socket IPC（本地套接字domain） · cpp

[TOC] # 簡介 socket API原本是為網絡通訊設計的，但后來在socket的框架上發展出一種IPC機制，就是UNIX Domain Socket。雖然網絡socket也可用于同一臺主機的進程間通訊（通過loopback地址127.0.0.1），但是UNIX Domain Socket用于IPC更有效率：不需要經過網絡協議棧，不需要打包拆包、計算校驗和、維護序號和應答等，只是將應用層數據從一個進程拷貝到另一個進程。這是因為，IPC機制本質上是可靠的通訊，而網絡協議是為不可靠的通訊設計的。UNIX Domain Socket也提供面向流和面向數據包兩種API接口，類似于TCP和UDP，但是面向消息的UNIX Domain Socket也是可靠的，消息既不會丟失也不會順序錯亂。 UNIX Domain Socket是全雙工的，API接口語義豐富，相比其它IPC機制有明顯的優越性，目前已成為使用最廣泛的IPC機制，比如X Window服務器和GUI程序之間就是通過UNIXDomain Socket通訊的。使用UNIX Domain Socket的過程和網絡socket十分相似，也要先調用socket()創建一個socket文件描述符，address family指定為AF_UNIX，type可以選擇SOCK_DGRAM或SOCK_STREAM，protocol參數仍然指定為0即可。 UNIX Domain Socket與網絡socket編程最明顯的不同在于地址格式不同，用結構體sockaddr_un表示，網絡編程的socket地址是IP地址加端口號，而UNIX Domain Socket的地址是一個socket類型的文件在文件系統中的路徑，這個socket文件由bind()調用創建，如果調用bind()時該文件已存在，則bind()錯誤返回。對比網絡套接字地址結構和本地套接字地址結構： ~~~ struct sockaddr_in { __kernel_sa_family_t sin_family; /* Address family */ 地址結構類型 __be16 sin_port; /* Port number */ 端口號 struct in_addr sin_addr; /* Internet address */ IP地址 }; struct sockaddr_un { __kernel_sa_family_t sun_family; /* AF_UNIX */ 地址結構類型 char sun_path[UNIX_PATH_MAX]; /* pathname */ socket文件名(含路徑) }; ~~~ 以下程序將UNIX Domain socket綁定到一個地址。 ~~~ 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); /* 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 ~~~ #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; } ~~~