创建一个守护进程,有几个关键的步骤,也有几个地方需要注意,
几个关键的步骤有:
1:清除文件创建权限
2:调用fork,然后使父进程退出
3:调用setsid以创建一个新的会话,有三个目的使调用进程 a:成为新会话的首进程,b:成为新进程的组长进程,c:没有控制终端
4:切换工作目录
5:关闭不需要的文件描述符
需要注意的地方
1:因为守护进程没有控制终端,所以不能与标准输入输出出错进行交互,不能使用printf,通常用syslog来解决守护进程的打印信息
参考代码如下:
- #include <signal.h>
- #include <stdlib.h>
- #include <syslog.h>
- #include <fcntl.h>
- #include <stdio.h>
- #include <sys/resource.h>
- void daemonize(const char *cmd)
- {
- int i, fd0, fd1, fd2;
- pid_t pid;
- struct rlimit rl;
- struct sigaction sa;
- /*
- * Clear file creation mask.
- */
- umask(0);
- /*
- * Get maximum number of file descriptors.
- */
- if (getrlimit(RLIMIT_NOFILE, &rl) < 0)
- printf("%s: can't get file limit", cmd);
- /*
- * Become a session leader to lose controlling TTY.
- */
- if ((pid = fork()) < 0)
- printf("%s: can't fork", cmd);
- else if (pid != 0) /* parent */
- exit(0);
- setsid();
- /*
- * Ensure future opens won't allocate controlling TTYs.
- */
- sa.sa_handler = SIG_IGN;
- sigemptyset(&sa.sa_mask);
- sa.sa_flags = 0;
- if (sigaction(SIGHUP, &sa, NULL) < 0)
- printf("can't ignore SIGHUP");
- if ((pid = fork()) < 0)
- printf("%s: can't fork", cmd);
- else if (pid != 0) /* parent */
- exit(0);
- /*
- * Change the current working directory to the root so
- * we won't prevent file systems from being unmounted.
- */
- if (chdir("/") < 0)
- printf("can't change directory to /");
- /*
- * Close all open file descriptors.
- */
- if (rl.rlim_max == RLIM_INFINITY)
- rl.rlim_max = 1024;
- for (i = 0; i < rl.rlim_max; i++)
- close(i);
- /*
- * Attach file descriptors 0, 1, and 2 to /dev/null.
- */
- fd0 = open("/dev/null", O_RDWR);
- fd1 = dup(0);
- fd2 = dup(0);
- /*
- * Initialize the log file.
- */
- openlog(cmd, LOG_CONS, LOG_DAEMON);
- if (fd0 != 0 || fd1 != 1 || fd2 != 2) {
- syslog(LOG_ERR, "unexpected file descriptors %d %d %d",fd0, fd1, fd2);
- exit(1);
- }
- syslog(LOG_DEBUG, "daem ok ");
-
- }
- int main()
- {
- daemonize("test");
- while(1);//守护进程所要干的事情
-
- }
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