在进程间传递文件描述符

#include<sys/socket.h>
#include<sys/param.h>
#include<fcntl.h>
#include<stdio.h>
#include<unistd.h>
#include<stdlib.h>
#include<assert.h>
#include<string.h>
/*
struct cmsghdr *CMSG_FIRSTHDR(struct msghdr *msgh);
struct cmsghdr *CMSG_NXTHDR(struct msghdr *msgh,struct cmsghdr *cmsg);
size_t CMSG_ALIGN(size_t length);
size_t CMSG_SPACE(size_t length);
size_t CMSG_LEN(size_t length);
unsigned char* CMSG_DATA(struct cmsghdr *cmsg);
struct cmsghdr{
        socklen_t cmsg_len;
        int cmsg_level;
        int cmsg_type;
};
*/
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域的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[1].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);
}
int recv_fd(int fd)
{
    struct iovec iov[1];
    struct msghdr msg;
    char buf[0];
 
    iov[0].iov_base = buf;
    iov[1].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;//文件描述符的传递值
        /*创建父，子进程间的管道，文件描述符 pipefd[0]和pipefd[1]*/
 
        int ret = socketpair( PF_UNIX,SOCK_DGRAM,0,pipefd);//管道函数
        assert(ret != -1);
 
        pid_t pid = fork();
        assert(pid >= 0);
 
        if(pid == 0)//子进程
        {
                close(pipefd[0]);//?????
                fd_to_pass = open("text.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 buff[128];
        memset(buff,0,sizeof(buff));
        read(fd_to_pass,buff,127);//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,buff);
        close(fd_to_pass);
        return 0;
}