Domain Socket本地进程间通信

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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UNIX
Domain 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。
以下程序将UNIX Domain socket绑定到一个地址。

技术图片

 

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

server

#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);
unlink(name); /* in case it already exists 否则bind的时候会出错*/
/* 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 会创建name*/
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 */
unlink(un.sun_path); /* we‘re done with pathname now */
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

#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);
unlink(un.sun_path); /* in case it already exists */
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;
}

 

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