Curious global variable bug? Bad assumptions on my part?

[Tom St Denis]

Tynan Wilke wrote:
> Hello list,
>
> If I am posting in the wrong place, I apologize. I was making some
> test scripts this morning and found a pretty glaring difference
> between my optimized (-O1) versus my unoptimized (-O0) test code.
> Below, is the a quick example that I made showing the problem:
> --------------------------------
> #include <pthread.h>
> #include <stdio.h>
> #include <stdlib.h>
>
> static int g_counter = 0;
>
> void *IncThread(void* nothing)
> {
>     printf("Thread started\n");
>     while( 1 )
>     {
>         g_counter++;
> //        printf("thread g_counter: %d\n", g_counter );
>     }
>     return NULL;
> }
>
> int main()
> {
>     pthread_t thread;
>     if ( 0 != pthread_create(&thread, NULL, IncThread, NULL ) )
>     {
>         perror("Failed to create thread");
>         exit(1);
>     }
>
>     printf("Waiting for counter...\n");
>     while ( g_counter == 0 );
>   

Make g_counter volatile.  And for the real spiffy, you should actually 
have a lock around it's access.  Congrats, you invented a race condition.

Basically what's happening is that g_counter is likely cached in a 
register, and never read from memory after the initial expression.  
Making it volatile will fix the bug on most platforms (where read/write 
of an int is atomic).  But the "correct" solution is to place a mutex 
around it's access.

Tom