code13_5.cpp

#include <sys/socket.h>
#include <fcntl.h>
#include <stdio.h>
#include <unistd.h>
#include <stdlib.h>
#include <assert.h>
#include <string.h>
#include <time.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);
}
/*接受目标文件描述符*/
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;/*设置辅助数据*/
	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("code13_5.cpp",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);/*这里关闭文件描述符，父进程也能读文件，是因为在send_fd上可能有文件描述符的临时变量，导致文件引用数不会为0，*/
		printf("www");
		exit(0);
	}
	close(pipefd[1]);
	int a = 1000000000;
	while(a--);
	fd_to_pass = recv_fd(pipefd[0]);/*父进程从管道接受目标文件描述符*/
	char buff[1024];
	memset(buff,'\0',1024);
	read(fd_to_pass,buff,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,buff);
	close(fd_to_pass);
	return 0;
}