子进程通过管道发送一个数字，然后等待

Question

我正在尝试生成两个通过管道连接的子进程。 孩子 1 应该通过管道向孩子 2 发送一个数字，孩子 2 应该打印它。我设法做到了，但是当我尝试循环执行此操作时，孩子 1 每次在发送另一个号码之前等待 1 秒，它不会发送任何内容。
我做错了什么？

#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <string.h>
#include <signal.h>
#include <windows.h>

int main(int argc, char *argv[]) {
    int fd[2];
    pipe(fd);
    pid_t pid1 = fork();
    if (pid1) {
        pid_t pid2 = fork();
        if (pid2) { //parent
            // do stuff as parent
        } else { //child 2
            close(fd[1]);
            char stringNumberReceived[10];
            while (1) {
                read(fd[0], stringNumberReceived, sizeof (stringNumberReceived));
                printf("number received: %s\n", stringNumberReceived);
            }
        }
    } else {//child 1
        close(fd[0]);
        int num;
        char stringnumber [10];
        while (1) {
            num = rand();
            snprintf(stringnumber, 10, "%d", num);
            printf("attempting to sent: %s\n", stringnumber);
            write(fd[1], stringnumber, strlen(stringnumber) + 1);
            Sleep(1000);
        }
    }
}

Answer 1

这是您的代码的轻微修改版本：

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

enum { MAX_COUNT = 20 };

int main(void)
{
    int fd[2];
    pipe(fd);
    pid_t pid1 = fork();
    if (pid1)
    {
        pid_t pid2 = fork();
        if (pid2)   // parent
        {
            printf("Child 1: %d; child 2: %d\n", (int)pid1, (int)pid2);
            close(fd[0]);   // Let the child detect EOF!
            close(fd[1]);   // Let the child detect EOF!
            int corpse;
            int status;
            while ((corpse = wait(&status)) > 0)
                printf("PID %d exited with status 0x%.4X\n", corpse, status);
        }
        else     // child 2
        {
            close(fd[1]);
            char stringNumberReceived[10];
            int nbytes;
            while ((nbytes = read(fd[0], stringNumberReceived, sizeof(stringNumberReceived))) > 0)
                printf("number received: %.*s\n", nbytes, stringNumberReceived);
            printf("Child 2: EOF or ERR\n");
        }
    }
    else    // child 1
    {
        close(fd[0]);
        int num;
        char stringnumber[10];
        for (int counter = 0; counter < MAX_COUNT; counter++)
        {
            num = rand();
            snprintf(stringnumber, sizeof(stringnumber), "%d", num);
            printf("attempting to send: [%s] (len %zu)\n", stringnumber, strlen(stringnumber));
            write(fd[1], stringnumber, strlen(stringnumber) + 1);
            sleep(1);
        }
    }
    return 0;
}

发生了什么变化？各种东西。您可以忽略大括号位置的变化；这纯粹是装饰性的。

1. 父代码已实现。
2. 父进程报告子进程 ID。
3. 父级关闭管道 - 既不读取也不写入。
4. 父母等待孩子死亡，同时报告他们的状态。
5. Child 记录它读取的字节数，并确保它打印的字节数不超过该字节数。
6. 当管道出现错误或 EOF（读取 0 字节）时，它会终止循环并在退出前报告。
7. 其他孩子在退出前进行了固定数量的循环（如图所示，为 20 个）。
8. 如果随机数生成器曾经生成过 10 位以上的数字，您可能会遇到数组边界问题。我得到的最大长度是 9，这很安全。
9. 生成孩子可以/应该在完成后报告。

在这些变化中，我怀疑关闭父管道中的管道可能是关键。

示例输出（在 Unix 机器上）：

$ pp43
attempting to send: [16807] (len 5)
Child 1: 69198; child 2: 69199
number received: 16807
attempting to send: [282475249] (len 9)
number received: 282475249
attempting to send: [162265007] (len 9)
number received: 162265007
attempting to send: [984943658] (len 9)
number received: 984943658
attempting to send: [114410893] (len 9)
number received: 114410893
attempting to send: [470211272] (len 9)
number received: 470211272
attempting to send: [101027544] (len 9)
number received: 101027544
attempting to send: [145785087] (len 9)
number received: 145785087
attempting to send: [145877792] (len 9)
number received: 145877792
attempting to send: [200723770] (len 9)
number received: 200723770
attempting to send: [823564440] (len 9)
number received: 823564440
attempting to send: [111543816] (len 9)
number received: 111543816
attempting to send: [178448449] (len 9)
number received: 178448449
attempting to send: [74243042] (len 8)
number received: 74243042
attempting to send: [114807987] (len 9)
number received: 114807987
attempting to send: [113752250] (len 9)
number received: 113752250
attempting to send: [144128232] (len 9)
number received: 144128232
attempting to send: [16531729] (len 8)
number received: 16531729
attempting to send: [823378840] (len 9)
number received: 823378840
attempting to send: [143542612] (len 9)
number received: 143542612
Child 2: EOF or ERR
PID 69198 exited with status 0x0000
PID 69199 exited with status 0x0000
$