进程间通信之进程间传递文件描述符

        本文将给出进程间传递文件描述符的代码，代码篇幅不长，我在其中添加了必要的注释。可是想要看懂这些代码依旧并不容易。如果大家在阅读代码时遇到困难，可以根据情况阅读下面几篇文章：

1. 高级IO函数之readv和writev-CSDN博客
2. 高级IO函数之recvmsg和sendmsg-CSDN博客
3. 进程间通信之管道-CSDN博客

示例代码：

#include 
#include 
#include 
#include 
#include 
#include 
#include 

static const int CONTROL_LEN = CMSG_LEN( sizeof(int) );// 计算 msg.msg_controllen = CONTROL_LEN;的宏

void send_fd( int fd, int fd_to_send )
{
    struct iovec iov[1]; // 关于该结构体大家可以看：高级IO函数之readv和writev
    struct msghdr msg;
    char buf[0];

    iov[0].iov_base = buf;
    iov[0].iov_len = 1;
    msg.msg_name    = NULL;// 关于该结构大家可以看：高级IO函数之recvmsg和sendmsg
    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;
    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[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 );// 取文件描述符数据的宏
    return fd_to_read;
}

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

    int ret = socketpair( PF_UNIX, SOCK_DGRAM, 0, pipefd );// 创建两个相互通信的域间套接字
    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 );
        send_fd( pipefd[1], ( fd_to_pass > 0 ) ? fd_to_pass : 0 );// 操作系统自动传递引用计数
        close( fd_to_pass );
        exit( 0 );
    }

    close( pipefd[1] );
    fd_to_pass = recv_fd( pipefd[0] );
    char buf[1024];
    memset( buf, '\0', 1024 );
    read( fd_to_pass, buf, 1024 );
    printf( "I got fd %d and data %s\n", fd_to_pass, buf );
    close( fd_to_pass );
}