下面的代码出自《Unix/Linux编程实践教程》，作用是用两个线程分别统计两个文件的单词的数目，并在主线程中计算总数。下面是运行截图：

但是看了半天还是难以理解下面代码中的加锁、解锁以及条件变量。
我想问：

1. 主线程中调用pthread_cond_wait时会释放互斥锁，然后挂起主线程，并等待条件变量的发生变化，当其他线程调用pthread_cond_signal时，如果互斥锁是被锁住的，那么主线程中的pthread_cond_wait会等互斥待锁被解锁后，然后再给互斥锁上锁后再返回吗？

2. 如果正如１中的描述那样的话，pthread_cond_wait收到了pthread_cond_signal发来的信号，但是未锁定互斥锁之前，又被其他线程抢了先，锁住了互斥锁，那不是pthread_cond_wait还得挂起等待互斥锁被解锁？

3. 如果可以的话，希望能帮助理清一下这个程序的执行流程。

万分感谢。

#include <stdio.h>
#include <stdlib.h>
#include <pthread.h>
#include <ctype.h>

struct arg_set {            /* two values int one arg */
    char    *filename;      /* file to examine */
    int     count;          /* number of words */
    int     code;
};

struct arg_set  *mailbox = NULL;
pthread_mutex_t lock = PTHREAD_MUTEX_INITIALIZER;
pthread_cond_t  flag = PTHREAD_COND_INITIALIZER;

void *count_words(void *);

int main(int argc, char *argv[])
{
    pthread_t t1, t2;               /* two threads */
    struct arg_set args1, args2;    /* two argsets */
    int reports_int = 0;
    int total_words = 0;

    if (argc != 3) {
        fprintf(stderr, "usage: %s file1 file2", argv[0]);
        exit(1);
    }

    pthread_mutex_lock(&lock);

    args1.filename  = argv[1];
    args1.count     = 0;
    args1.code      = 1;
    pthread_create(&t1, NULL, count_words, (void *)&args1);

    args2.filename  = argv[2];
    args2.count     = 0;
    args2.code      = 2;
    pthread_create(&t2, NULL, count_words, (void *)&args2);

    while (reports_int < 2) {　//　等待其他线程结束
        printf("MAIN: waiting for flag to go up\n");
        pthread_cond_wait(&flag, &lock);
        printf("MAIN: Wow! flag was raised, I have the lock\n");
        printf("%7d: %s\n", mailbox->count, mailbox->filename);
        total_words += mailbox->count;
        if (mailbox == &args1)
            pthread_join(t1, NULL);
        if (mailbox == &args2)
            pthread_join(t2, NULL);
        mailbox = NULL;
        pthread_cond_signal(&flag);
        reports_int++;
    }

    printf("%7d: total words\n", total_words);

    return 0;
}

void *count_words(void *a)
{
    struct arg_set *args = a;
    FILE *fp;
    int c, prevc = 'rrreee';

    if ((fp = fopen(args->filename, "r")) != NULL) {　//　统计单词个数
        while ((c = getc(fp)) != EOF) {
            if (!isalnum(c) && isalnum(prevc))
                args->count++;
            prevc = c;
        }
        fclose(fp);
    } else
        perror(args->filename);

    printf("COUNT %d: waiting to get lock\n", args->code);
    pthread_mutex_lock(&lock);
    printf("COUNT %d: have lock, storing data\n", args->code);
    if (mailbox != NULL)
        pthread_cond_wait(&flag, &lock);
    mailbox = args;
    printf("COUNT %d: raising flag\n", args->code);
    pthread_cond_signal(&flag);
    printf("COUNT %d: unlocking box\n", args->code);
    pthread_mutex_unlock(&lock);

    return NULL;
}