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 <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) );// 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writevstruct 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sendmsgmsg.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 );
}