Thread: C Processing Issues (oh fork()!)

Originally Posted by jeffcobb

So if you number your processes 1-8, only 2,3,4 need to spawn 2 children (grandchildren)? Do they have to report anything? like actual process ID or logical process ID (ie 1-8)?

I think actual process ID. The instructions specifically say to use fork() and getpid(). Man it would be so easy if it were just logical process IDs!

You know what the biggest problem I think I'm having? getpid() doesn't work the way it's supposed to. It doesn't store or print the current process. It prints the processes all the way up to the first, even though it's not supposed to. How do I get rid of the parents?

I can't install anything, it's a university server and even if I did get approval, the college is closed today for a snowday.

My professor's answer: "check man -s 2 fork and man -s 2 getpid". As if that link you posted didn't do a better job of it or anything.

I'm starting to get the idea that we're making a process tree. I'm also starting to get the idea of using process specific code. However, I'm still stumped on making one process fork into 2. I can do grandchildren fine (ie 1 makes 2 makes 3) but not what he wants.

Ok, so I found waitpid(). I also found an example that was supposed to bear 3 children from the same process.

Code:

#include <stdio.h>
#include <sys/types.h>
#include <unistd.h>

int main(void) 
{
	int i;
	pid_t one, two, three;
	for(i = 0; i < 2; i++)
	{
		one = fork();
		switch(one)
		{
			case 0:printf("My PID is %d ", getpid());
				   printf("parent is %d\n", getppid());
			default: waitpid(one, NULL, 0);
		}
	}
}

So I took the liberty to make it 2 children. Simpler, right? Well, here's the output:

Code:

My PID is 17438 parent is 17437
My PID is 17439 parent is 17438
My PID is 17438 parent is 17437
My PID is 17440 parent is 17437

So it prints out line 1 and 3 which are the same. Why?

Case = 0 means that it's the child. Otherwise, it's the adult. I'm not checking to see if it's negative, cause it wont be.

Originally Posted by MK27

I actually get this from that:

My PID is 1904 parent is 1903
My PID is 1905 parent is 1904
My PID is 1906 parent is 1903

The second one is the for loop completing in the child, producing a grandchild -- you have

parent i = 0 child1 = 0,
child1 i =1, grandchild1 i = 1,
parent i = 1 child2 i = 1.

I don't mean to be rude, but could you elaborate? I've been working on this all day and it's getting to the point where I'm starting to lose focus.

Also, how you'd only get 3 lines? Did you include a library when you compiled?


Edit: Are you saying the loop is being run in the child? How do I stop that?

Originally Posted by MK27

I'd say it's a better question as to why you get four lines! Are you sure you are running exactly the code you posted?

Anyway, to elaborate, the reason the loop continues in the child is because a child is an exact copy of it's parent; it includes the variable i set to it's current value. You can see this more easily by tweaking that code a bit:

Code:

	for(i = 0; i < 3; i++)
	{
		one = fork();
		switch(one)
		{
			case 0:printf("My PID is %d, i = %d ", getpid(), i);

Now watch what happens.

Code:

My PID is 17772, i = 0
 My PID is 17773, i = 1
 My PID is 17774, i = 2
 My PID is 17772, i = 0
 My PID is 17773, i = 1
 My PID is 17772, i = 0
 My PID is 17775, i = 2
 My PID is 17772, i = 0
 My PID is 17776, i = 1
 My PID is 17777, i = 2
 My PID is 17776, i = 1
 My PID is 17778, i = 2
 WinSCP: this is end-of-file:1

Hmm, but I don't want the child to be a complete clone. Otherwise I can't get anything done! I thought vfork() was the solution, but it's not.

I also did ask my professor for help, but he referred me to a page that explains how to seperate what happens by if statements....like I've tried before. I can't find any in-depth examples that are more than 2 processes. 2 processes are easy. I get that, but when you start adding loops or more than one fork() call, it all seems to go haywire.