文章目录

• 本地套接字
• 代码
• • server
  • client

socket API原本是为网络通讯设计的，但后来在socket的框架上发展出一种IPC机制，就是UNIX Domain Socket。虽然网络socket也可用于同一台主机的进程间通讯（通过loopback地址127.0.0.1），但是UNIX Domain Socket用于IPC更有效率：不需要经过网络协议栈，不需要打包拆包、计算校验和、维护序号和应答等，只是将应用层数据从一个进程拷贝到另一个进程。这是因为，IPC机制本质上是可靠的通讯，而网络协议是为不可靠的通讯设计的。UNIX Domain Socket也提供面向流和面向数据包两种API接口，类似于TCP和UDP，但是面向消息的UNIX Domain Socket也是可靠的，消息既不会丢失也不会顺序错乱。

UNIX Domain Socket是全双工的，API接口语义丰富，相比其它IPC机制有明显的优越性，目前已成为使用最广泛的IPC机制，比如X Window服务器和GUI程序之间就是通过UNIXDomain Socket通讯的。

使用UNIX Domain Socket的过程和网络socket十分相似，也要先调用socket()创建一个socket文件描述符，address family指定为AF_UNIX，type可以选择SOCK_DGRAM或SOCK_STREAM，protocol参数仍然指定为0即可。

UNIX Domain Socket与网络socket编程最明显的不同在于地址格式不同，用结构体sockaddr_un表示，网络编程的socket地址是IP地址加端口号，而UNIX Domain Socket的地址是一个socket类型的文件在文件系统中的路径，这个socket文件由bind()调用创建，如果调用bind()时该文件已存在，则bind()错误返回。

对比网络套接字地址结构和本地套接字地址结构：

struct sockaddr_in {
__kernel_sa_family_t sin_family; 			/* Address family */  	地址结构类型
__be16 sin_port;					 	/* Port number */		端口号
struct in_addr sin_addr;					/* Internet address */	IP地址
};
struct sockaddr_un {
__kernel_sa_family_t sun_family; 		/* AF_UNIX */			地址结构类型
char sun_path[UNIX_PATH_MAX]; 		/* pathname */		socket文件名(含路径)
};

以下程序将UNIX Domain socket绑定到一个地址。

	size = offsetof(struct sockaddr_un, sun_path) + strlen(un.sun_path);
	#define offsetof(type, member) ((int)&((type *)0)->MEMBER)

server

/*************************************************************************
    > File Name: server.c
    > Author: 杨永利
    > Mail: 1795018360@qq.com 
    > Created Time: 2020年10月30日 星期五 23时18分39秒
 ************************************************************************/

#include <stdlib.h>
#include <stdio.h>
#include <stddef.h>
#include <sys/socket.h>
#include <sys/un.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <unistd.h>
#include <errno.h>

#define QLEN 10
/*
* Create a server endpoint of a connection.
* Returns fd if all OK, <0 on error.
*/
int serv_listen(const char *name)
{
	int fd, len, err, rval;
	struct sockaddr_un un;

	/* create a UNIX domain stream socket */
	if ((fd = socket(AF_UNIX, SOCK_STREAM, 0)) < 0)
		return(-1);
	/* in case it already exists */
	unlink(name);

	/* fill in socket address structure */
	memset(&un, 0, sizeof(un));
	un.sun_family = AF_UNIX;
	strcpy(un.sun_path, name);
	len = offsetof(struct sockaddr_un, sun_path) + strlen(name);

	/* bind the name to the descriptor */
	if (bind(fd, (struct sockaddr *)&un, len) < 0) {
		rval = -2;
		goto errout;
	}
	if (listen(fd, QLEN) < 0) { /* tell kernel we're a server */
		rval = -3;
		goto errout;
	}
	return(fd);

errout:
	err = errno;
	close(fd);
	errno = err;
	return(rval);
}
int serv_accept(int listenfd, uid_t *uidptr)
{
	int clifd, len, err, rval;
	time_t staletime;
	struct sockaddr_un un;
	struct stat statbuf;

	len = sizeof(un);
	if ((clifd = accept(listenfd, (struct sockaddr *)&un, &len)) < 0)
		return(-1); /* often errno=EINTR, if signal caught */

	/* obtain the client's uid from its calling address */
	len -= offsetof(struct sockaddr_un, sun_path); /* len of pathname */
	un.sun_path[len] = 0; /* null terminate */

	if (stat(un.sun_path, &statbuf) < 0) {
		rval = -2;
		goto errout;
	}
	if (S_ISSOCK(statbuf.st_mode) == 0) {
		rval = -3; /* not a socket */
		goto errout;
	}
	if (uidptr != NULL)
		*uidptr = statbuf.st_uid; /* return uid of caller */
	/* we're done with pathname now */
	unlink(un.sun_path);
	return(clifd);

errout:
	err = errno;
	close(clifd);
	errno = err;
	return(rval);
}
int main(void)
{
	int lfd, cfd, n, i;
	uid_t cuid;
	char buf[1024];
	lfd = serv_listen("foo.socket");

	if (lfd < 0) {
		switch (lfd) {
			case -3:perror("listen"); break;
			case -2:perror("bind"); break;
			case -1:perror("socket"); break;
		}
		exit(-1);
	}
	cfd = serv_accept(lfd, &cuid);
	if (cfd < 0) {
		switch (cfd) {
			case -3:perror("not a socket"); break;
			case -2:perror("a bad filename"); break;
			case -1:perror("accept"); break;
		}
		exit(-1);
	}
	while (1) {
r_again:
		n = read(cfd, buf, 1024);
		if (n == -1) {
		if (errno == EINTR)
		goto r_again;
	}
	else if (n == 0) {
		printf("the other side has been closed.\n");
		break;
	}
	for (i = 0; i < n; i++)
		buf[i] = toupper(buf[i]);
		write(cfd, buf, n);
	}
	close(cfd);
	close(lfd);
	return 0;
}

client

/*************************************************************************
    > File Name: client.c
    > Author: 杨永利
    > Mail: 1795018360@qq.com 
    > Created Time: 2020年10月30日 星期五 23时18分59秒
 ************************************************************************/

#include <stdio.h>
#include <stdlib.h>
#include <stddef.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <unistd.h>
#include <sys/socket.h>
#include <sys/un.h>
#include <errno.h>

#define CLI_PATH "/var/tmp/" /* +5 for pid = 14 chars */
/*
* Create a client endpoint and connect to a server.
* Returns fd if all OK, <0 on error.
*/
int cli_conn(const char *name)
{
	int fd, len, err, rval;
	struct sockaddr_un un;

	/* create a UNIX domain stream socket */
	if ((fd = socket(AF_UNIX, SOCK_STREAM, 0)) < 0)
		return(-1);

	/* fill socket address structure with our address */
	memset(&un, 0, sizeof(un));
	un.sun_family = AF_UNIX;
	sprintf(un.sun_path, "%s%05d", CLI_PATH, getpid());
	len = offsetof(struct sockaddr_un, sun_path) + strlen(un.sun_path);

	/* in case it already exists */
	unlink(un.sun_path);
	if (bind(fd, (struct sockaddr *)&un, len) < 0) {
		rval = -2;
		goto errout;
	}

	/* fill socket address structure with server's address */
	memset(&un, 0, sizeof(un));
	un.sun_family = AF_UNIX;
	strcpy(un.sun_path, name);
	len = offsetof(struct sockaddr_un, sun_path) + strlen(name);
	if (connect(fd, (struct sockaddr *)&un, len) < 0) {
		rval = -4;
		goto errout;
	}
return(fd);
	errout:
	err = errno;
	close(fd);
	errno = err;
	return(rval);
}
int main(void)
{
	int fd, n;
	char buf[1024];

	fd = cli_conn("foo.socket");
	if (fd < 0) {
		switch (fd) {
			case -4:perror("connect"); break;
			case -3:perror("listen"); break;
			case -2:perror("bind"); break;
			case -1:perror("socket"); break;
		}
		exit(-1);
	}
	while (fgets(buf, sizeof(buf), stdin) != NULL) {
		write(fd, buf, strlen(buf));
		n = read(fd, buf, sizeof(buf));
		write(STDOUT_FILENO, buf, n);
	}
	close(fd);
	return 0;
}