如何仅使用 pthread 互斥锁同步多个线程？

问：

我试图制作三个线程，每个线程打开一个不同的输入文件，从中读取一个字符并将其设置为全局变量，然后等待下一个线程从它打开的文件中读取一个字符，并将其设置为该全局变量，然后移动到下一个线程。

总的来说，我打开了一个来写字符。因此，main 将只从全局变量中读取并按照 thread1、thread2、thread3、thread1、thread2、3..等读取的字符顺序写入文件。file.out.out

这是我到目前为止所拥有的：

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


//global char variables
char globalVAR1;
char globalVAR2;
char globalVAR3;

//define mutexes
pthread_mutex_t mutexOne;
pthread_mutex_t mutexTwo;
pthread_mutex_t mutexThree;


void *threadOne(void *arg){
    FILE *fileOne;
    fileOne = fopen("hw5-1.in", "r");

    char mChr1;

    //read each char
    if(fileOne == NULL){
        perror("file one error\n");
    }

    while(!feof(fileOne)){
        fscanf(fileOne, "%c\n", &mChr1);
        printf("char read: %c \n", mChr1);

        pthread_mutex_lock(&mutexOne);
        globalVAR1 = mChr1;
        pthread_mutex_unlock(&mutexOne);

    }

}

//thread two
void *threadTwo(void *arg){
    FILE *fileTwo;
    fileTwo = fopen("hw5-2.in", "r");

    char mChr2;

    //read each char
    if(fileTwo == NULL){
        perror("file two error\n");
    }

    while(!feof(fileTwo)){
        fscanf(fileTwo, "%c\n", &mChr2);
        printf("char read: %c \n", mChr2);

        pthread_mutex_lock(&mutexTwo);
        globalVAR2 = mChr2;
        pthread_mutex_unlock(&mutexTwo);

    }

}
//thread three
void *threadThree(void *arg){
    FILE *fileThree;
    fileThree = fopen("hw5-3.in", "r");

    char mChr3;

    //read each char
    if(fileThree == NULL){
        perror("file three error\n");
    }

    while(!feof(fileThree)){
        fscanf(fileThree, "%c\n", &mChr3);
        printf("char read: %c \n", mChr3);

        pthread_mutex_lock(&mutexThree);
        globalVAR3 = mChr3;
        pthread_mutex_unlock(&mutexThree);

    }

}

//main should open an out file for writing
int main(){

    //open a file for writing
    FILE *mainWriting;
    mainWriting = fopen("hw5.out", "w");

    FILE *count;
    count = fopen("hw5-1.in", "r");
    char cc;
    int nu=0;

    while(fscanf(count, "%c\n", &cc) == 1){
        nu += 1;
    }
    rewind(count);
    fclose(count);

    //write the global value into the outfile

    //the mutexes initialized
    pthread_mutex_init(&mutexOne, NULL);
    pthread_mutex_init(&mutexTwo, NULL);
    pthread_mutex_init(&mutexThree, NULL);

    pthread_t pthreadOne, pthreadTwo, pthreadThree;
    pthread_create(&pthreadOne, NULL, threadOne, NULL);
    pthread_create(&pthreadTwo, NULL, threadTwo, NULL);
    pthread_create(&pthreadThree, NULL, threadThree, NULL);


    while(nu != 0){
        //write into hw5.out
        pthread_mutex_lock(&mutexOne);
        fputc(globalVAR1, mainWriting);
        fputc('\n', mainWriting);
        pthread_mutex_unlock(&mutexOne);

        sleep(1);
    
        pthread_mutex_lock(&mutexTwo);
        fputc(globalVAR2, mainWriting);
        fputc('\n', mainWriting);
        pthread_mutex_unlock(&mutexTwo);

        sleep(1);
  
        pthread_mutex_lock(&mutexThree);
        fputc(globalVAR3, mainWriting);
        fputc('\n', mainWriting);
        pthread_mutex_unlock(&mutexThree);

        nu -=1;
    }


    sleep(1);
    pthread_join(pthreadOne, NULL);
    pthread_join(pthreadTwo, NULL);
    pthread_join(pthreadThree, NULL);

    fclose(mainWriting);

}

输入文件的行数相同，每行都有一个字符。

我的程序的输出预计为：

a
1
@
b
2
$
c
3
%

（输入文件实际上有点长。为简单起见，我没有包含整个输入文件。该程序应该处理任意长度的文件，假设它们都是相同的长度）

对于输入文件：

hw5-1.in

a
b
c

hw5-2.in

1
2
3

hw5-3.in

@
$
%

我只应该使用互斥锁来同步三个线程，而不使用标志、全局计数变量、信号量或条件。

我的输出是这样的：

c
3
%
c
3
%
c
3
%

我已经坚持了一段时间，我可能会把它弄得太复杂了，但不确定下一步该怎么做。我非常感谢任何建议或提示。谢谢。