这样加锁来保证线程同步为什么不行？

#include <stdio.h>
#include <unistd.h>
#include <assert.h>
#include <pthread.h>

#define TIMES        1000

int num = 0;

pthread_mutex_t mutex;

void * do_add(void *args)
{
    int i;

    for(i = 0; i < TIMES; i ++)
    {
        pthread_mutex_lock(&mutex);
        num ++;
        pthread_mutex_unlock(&mutex);
        printf("A");
    }
    return NULL;
}

int main(int argc, char *argv[])
{
    int ret;
    pthread_t ptid[2];

    pthread_mutex_init(&mutex, NULL);

    ret = pthread_create(&ptid[1], NULL, do_add, NULL);
    if(ret != 0)
    {
        printf("Error: create thread failed.\n");
    }

    ret = pthread_create(&ptid[1], NULL, do_add, NULL);
    if(ret != 0)
    {
        printf("Error: create thread failed.\n");
    }

    pthread_join(ptid[0], NULL);
    pthread_join(ptid[1], NULL);
    pthread_mutex_destroy(&mutex);

    printf("Num = %d\n", num);
    return 0;
}

gcc sync.c -Wall -g -lpthread -o ap

OS: Ubuntu 10.04

反复运行10次左右，总有两三次num的值不为2000.