下面的程式碼出自《Unix/Linux程式設計實作教學》，作用是用兩個執行緒分別統計兩個檔案的單字的數目，並在主執行緒中計算總數。下面是運行截圖：

#但是看了半天還是難以理解下面程式碼中的加鎖、解鎖以及條件變數。
我想問：

1. 主執行緒中呼叫pthread_cond_wait時會釋放互斥鎖，然後掛起主執行緒，並等待條件變數的變化，當其他執行緒呼叫pthread_cond_signal時，如果互斥鎖是被鎖住的，那麼主線程中的pthread_cond_wait會等互斥待鎖被解鎖後，然後再給互斥鎖上鎖後再返回嗎？

2. 如果正如１中的描述那樣的話，pthread_cond_wait收到了pthread_cond_signal發來的訊號，但是未鎖定互斥鎖之前，又被其他執行緒搶了先，鎖住了互斥鎖，那不是pthread_cond_wait還得掛起等待互斥鎖被解鎖？

3. 如果可以的話，希望能幫忙釐清這個程式的執行流程。

萬分感謝。

#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;
}