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>Why does this strange 'x io error' occur when I

fork() in my GTK+ app? [GTK 2.x]

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NAME="AEN496"

>Why does this strange 'x io error' occur when I

CLASS="LITERAL"

>fork()

> in my GTK+ app? 

CLASS="EMPHASIS"

>[GTK 2.x]

>

>

>

>This is not really a GTK+ problem, and the problem is

not related to 

CLASS="LITERAL"

>fork()

> either. If the 'x io

error' occurs then you probably use the 

CLASS="LITERAL"

>exit()

> function

in order to exit from the child process.

>

>When GDK opens an X display, it creates a socket file

descriptor. When you use the 

CLASS="LITERAL"

>exit()

>

function, you implicitly close all the open file descriptors,

and the underlying X library really doesn't like this.

>

>The right function to use here is

CLASS="LITERAL"

>_exit()

>.

>

>Erik Mouw contributed the following code example to

illustrate handling fork() and exit().

>

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CLASS="PROGRAMLISTING"

>/*-------------------------------------------------------------------------

 * Filename:      gtk-fork.c

 * Version:       0.99.2

 * Copyright:     Copyright (C) 1999, Erik Mouw

 * Author:        Erik Mouw <J.A.K.Mouw@its.tudelft.nl>

 * Description:   GTK+ fork example

 * Created at:    Thu Sep 23 21:37:55 1999

 * Modified by:   Erik Mouw <J.A.K.Mouw@its.tudelft.nl>

 * Modified at:   Thu Sep 23 22:39:39 1999

 * Modified by:   Tony Gale <gale@gtk.org>

 * Modified at:   Wed Jan 14 12:38:00 2004

 *-----------------------------------------------------------------------*/

/*

 * Compile with:

 *

 * cc -o gtk-fork gtk-fork.c `pkg-config gtk+-2.0 --cflags --libs`

 *

 */

#include <stdio.h>

#include <stdlib.h>

#include <signal.h>

#include <sys/types.h>

#include <sys/wait.h>

#include <unistd.h>

#include <gtk/gtk.h>

void sigchld_handler(int num)

{

  sigset_t set, oldset;

  pid_t pid;

  int status, exitstatus;

  /* block other incoming SIGCHLD signals */

  sigemptyset(&set);

  sigaddset(&set, SIGCHLD);

  sigprocmask(SIG_BLOCK, &set, &oldset);

  /* wait for child */

  while((pid = waitpid((pid_t)-1, &status, WNOHANG)) > 0)

    {

      if(WIFEXITED(status))

	{

	  exitstatus = WEXITSTATUS(status);

	  fprintf(stderr, 

		  "Parent: child exited, pid = %d, exit status = %d\n", 

		  (int)pid, exitstatus);

	}

      else if(WIFSIGNALED(status))

	{

	  exitstatus = WTERMSIG(status);

	  fprintf(stderr,

		  "Parent: child terminated by signal %d, pid = %d\n",

		  exitstatus, (int)pid);

	}

      else if(WIFSTOPPED(status))

	{

	  exitstatus = WSTOPSIG(status);

	  fprintf(stderr,

		  "Parent: child stopped by signal %d, pid = %d\n",

		  exitstatus, (int)pid);

	}

      else

	{

	  fprintf(stderr,

		  "Parent: child exited magically, pid = %d\n",

		  (int)pid);

	}

    }

  /* re-install the signal handler (some systems need this) */

  signal(SIGCHLD, sigchld_handler);

  /* and unblock it */

  sigemptyset(&set);

  sigaddset(&set, SIGCHLD);

  sigprocmask(SIG_UNBLOCK, &set, &oldset);

}

gint delete_event(GtkWidget *widget, GdkEvent *event, gpointer data)

{

  return(FALSE);

}

void destroy(GtkWidget *widget, gpointer data)

{

  gtk_main_quit();

}

void fork_me(GtkWidget *widget, gpointer data)

{

  pid_t pid;

  pid = fork();

  if(pid == -1)

    {

      /* ouch, fork() failed */

      perror("fork");

      exit(-1);

    }

  else if(pid == 0)

    {

      /* child */

      fprintf(stderr, "Child: pid = %d\n", (int)getpid());

      execlp("ls", "ls", "-CF", "/", NULL);

      /* if exec() returns, there is something wrong */

      perror("execlp");

      /* exit child. note the use of _exit() instead of exit() */

      _exit(-1);

    }

  else

    {

      /* parent */

      fprintf(stderr, "Parent: forked a child with pid = %d\n", (int)pid);

    }

}

int main(int argc, char *argv[])

{

  GtkWidget *window;

  GtkWidget *button;

  gtk_init(&argc, &argv);

  /* the basic stuff: make a window and set callbacks for destroy and

   * delete events 

   */

  window = gtk_window_new(GTK_WINDOW_TOPLEVEL);

  g_signal_connect(G_OBJECT (window), "delete_event",

		     G_CALLBACK(delete_event), NULL);

  g_signal_connect(G_OBJECT (window), "destroy",

		     G_CALLBACK(destroy), NULL);

#if (GTK_MAJOR_VERSION == 1) && (GTK_MINOR_VERSION == 0)

  gtk_container_border_width(GTK_CONTAINER (window), 10);

#else  

  gtk_container_set_border_width(GTK_CONTAINER (window), 10);

#endif

  /* add a button to do something useful */

  button = gtk_button_new_with_label("Fork me!");

  g_signal_connect(G_OBJECT (button), "clicked",

		     G_CALLBACK(fork_me), NULL);

  gtk_container_add(GTK_CONTAINER(window), button);

  /* show everything */

  gtk_widget_show (button);

  gtk_widget_show (window);

  /* install a signal handler for SIGCHLD signals */

  signal(SIGCHLD, sigchld_handler);

  /* main loop */

  gtk_main ();

  exit(0);         

}

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