Define and test custom atomic structure and functions in the absence of gcc intrinsics. atomic_t, atomic_set(), atomic_read(), atomic_inc().

atomic-test.c

/* atomic-test.c
 *
 * Compiled using "gcc (Ubuntu/Linaro 4.6.3-1ubuntu5) 4.6.3"
 * Compiler cmd : `gcc atomic-test.c -lpthread -o atomic-test`
 *
 * objdump -S shows that the lock prefix in properly inserted
 *
 * 08048674 <atomic_inc>:
 * 8048674:       55                      push   %ebp
 * 8048675:       89 e5                   mov    %esp,%ebp
 * 8048677:       8b 45 08                mov    0x8(%ebp),%eax
 * 804867a:       8b 55 08                mov    0x8(%ebp),%edx
 * 804867d:       f0 ff 00                lock incl (%eax)
 * 8048680:       5d                      pop    %ebp
 * 8048681:       c3                      ret
 *
 * Multiple runs on an Intel Core2 - 4cores
 * Result : "atomic_count:300000" (as expected)
 *
 */

#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <stdint.h>
#include <pthread.h>

/* atomics */
typedef struct {
	volatile int counter;
} atomic_t;

#define atomic_set(v,i) (((v)->counter) = (i))

#define atomic_read(v) ((v)->counter)

static inline void atomic_inc(atomic_t *v)
{
	asm volatile( "lock incl %0"
			: "+m" (v->counter));
}

/* globals */
#define THRD_NUM 15
static atomic_t atomic_count;
static pthread_cond_t g_cond;
static pthread_mutex_t g_mutex;

void* thread_func(void*p) {
	pthread_mutex_t* lock = (pthread_mutex_t*)p;
	pthread_mutex_lock(lock);
	pthread_cond_wait(&g_cond, lock);
	pthread_mutex_unlock(lock);

	int i;

	for (i=0;i<20000;++i){
		atomic_inc(&atomic_count);
	}
	return NULL;
}

int main() {
	atomic_set(&atomic_count, 0);
	pthread_cond_init(&g_cond, NULL);
	pthread_mutex_init(&g_mutex, NULL);
	pthread_t pid[THRD_NUM];

	int i;

	for (i=0; i<THRD_NUM; i++) {
		pthread_create(&pid[i], NULL, thread_func, &g_mutex);
	}

	sleep(3);
	pthread_cond_broadcast(&g_cond);

	for (i=0; i<THRD_NUM; i++) {
		pthread_join(pid[i], NULL);
	}

	long ans = atomic_read(&atomic_count);
	printf("atomic_count:%ld \n", ans);
}

Originally written to test http://stackoverflow.com/q/17861335/319204