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
