SIGNAL Suspended?

**Victor_November** · 11-07-2009

Hi,

First off I am doing homework. I have been looking at this site to view code examples, and I am happy to see the technical caliber on these forums. This is the first time I am programming with signals and I must say that I am "familiar" at best when coding with them.

My assignment is to sync a parent and a child using signals. I have got that to work (sort of). But the output is quite strange. Anyhow, if anyone can tell me if I am using signals right, and if anyone could explain why I am getting this crazy output, would be awesome.

Thanks!

code file w8.c:

Code:

#include <stdio.h>
#include <signal.h>
#include <unistd.h>
#include <sys/wait.h>

void mySignalHandler(int sig);
void sigParent(void);
void sigChild(void);
int wait_status;
pid_t parentPid, childPid, pid;

int main()
{
	int i;
	wait_status = 1; 
	pid  = fork();
	switch(pid)
		{
		case -1: /* an error occurred */
		printf("Fork error\n");
		break;

		case 0: /* this code is executed by the child process */
		childPid = getpid();
		signal(SIGCONT, mySignalHandler);
		signal(SIGSTOP, mySignalHandler);
			for(i=1; i<10; i++)
			{
				if(wait_status == 0)
				{
				waitpid(parentPid, NULL, WUNTRACED);
				}
				else
				{
				printf("Child process, iteration: %d\n", i);
				sigParent();
				}
			}
		break;

		default: /* this code is executed by the parent process */
		parentPid = getpid();
		signal(SIGCONT, mySignalHandler);
		signal(SIGSTOP, mySignalHandler);
			for(i=1; i<10; i++)
			{
				if(wait_status == 0)
				{
				waitpid(childPid, NULL, WUNTRACED);
				}
				else
				{
				printf("Parent process, iteration: %d\n", i);
				sigChild();
				}
			}/*end for*/
		break;
		
		} /*end switch*/
}/*end main*/

void mySignalHandler(int sig)
{
	if(sig == SIGSTOP)
		wait_status = 0;
	else if(sig == SIGCONT)
		wait_status = 1;

}

void sigParent(void)
{	
	kill(parentPid, SIGCONT);
	raise(SIGSTOP);
}
void sigChild(void)
{
	kill(childPid, SIGCONT);
	raise(SIGSTOP);
}

and then the output looks like this. Its not always the same either. But it does always say signal suspended:

Code:

Parent process, iteration: 1

Child process, iteration: 1
Parent process, iteration: 2
Child process, iteration: 2
Parent process, iteration: 3
Child process, iteration: 3
Parent process, iteration: 4
Child process, iteration: 4
Parent process, iteration: 5
Suspended (signal)
Child process, iteration: 5
Parent process, iteration: 6
Child process, iteration: 6
Parent process, iteration: 7
Child process, iteration: 7
Parent process, iteration: 8
Child process, iteration: 8
Parent process, iteration: 9
Child process, iteration: 9
[8]  + Suspended (signal)            ./a.out

**Dino** · 11-07-2009

The last time anyone used caliber to describe my skills was back in high school when I tried out for the Tennis team. After my try out, the coach came up to me, thanked me for trying out, and then said he didn't need anyone of my caliber on his team at the moment.

**MK27** · 11-07-2009

You should check the return value of waitpid. What happens with this is that as soon as waitpid() occurs in the child, it returns an error immediately because parentPid is not it's child. It does not wait for anything or perform any purpose.

And as soon as it occurs in the parent, the parent must wait until the child has completed it's 10 iterations and terminated.

You should be aware that unless you use this after your printf statements:

Code:

fflush(stdout);

your output may not reflect the real order of events.

**Victor_November** · 11-07-2009

I took out if logic and the wait statement from the child. I also added the flush functions after the printf's. The output still looks the same.

Did the use of the signals seem correct? They remind of something like a try-catch in Java.

**MK27** · 11-07-2009

Originally Posted by Victor_November

IThe output still looks the same.

I'm not sure what you are trying to accomplish. I get different output than what you report, so if you are going for a consistent effect, you are not getting it. Eg:

Code:

C 1 (0)
Child process, iteration: 1
C 2 (0)
C 3 (0)
C 4 (0)
C 5 (0)
C 6 (0)
C 7 (0)
C 8 (0)
C 9 (0)
P 1 (0)
Parent process, iteration: 1
P 2 (0)
P 3 (0)
P 4 (0)
P 5 (0)
P 6 (0)
P 7 (0)
P 8 (0)
P 9 (0)

The "P" & "C" lines I added in to track the iterations:

Code:

printf("P %d (%d)\n",i, x); fflush(stdout);

"x" was the return value from waitpid, but I removed that too.

As you can see, not much happens. parentPid remains UNDEFINED as it does in your previously posted code, so of course the parent never receives any signal, and by the time the parent loops, the child has finished and exited also so there is no signal for it.

The plain truth is there is absolutely no way you can rely on two seperate processes performing a simple for loop in a synchronized manner, as you may have presupposed, without 1) a specific time delay, or 2) some IPC such as signal handling. The code you posted does not really do the later, altho it may appear to in some sense.

Perhaps the output you posted is from some different code? Anyway, without an actual value for parentPid, you've got nothing substantial going on. This line:

Code:

parentPid = getpid();

is presumably in the wrong place. Forked children DO NOT share variables as with threading. If you print the value of "parentPid" in the child process, you will see it is meaningless, and different from it's value in the parent, where it is set AFTER the fork.

**Victor_November** · 11-07-2009

Originally Posted by MK27

I'm not sure what you are trying to accomplish. I get different output than what you report, so if you are going for a consistent effect, you are not getting it.

I am trying to fork a process then using two for loops have them alternate by printing a sequences of iterations

"x" was the return value from waitpid, but I removed that too.

I added in this statement to the parent

Code:

x = waitpid(childPid, NULL, WUNTRACED);

I print it after the printf stating the iteration number.

As you can see, not much happens. parentPid remains UNDEFINED as it does in your previously posted code, so of course the parent never receives any signal, and by the time the parent loops, the child has finished and exited also so there is no signal for it.

I am getting a value for parentPid. See output below.

The plain truth is there is absolutely no way you can rely on two seperate processes performing a simple for loop in a synchronized manner, as you may have presupposed, without 1) a specific time delay, or 2) some IPC such as signal handling. The code you posted does not really do the later, altho it may appear to in some sense.

I am trying to use signals to achieve this effect. I take it though from your answer that I am not using them correctly.

Perhaps the output you posted is from some different code?

I know I sound new, and its true I am pretty much a noob; but, not that new.

OK here is the new output:
The pid: 4 is the value of x set by waitpid. As far as our outputs are concerned, I think it has to do with the fact that I am logged remotely to a university server(?). It is Solaris. I mean: perhaps it is machine differences?

Code:

Parent process, iteration: 1 pid: 4
Parent pid: 9537

Child process, iteration: 1
Suspended (signal)
Parent process, iteration: 2 pid: 4
Parent pid: 9537
Child process, iteration: 2
Parent process, iteration: 3 pid: 4
Parent pid: 9537
Child process, iteration: 3
Parent process, iteration: 4 pid: 4
Parent pid: 9537
Child process, iteration: 4
Parent process, iteration: 5 pid: 4
Parent pid: 9537
Child process, iteration: 5
Parent process, iteration: 6 pid: 4
Parent pid: 9537
[1]  + Suspended (signal)            ./a.out
Child process, iteration: 6
Parent process, iteration: 7 pid: 4
Parent pid: 9537
Child process, iteration: 7
Parent process, iteration: 8 pid: 4
Parent pid: 9537
Child process, iteration: 8
Parent process, iteration: 9 pid: 4
Parent pid: 9537
Child process, iteration: 9

**Victor_November** · 11-07-2009

At this stage, just getting something to work for you is hard enough, so if something works, you consider it "error-free".

I can assure you that this is not the case.

Linux and I believe Solaris both conform to POSIX, so it is hard to see how "4" is an indication of success here...

I am not claiming success. Perhaps I did not make myself clear enough the first time: I am really new at this stuff. MK27 I am not trying to accuse of being wrong; quite the contrary in fact. I am trying to explain to you what I am seeing, and I was just curious why our outputs are different especially if we both have POSIX complaint machines.

With that said I am going to get my modified code in post it here.

**Victor_November** · 11-07-2009

Thanks for looking at it for me. As a part of the assignment I can't use sleep. But I will keep plugging away at it.

**Victor_November** · 11-10-2009

Here is some updated code:

It works but I still get a Signal Suspended message at a random (or so it appears) place in the output.

Code:

#include <stdio.h>
#include <signal.h>
#include <unistd.h>
#include <sys/wait.h>

void sigParent(void);
void sigChild(void);
int wait_status;
pid_t parentPid, childPid, pid;

int main()
{
	int i;
	wait_status = 1;
	parentPid = getpid();
	pid  = fork();
	switch(pid)
		{
		case -1: /* an error occurred */
		printf("Fork error\n");
		break;

		case 0: 	/* this code is executed by the child process */

			childPid = getpid();

			signal(SIGCONT, SIG_DFL);
			signal(SIGSTOP, SIG_DFL);

			for(i=1; i<10; i++)
			{
				printf("Child process, iteration: %d\n", i);
				fflush(stdout);
				sigParent();
			}
		break;

		default: 	/* this code is executed by the parent process */
			
			waitpid(childPid,NULL, WUNTRACED);
			signal(SIGCONT, SIG_DFL);
			signal(SIGSTOP, SIG_DFL);

			for(i=1; i<10; i++)
			{
				printf("Parent process, iteration: %d\n", i);
				fflush(stdout);
				sigChild();
			}/*end for*/
		break;
		
		} /*end switch*/
}/*end main*/



void sigParent(void)
{	
	kill(parentPid, SIGCONT);
	raise(SIGSTOP);
}
void sigChild(void)
{
	kill(childPid, SIGCONT);
	raise(SIGSTOP);
}

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