[apue] 作为 daemon， 启动 Unix Domain Socket 侦听失败?

前段时间写一个传递文件句柄的小 demo，有 server 端、有 client 端，之间通过 Unix Domain Socket 通讯。

在普通模式下，双方可以正常建立连接，当server端作为daemon启动时，则第一次启动成功，之后再启动， listen 会连接报 ENOTSUPP 错误，导致启动失败。

spipe.c

 int cli_conn(const char *name)
 {
     int fd, len, err, rval;
     struct sockaddr_un un; 

     if ((fd = socket (AF_UNIX, SOCK_STREAM, )) < ) {
         printf ("create socket failed\n");
         return -;
     }

     printf ("create socket ok\n");
     memset (&un, , 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) < ) {
         err = errno;
         printf ("connect failed\n");
         rval = -;
         goto errout;
     }

     printf ("connect to server ok\n");
     return fd;
 errout:
     close (fd);
     errno = err;
     return rval;
 }

 int serv_listen (const char *name)
 {
     int fd, len, err, rval;
     struct sockaddr_un un; 

     if ((fd = socket (AF_UNIX, SOCK_STREAM, )) < ) {
         printf ("socket failed\n");
         return -;
     }

     printf ("create socket ok\n");
     unlink (name);
     memset (&un, , sizeof(un));
     un.sun_family = AF_UNIX;
     strcpy (un.sun_path, name);
     len = offsetof (struct sockaddr_un, sun_path) + strlen (name); 

     if (bind (fd, (struct sockaddr *)&un, len) < ) {
         err = errno;
         printf ("bind failed\n");
         rval = -;
         goto errout;
     }

     printf ("bind socket to path ok\n");
     if (listen (fd, QLEN) < ) {
         err = errno;
         printf ("listen failed, errno %d\n", errno);
         rval = -;
         goto errout;
     }

     printf ("start listen on socket ok\n");
     return fd;
 errout:
     close (fd);
     errno = err;
     return rval;
 }

 int serv_accept (int listenfd, uid_t *uidptr)
 {
     int clifd, err, rval;
     time_t staletime;
     struct sockaddr_un un;
     struct stat statbuf; 

     size_t len = sizeof (un);
     if ((clifd = accept (listenfd, (struct sockaddr *)&un, &len)) < ) {
         printf ("accept failed\n");
         return -;
     }

     len -= offsetof (struct sockaddr_un, sun_path);
     un.sun_path[len] = ;
     printf ("accept %s ok\n", un.sun_path); 

     unlink (un.sun_path);
     return clifd; 

 errout:
     close (clifd);
     errno = err;
     return rval;
 }

出错的位置在 serv_listen (line 57) 处，出错时的 server 端输出为：

Jan 17 00:24:44 localhost opend: create socket ok
Jan 17 00:24:44 localhost opend: bind socket to path ok
Jan 17 00:24:44 localhost opend: listen failed, errno 95
Jan 17 00:24:44 localhost opend: serv_listen error: Operation not supported

errno 95 为 ENOTSUPP。不以 daemon 运行时正常的输出如下：

create socket ok
bind socket to path ok
start listen on socket ok
accept  ok
new connection: uid 0, fd 4

可能细心的读者会觉得，以 daemon 方式运行 printf 怎么还可以输出呢，是有以下宏定义做了处理：

 #ifdef USE_APUE
 #include "../apue.h"
 #define printf log_msg
 #endif

以 daemon 运行时会定义 USE_APUE 宏，从而将 printf 重定义为 log_msg 输出到 syslog。

下面是 server 端的代码：

csopend2.c

 int main (int argc, char *argv[])
 {
     int c = ;
     log_open ("open.serv", LOG_PID, LOG_USER); 

     opterr = ; // don't want getopt() writting to stderr !
     while ((c = getopt (argc, argv, "d")) != EOF) {
         switch (c) {
             case 'd':
                 debug = log_to_stderr = ;
                 break;
             case '?':
                 err_quit ("unrecongnized option: -%c", optopt);
         }
     }

     if (debug == )
     {
         log_to_stderr = ;
         daemonize ("opend");
     }

     loop ();
     return ;
 }

不使用 -d 时表示 daemon 运行(与常识相反?)，上面标黄的代码就是。

对应的 client 端代码：

csopenc.c

一开始怀疑是用于 listen 的本地 socket 文件已经存在，于是去 /tmp 目录看了下，果然有 opend 这个文件，删除之，再运行，不行；

然后怀疑是没有复用端口(?)所致，于是在 listen 之前添加了以下代码段：

     int opt = ;
     if (setsockopt(fd, SOL_SOCKET, SO_REUSEADDR, (const void *)&opt, sizeof(opt)) < ) {
         err = errno;
         printf ("setsockopt failed\n");
         rval = -;
         goto errout;
     }

设置端口复用。编译、运行，输出如下：

Jan 17 00:43:11 localhost opend: create socket ok
Jan 17 00:43:11 localhost opend: bind socket to path ok
Jan 17 00:43:11 localhost opend: set socket option ok
Jan 17 00:43:11 localhost opend: listen failed, errno 95
Jan 17 00:43:11 localhost opend: serv_listen error: Operation not supported

设置成功了，但还是不行

难道 daemon 与普通进程使用 Unix 域套接字还有什么区别么?

暂时存疑……