Setting process group id with a C code

zvivered · 10-18-2014, 11:27 AM

Dear Members,

The following code creates a child process and sets it process group id to the same process group id of its parent.

Then, every 1 sec, the child and the parent prints their process group id.

For some reason, on the first print, the process group id is as specified but from this point, the process group id changes to 1.

Can you help ?

Thank you in advance,
Z.V

Code:

#include <stdio.h>
#include <stdlib.h>
#include <string.h>

main()
{
	char str_id[16];
	int rc;
			
	if (fork ()==0)
	{
		//This is the child process
		rc=setpgid (getpid(),getppid());
		if (rc==-1)
			printf ("child: setpgid faild\n");
		
		strcpy (str_id,"child");
	}
	else
	{
		//This is the parent process
		rc=setpgid (getpid(),getpid());
		if (rc==-1)
			printf ("parent: setpgid faild\n");
		strcpy (str_id,"parent");
	}

	while (1)
	{
		printf ("from process:%s %d\n",str_id,getpgid());
		sleep (1);
	}
}

rknichols · 10-19-2014, 10:25 AM

getpgid() requires an argument. Since you did not supply one, it is picking up garbage from the stack. You should include "-Wall" in your C compiler arguments. Then you would see evidence that you are missing a header file.

Code:

#define _XOPEN_SOURCE 500
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>

int main()
{
.
.
.
	while (1)
	{
		printf ("from process:%s %d\n",str_id,getpgid(0));
		sleep (1);
	}
}

zvivered · 10-21-2014, 02:00 PM

Dear Member,

Thank for your help ! Now it works OK.
I can see now that both processes (parent and child) has the same process group id.

Using gdb I attached to the parent process.
I expected the child to stop also because it is in the same process group.

But only the parent stops.
Can you tell why ?

Thank you in advance,
Z.V