C - 在进程之间来回发送数据

Question

我正在研究下面的程序。它应该做的是从子进程获取随机整数，并根据 item_count 变量，return 通过使用管道向子进程发送适当的消息。问题是我在执行时遇到了很多意想不到的行为，见下文。 myCatalog 是一个结构。

#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <time.h>
#include <sys/wait.h>
#include <string.h>

struct catalog {
    char description[128];
    int price;
    int item_count;
};

int main() {
    int i, j, k, fd1[5][2], fd2[5][2];
    struct catalog myCatalog[3];
    pid_t pid;
    int itemNo;
    char message[24];
    char messageFailure[] = "Failure.", messageSuccess[] = "Success!";;        

 

    for(i = 0; i < 5; i++) {
        if(pipe(fd1[i]) == -1) {
                printf("Error opening pipe 1!\n");
                exit(1);
        }

        if(pipe(fd2[i]) == -1) {
            printf("Error opening pipe 2!\n");
            exit(1);
        }

        pid = fork();

        if(pid == 0) { //Child
            time_t t;
            srand((int)time(&t) % getpid());
            
            close(fd1[i][0]);  //Close reading end of fd1
            close(fd2[i][1]);  //Close writing end of fd2
            
            for(k = 0; k < 10; k++) {
                itemNo = rand() % 21;
                write(fd1[i][1], &itemNo, sizeof(int));
                sleep(1);
                read(fd2[i][0], message, sizeof(message));
                printf("%s\n", message);
            }
            printf("Hello from child process %d\n", getpid());
            close(fd1[i][1]);
            close(fd2[i][0]);
            exit(0);
        }
        else if(pid > 0) { //Parent
            int item;

            close(fd1[i][1]);  //Close writing end of fd1
            close(fd2[i][0]);  //Close reading end of fd2

            if(i == 0) {
                for(j = 0; j < 20; j++) {
                    snprintf(myCatalog[j].description, sizeof(myCatalog[j].description), "Item #%d", j);
                    myCatalog[j].price = (rand() % 10) + 1;
                    myCatalog[j].item_count = 2;
                }
                for(j = 0; j < 20; j++) {
                    printf("Description: %s\n", myCatalog[j].description);
                    printf("Price: %d\n", myCatalog[j].price);
                    printf("Count: %d\n", myCatalog[j].item_count);
                }   
            }
            for(j = 0; j < 10; j++) {
                read(fd1[i][0], &item, sizeof(int));
                printf("%d\n", item);
                myCatalog[item].item_count = myCatalog[item].item_count - 1;
                if(myCatalog[item].item_count <= 0) {
                    write(fd2[i][1], messageFailure, sizeof(messageFailure));
                }
                else {
                    write(fd2[i][1], messageSuccess, sizeof(messageSuccess));                
                }
                sleep(1);
            }
            printf("Hello from parent process %d\n", getpid());
            close(fd1[i][0]);
            close(fd2[i][1]);
            wait(NULL);
        }
        else {
            printf("Error forking!");
            exit(1);
        }

    }
    return 0;
}

一个子进程的示例输出如下所示：

Hello from parent process 11868
Hello from child process 11890
6
Success!
4
Success!
3
Success!
4
3
16
Success!
14
3
6
3
18
3
20
3
18
3

它应该做的是打印数字，然后是成功或失败等等。另外，它有时会输出 13 个数字而不是 10 个。问题是，问题在哪里？是我的管道吗？我的循环使用完全错误吗？任何帮助将不胜感激。

Answer 1

这看起来肯定不对。为 n > 2 访问 myCatalog[n] 会调用未定义的行为。

  struct catalog myCatalog[3];
  ...
  for(j = 0; j < 20; j++) {
          printf("Description: %s\n", myCatalog[j].description);

C - 在进程之间来回发送数据

C - Sending data back and forth between processes

c

fork

ipc

pipe