阿里的一个面试题，考察linux下的条件变量的用法，pthread_cond_wait（）

Question

参加了阿里的一个面试，面试官从正常代码里删除了几行，要求我找出下面这段代码的错误。我看了半天也没找到问题。其中worker()函数是7个线程的执行函数。各位爸爸,谁知道下面这段代码错在哪,原题如下：

假设多核的CPU，没有任何竞争问题。希望print_a, print_b, ... print_g在结束的时候都是1000。下面的代码是否可以做到？如果做不到，是什么原因，如何修改？

int n_threads=7;
int print_a=0;
int print_b=0;
int print_c=0;
int print_d=0;
int print_e=0;
int print_f=0;
int print_g=0;
int assigned_job[7]={PRINTA, PRINTB, PRINTC, PRINTD, PRINTE, PRINTF, PRINTG};
void *worker(void *t)
{
wait_for_new_work:
    pthread_mutex_lock(&threads_wait_for_new_cmd_mutex[my_id]);
    pthread_cond_wait(&threads_wake_up_cv[my_id], &threads_wait_for_new_cmd_mutex[my_id]);
    pthread_mutex_unlock(&threads_wait_for_new_cmd_mutex[my_id]);
    if (thread_control->pls_exit){
        pthread_exit(&(thread_control->retval));
    }
    if (assigned_job[my_id]==PRINTA} print_a++;
    else (assigned_job[my_id]==PRINTB} print_b++;
    else (assigned_job[my_id]==PRINTC} print_c+;
    else (assigned_job[my_id]==PRINTD} print_d++;
    else (assigned_job[my_id]==PRINTE} print_e++;
    else (assigned_job[my_id]==PRINTF} print_f++;
    else (assigned_job[my_id]==PRINTG} print_g++;
    pthread_mutex_lock(&threads_done_count_mutex);
    threads_done_count++;
    if (threads_done_count==n_threads)
        pthread_cond_signal(&all_threads_done_cv);
    pthread_mutex_unlock(&threads_done_count_mutex);
    goto wait_for_new_work;
}
int workers_go(int clear_res)
{
    pthread_mutex_lock(&threads_done_count_mutex);
    threads_done_count=0;
    pthread_mutex_unlock(&threads_done_count_mutex);
    for (i=0; i<n_threads; i++){
        pthread_mutex_lock(&threads_wait_for_new_cmd_mutex[i]);
        pthread_cond_signal(&threads_wake_up_cv[i]);
        pthread_mutex_unlock(&threads_wait_for_new_cmd_mutex[i]);
    }
    pthread_mutex_lock(&threads_done_count_mutex);
    if (threads_done_count<n_threads){
        printf("n_threads=%d, threads_done_count=%d, waiting for all threads done...
",
            n_threads,
            threads_done_count);
        pthread_cond_wait(&all_threads_done_cv, &threads_done_count_mutex);
    }
    pthread_mutex_unlock(&threads_done_count_mutex);
    if (verbose)
        printf("master: all threads done work
");
}
int main(int argc, char **argv)
{
/*
    Create threads pool
*/
cnt=1000;
    while(cnt--){
        workers_go(1);
        sleep(10);
    }
}

Answer 1

等待大神解答