Subj : Re: Futex problem
To   : comp.programming.threads
From : Joe Seigh
Date : Sat Oct 01 2005 12:50 pm

JK wrote:
> Hi, I am trying to learn how to use futex and I just dont get it ;)
> What is problem with this code example? The child process never stops
> waiting. This is only a test, thats why it does not have any error
> checking.
> 
> 
> #include <sys/types.h>
> #include <unistd.h>
> #include <stdlib.h>
> #include <stdio.h>
> #include <linux/futex.h>
> #include <asm/unistd.h>
> #include <errno.h>
> 
> #define __NR_sys_futex __NR_futex
> _syscall4(int, sys_futex, volatile int *, futex, int, op,
>     int, val, struct timespec *, rel);
> 
> 
> int main(int argc, char *argv[])
> {
> 	int counter = 1;
> 	pid_t p = fork();
> 
> 	if (p == 0)
> 	{
> 		printf ("before wait\n");
> 		sys_futex(&counter, FUTEX_WAIT, 1, NULL);
> 		printf ("after wait\n");
> 	}
> 
> 	sleep (2);
> 
> 	counter = 0;
> 	printf ("wake (%d) : %d\n", sys_futex(&counter,
>              FUTEX_WAKE, 1, NULL), counter);
> 
> 	sleep (2);
> 	return EXIT_SUCCESS;
> }
> 

The futex needs to be in shared memory.  Forked processes don't share memory.


-- 
Joe Seigh

When you get lemons, you make lemonade.
When you get hardware, you make software.

.