13-5-send-fd.c

#include <stdio.h>
#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)
{
    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;
    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);
}

//接收目标文件描述符
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 argc, const char *argv[])
{
    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text.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 );
        char buf[1024];
        memset(buf, '\0', 1024);  
        read(fd_to_pass, buf, 1024); //读取目标文件描述符，以验证其有效性
        printf("I got fd %d and data %s\n", fd_to_pass, buf);
        close(fd_to_pass);

    }
    return 0;
}