c - 使用 Write() 写入管道总是返回 0

Question

我试图让两个进程通过以下方式相互写入: 1. 父进程写信给子进程 2. 子进程接收来自父进程的消息 3. child 写信给 parent 4.父进程接收子进程的消息

我不明白为什么当我尝试发送字母“A”时，写入函数总是返回零。有谁知道为什么？我查看了手册页，它只是说它返回 0 字节，但“A”是一个应该是一个字节的字符？

int main()
{
    int fdP[2]; //used for child to contact parent 
    int fdC[2]; //used by parent to contact child
    int nBytes;
    char readBuffer[2];
    char writeBuffer[2];

    pipe(fdP); //Child >> Parent
    pipe(fdC); //Parent >> Child

    pid_t  pid;
    pid = fork();
    if( pid < 0)
    {
        perror("There was an error during the fork");
        exit(1);
    }

    //two processes running here
    if (pid == 0) //child process
    {   
        close(fdP[0]);
        close(fdC[1]);
        while (nBytes != 0) //when nbytes is nonzero it received a message
        {
            nBytes = read(fdP[0], readBuffer, sizeof(readBuffer));
        }
        printf("Child received: %s\n", readBuffer);

        write(fdP[1], "B", sizeof(readBuffer));
        exit(0);
    }
    else //parent process
    {
        close(fdC[1]);
        close(fdP[0]);

        while(write(fdC[1], "A", sizeof("A")) != 0) //THIS ALWAYS RETURNS ZERO
        {
            printf("%s\n", "I'M STUCK HERE");
        }

        while (nBytes != 0) //when nbytes is nonzero it received a message
        {
            nBytes = read(fdC[0], readBuffer, sizeof(readBuffer));
        }
        printf("Parent received: %s\n", readBuffer);
        wait(NULL);
    }
    printf("%s\n", "finished");
}

Answer 1

我发现一些问题:

• 在初始化之前使用 nBytes。这是未定义的行为。
• sizeof("A") 是错误的。这给出了 const char* 的大小，而不是字符串的实际大小。
• 用 0 测试 read() 和 write() 的逻辑是向后的。 (实际上，对于这个玩具程序，您可以完全删除循环和 nBytes 变量，而只需执行每个 read() 和 write() 一次。)

此外，您还混淆了文件描述符。假设您对它们的评论是正确的，详细信息如下:

• 子进程应该从 fdC[0] 读取，而不是 fdP[0]。
• 父进程关闭与预期相反的文件描述符。
• 父进程应该从 fdP[0] 读取，而不是 fdC[0]。

当我修复上述所有问题后，您的代码似乎可以执行您想要的操作。这是我最终得到的结果:

int main()
{
    int fdP[2]; //used for child to contact parent 
    int fdC[2]; //used by parent to contact child
    char readBuffer[2];

    pipe(fdP); //Child >> Parent
    pipe(fdC); //Parent >> Child

    pid_t  pid;
    pid = fork();
    if( pid < 0)
    {
        perror("There was an error during the fork");
        exit(1);
    }

    //two processes running here
    if (pid == 0) //child process
    {   
        close(fdP[0]);
        close(fdC[1]);
        read(fdC[0], readBuffer, sizeof(readBuffer));
        printf("Child received: %s\n", readBuffer);
        write(fdP[1], "B", 2);
        exit(0);
    }
    else //parent process
    {
        close(fdC[0]);
        close(fdP[1]);
        write(fdC[1], "A", 2);
        read(fdP[0], readBuffer, sizeof(readBuffer));
        printf("Parent received: %s\n", readBuffer);
        wait(NULL);
    }
    printf("%s\n", "finished");
}