13_5.cpp

// 在进程间传递文件描述符
#include<sys/socket.h>
#include<fcntl.h>
#include<stdio.h>
#include<unistd.h>
#include<stdlib.h>
#include<assert.h>
#include<string.h>

static const int CONTROL_LEN = CMSG_LEN(sizeof(int));


// 发送文件描述符
// fd参数是用来传递信息的UNIX域socket
// fd_to_send参数是待发送的文件描述符
void send_fd(int fd, int fd_to_send){
    // iovec和msghdr数据结构见5.8.3节——通用数据读写函数
    struct iovec iov[1];
    struct msghdr msg;
    char buf[0];

    iov[0].iov_base = buf;
    iov[0].iov_len = 1;
    msg.msg_name = NULL;
    msg.msg_namelen = 0;
    msg.msg_iov = iov;
    msg.msg_iovlen = 1;

    // 这个cmsghdr是啥
    cmsghdr cm;
    cm.cmsg_len = CONTROL_LEN;
    cm.cmsg_level = SOL_SOCKET;
    cm.cmsg_type = SCM_RIGHTS;
    *(int *)CMSG_DATA(&cm) = fd_to_send;
    msg.msg_control = &cm; // 设置辅助数据
    msg.msg_controllen = CONTROL_LEN;

    sendmsg(fd, &msg, 0);
}


// 接收目标文件符
int recv_fd(int fd){
    struct iovec iov[1];
    struct msghdr msg;
    char buf[0];
    iov[0].iov_base = buf;
    iov[0].iov_len = 1;
    msg.msg_name = NULL;
    msg.msg_namelen = 0;
    msg.msg_iov = iov;
    msg.msg_iovlen = 1;

    cmsghdr cm;
    msg.msg_control = &cm;
    msg.msg_controllen = CONTROL_LEN;

    recvmsg(fd, &msg, 0);
    
    int fd_to_read = *(int *)CMSG_DATA(&cm);
    return fd_to_read;
}

int main(){
    int pipefd[2];
    int fd_to_pass = 0;

    // 创建父、子进程间的管道，文件描述符pipefd[0]和pipefd[1]都是UNIX域socket
    int ret = socketpair(PF_UNIX, SOCK_DGRAM, 0, pipefd);
    assert(ret != -1);

    pid_t pid = fork();
    assert(pid >= 0);

    if(pid == 0){
        close(pipefd[0]);
        fd_to_pass = open("test.txt", O_RDWR, 0666);
        // 子进程通过管道将文件描述符发送到父进程
        // 如果文件test.txt打开失败，则子进程将标准输入文件描述符发送到父进程
        send_fd(pipefd[1], (fd_to_pass > 0) ? fd_to_pass : 0);
        close(fd_to_pass);
        exit(0);
    }

    close(pipefd[1]);
    fd_to_pass = recv_fd(pipefd[0]); // 父进程从管道接收目标文件描述符
    char buf[1024];
    memset(buf, '\0', 1024);
    read(fd_to_pass, buf, 1024); // 读目标文件描述符，以验证其有效性
    printf("I got fd %d and data %s\n", fd_to_pass, buf);
    close(fd_to_pass);
}