How to fork a program and tell when it's done?

tonyfreeman · 08-29-2004, 10:55 AM

I have a backup command that I need to run from a program that I'm writing using GTK/Glib. The problem is that I'm using the system() call ... which causes my GUI to feeze up until the backup command is finished.

How can I properly fork this command and have my gtk_progress_bar just run back and forth until the backup command is finished?

bruce ford · 08-29-2004, 11:20 AM

Hi,

use fork(), then, in the child use execve/execp to run your program and set up a signal handler for SIGCHLD in the parent.
Very short answer, but that's it - consult the man pages of the respective command for details.

Good luck
bruce

tonyfreeman · 08-29-2004, 02:33 PM

Thanks for the note ...

I was using fork with a command inside of it. Ex: fork("find /home/tony"). Here's the code I came up with today:

Code:

pid = fork();
if (pid == -1) {
     g_error("Forking Failed");
     exit(-1);
} else if (pid == 0) {
     execlp("find", "find", "/home/tony/", NULL);
     /* I guess there's something wrong if I get this far? */
     g_error("I'm past execlp");
     exit(-1);
}

I need to know two things: How can I tell when the example find command is finished and how do I make a gtk progress bar start when the find command starts and stop when the find command stops?

Any help would be great :-)

--Tony

bruce ford · 08-30-2004, 02:18 AM

Hi,

I'm not into GTK programming but I think your program has some sort of main loop.
Then you could poll for process termination as well as setting up a signal handler, e.g. like this

Code:

// your main loop
while( true ) {
...
// begin your posted code
pid = fork();
if (pid == -1) {
     g_error("Forking Failed");
     exit(-1);
} else if (pid == 0) {
     execlp("find", "find", "/home/tony/", NULL);
     /* I guess there's something wrong if I get this far? */
     g_error("I'm past execlp");
     exit(-1);
}
// end you posted code

// your parent process continues here, so start you progress bar at this point

// then poll for process termination:
int child_stat = 0;
int process_status = waitpid((pid_t)pid, &child_stat, WNOHANG);
if( process_status == 0 ) {
  // process is still running
}
else if ( process_status == pid ) { 
  // process exited with exit status WEXITSTATUS( child_stat )
  // stop your progress bar here
}
else {
  // error, examine errno to tell what happened
}
...
// end of your main loop
}

You may want to add some if() conditions which I left out

Hope it helps,
bruce

bruce ford · 08-30-2004, 02:30 AM

BTW, if you're interested in the output that your find process generates it is more
convenient to use popen() instead of fork().

so long...
bruce

tonyfreeman · 09-01-2004, 10:25 PM

Thanks for the great reply! With your help I've finally worked through the answer to my original question: How to fork a program and tell when it's done. Here's the working code that I'm using.

Code:

#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <errno.h>
#include <sys/types.h>
#include <sys/wait.h>
#include <glib.h>

int main ()
{
	pid_t pid;
	gboolean test = TRUE;
	gint child_stat = 0;
        gint process_status;

	g_print("\nSTART OF MAIN PROGRAM\n");

	pid = fork();

	if (pid == -1) {
		g_error("Forking Failed");
		exit(1);
		return EXIT_FAILURE;
	} else if (pid == 0) {
		// start progress bar and run the command
		g_print("Start progress bar\n");
		if (execlp("find", "find", "/home/tony/glib/docs", NULL) < 0) {
			g_error("*** Something wrong with the find command *** \n");
			exit(1);
		}
	} else {
		while (test) {
			process_status= waitpid((pid_t)pid, &child_stat, WNOHANG);
			if (process_status == 0) {
				// process is still running
				// increment the progress bar after doing some sort of timeout thingy
				test = TRUE;
			} else if (process_status == pid) {
				// stop progress bar
				g_print("Process stopped so stop progress bar\n");
				test = FALSE;
			} else {
				g_error("Some sort of error that needs to be examined");
				test = FALSE;
				return EXIT_FAILURE;
			}
		}
	}
	g_print("\nEND OF MAIN PROGRAM\n");
	return EXIT_SUCCESS;
}

Now I need to figure out how to make the progress bar run back and forth.

--Tony