linux pipe 互斥

#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <fcntl.h>
#include <sys/types.h>
#include <sys/stat.h>

int main() {
    int pipefd[2];
    pid_t cpid;
    char buf;

    if (pipe(pipefd) == -1) {
        perror("pipe");
        exit(EXIT_FAILURE);
    }

    cpid = fork();
    if (cpid == -1) {
        perror("fork");
        exit(EXIT_FAILURE);
    }

    if (cpid == 0) {    // 子进程
        close(pipefd[1]);  // 关闭写端

        struct flock fl;
        fl.l_type = F_WRLCK;
        fl.l_whence = SEEK_SET;
        fl.l_start = 0;
        fl.l_len = 0;

        if (fcntl(pipefd[0], F_SETLKW, &fl) == -1) {
            perror("fcntl");
            exit(EXIT_FAILURE);
        }

        read(pipefd[0], &buf, 1);
        printf("Child read %c\n", buf);

        fl.l_type = F_UNLCK;
        if (fcntl(pipefd[0], F_SETLK, &fl) == -1) {
            perror("fcntl");
            exit(EXIT_FAILURE);
        }

        close(pipefd[0]);
    } else {            // 父进程
        close(pipefd[0]);  // 关闭读端

        struct flock fl;
        fl.l_type = F_WRLCK;
        fl.l_whence = SEEK_SET;
        fl.l_start = 0;
        fl.l_len = 0;

        if (fcntl(pipefd[1], F_SETLKW, &fl) == -1) {
            perror("fcntl");
            exit(EXIT_FAILURE);
        }

        write(pipefd[1], "a", 1);
        printf("Parent wrote 'a'\n");

        fl.l_type = F_UNLCK;
        if (fcntl(pipefd[1], F_SETLK, &fl) == -1) {
            perror("fcntl");
            exit(EXIT_FAILURE);
        }

        close(pipefd[1]);
        wait(NULL); // 等待子进程结束
    }

    return 0;
}