Distribution: na
Organization: Amoco Production Company, Tulsa Research Center
Lines: 104

In an effort to separate functionality between programs, one of my 
colleagues (cohorts?) produced code using popen to start another
process which would use X to plot data created by the initial 
process. He then found the initial process had to be forked so
that the command widget which spawned the popen would return to
its sensitive state. Everything works hunky-dory except for one 
thing-the initial process dies when the quit button is pushed,
but the window doesn't go away until the spawned-off process is
also exited. Killing the window manager didn't help, although it 
seems as though that would probably be the source of the difficulty.
Below is a short piece of code demonstrating the problem. It produces
two buttons, fork and quit. If you hit fork, it does a popen of 
this same process. Try using the quit button of the original process
and its pid disappears, but the windows remain.

Pertinent info: Sun 3/60 OS 4.0.1, X11R3, twm

#include <stdio.h>
#include <math.h>
#include <X11/Xlib.h>
#include <X11/Intrinsic.h>
#include <X11/StringDefs.h>
#include <X11/Xatom.h>
#include <X11/Cardinals.h>
#include <X11/Box.h>
#include <X11/Command.h>
#include <X11/Shell.h>

void quit();
void fork_process();

main(argc, argv)
int argc;
char *argv[];
    {
    extern void time_out();

    Widget toplevel, outerform, quitter, forker;
    int screen;
    Display *display;
    Arg arg[5];
    int fork_value;

    /*
     * fork this process 
     */
    if ( (fork_value = fork()) != 0 )
        exit(0);

    toplevel = XtInitialize(NULL, "XLabel", NULL, 0, &argc, argv);
    display = XtDisplay(toplevel);
    screen = XDefaultScreen(display);

    outerform = XtCreateManagedWidget("outerform", boxWidgetClass, toplevel,
                                   (Arg*) arg, NULL);

    XtSetArg(arg[0], XtNlabel, (String) "Quit");
    quitter = XtCreateManagedWidget("Quit", commandWidgetClass,
                                    outerform, (Arg*) arg, ONE);

    /*
     * Create a command widget to popen a new process
     */
    XtSetArg(arg[0], XtNlabel, (String) "Fork");
    forker = XtCreateManagedWidget("Fork", commandWidgetClass,
                                    outerform, (Arg*) arg, ONE);

    XtAddCallback(quitter, XtNcallback, quit, &toplevel);
    XtAddCallback(forker, XtNcallback, fork_process, NULL);

    XtRealizeWidget(toplevel);

    time_out();
    XtMainLoop();
    }

void fork_process()
    {
    FILE *fptr;
    /*
     * start a new process
     */
    fptr = popen( "fork", "r");
    pclose( fptr );
    return;
    }

void quit ( button, top_widget )
Widget button;
caddr_t *top_widget;
/* 
 * destroy the top level widget and exit 
 */
    {
    XtDestroyWidget( *((Widget *) top_widget) );
    exit (0);
    }

Any info/input would be greatly appreciated.

*  *  *  *  *  *  *  *  *  *  *  *  *  *  *  *  *  *  *  *  *  *
*  Joe M. Wade (zjmw0a@apctrc.trc.amoco.com) (918) 660-4387    *
*  Amoco Research Center * 4502 E. 41st St. * Tulsa, OK  74102 *
*  *  *  *  *  *  *  *  *  *  *  *  *  *  *  *  *  *  *  *  *  *