守护进程

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#include <stdio.h>

#include <stdlib.h>
#include <unistd.h>       //open()
#include <signal.h>       //sigaction()
#include <fcntl.h>
#include <syslog.h>       //openlog()
#include <stdarg.h>       //openlog() arguments
#include <sys/time.h>       //getrlimit()
#include <sys/resource.h>   //getrlimit()
#include <sys/stat.h>       //umask()
#include <sys/types.h>       //umask
#include <string.h>
#include <errno.h>

void daemonize(const char *cmd);

int main(int argc, char *argv[])
{
   fprintf(stdout,
           "parent id:%d, parent group id: %d, parent session id: %d\n",
           getpid(), getgid(), getsid());

   daemonize("daemon");

   return 0;
}

void daemonize(const char *cmd)
{
   int i, fd0, fd1, fd2;
   pid_t pid;
   struct rlimit rl;
   struct sigaction sa;

   /*
   * Clear file creation mask; 创建守护进程第一步，设置文件创建屏蔽字
   * */
   umask(0);

   /*
   * Get maximum number of file descriptors
   * */
   if (getrlimit(RLIMIT_NOFILE, &rl) < 0)
   {
       fprintf(stderr, "%s can't get file limit.%s\n", cmd, strerror(errno));
       exit(EXIT_FAILURE);
   }

   /*
   * Become a session leader to lose controling TTY. 创建守护进程第1.5步，创建子进程，父进程退出
   * */
   if ((pid = fork()) < 0)
   {
       fprintf(stderr, "%s can't fork:%s", cmd, strerror(errno));
       exit(EXIT_FAILURE);
   } else if (pid != 0)
   {
       //parent exit
       exit(0);
   }
//创建守护进程第3步，将子进程加入新会话，
   setsid();

   /*
   * Ensure future opens won't allocate controlling TTYs. 忽略SIGHUP信号，此信号是终端创建和退出时发生的
   * */
#if 0
   struct sigaction
   {
       void (*sa_handler) (int);
       void (*sa_sigaction) (int, siginfo_t *, void *);
       sigset_t sa_mask;
       int sa_flags;
       void (*sa_restorer) (void);
   };
#endif
   sa.sa_handler = SIG_IGN;
   sigemptyset(&sa.sa_mask);
   sa.sa_flags = 0;
   if(sigaction(SIGHUP, &sa, NULL) < 0)
   {
       fprintf(stdout, "%s can't ignore SIGUP\n", cmd);

exit(EXIT_FAILURE);

}

if((pid = fork()) < 0)   //创建守护进程第2步，创建子进程，父进程退出
   {
       fprintf(stderr, "%s can't fork:%s", cmd, strerror(errno));
       exit(EXIT_FAILURE);
   }
   else if(pid != 0)
   {
       //parent exit.
       exit(0);
   }

   /*
   * Change the current workint directory to the root so we won't prevent
   * file systems from begin unmounted.创建守护进程第4步，改变工作路径到根，预防文件系统卸载
   * */
   if(chdir("/") < 0)
   {
       fprintf(stderr, "%s: can't change directory to /\n", strerror(errno));
       exit(EXIT_FAILURE);
   }

   /*
   * Close all open file descriptors, RLIM_INFINITY is 资源无限制，创建守护进程第5步，关闭所有的文件描述符
   * */
   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.创建守护进程第6步，重定向0，1，2到/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, "unexcptioned file descriptors %d %d %d",fd0, fd1, fd2);
       exit(1);
   }
}