守护进程及守护进程输出

#include <stdio.h>

int main(int argc, char** argv)
{
     if(argc == 1)
     {
          printf("only one parameter \n");
     }
     else if(argc == 2)
     {
          printf("%s \n", argv[1]);
     }
     else
     {
          printf("too more parameter: %d\n", argc);
     }

     return 0;
}

int daemon_init(const char* pname, int facility)
{
     umask(0);

     pid_t pid;

     if((pid = fork()) < 0)
          return -1;
     else if(pid > 0)
          _exit(0); // parent terminates

     // child 1 continues
    
     // become session leader
     if(setsid() < 0)
          return -1;

     // ignore SIGHUP
     // signal(SIGHUP, SIG_IGN);
     struct sigaction sa;
     sa.sa_handler = SIG_IGN;
     sigemptyset(&sa.sa_mask);
     sa.sa_flags = 0;

     if(sigaction(SIGHUP, &sa, nullptr) < 0)
          _exit(0);

     if((pid = fork()) < 0)
          return -1;
     else if(pid > 0)
          _exit(0);     // child 1 terminates

     // child 2 continues
    
     helpguy::daemon_flag = 1;

     // changes working directory
     chdir("/");

     // close off file descriptors
     rlimit rl;
     if(getrlimit(RLIMIT_NOFILE, &rl) < 0)
          return -1;

     int fdMax = rl.rlim_max == RLIM_INFINITY ? 1024 : rl.rlim_max;
     for(int i = 0; i < fdMax; ++i)
          close(i);

     // redirect stdin, stdout and stderr to /dev/null
     open("/dev/null", O_RDONLY);
     open("/dev/null", O_RDWR);
     open("/dev/null", O_RDWR);

     openlog(pname, LOG_PID, facility);

     return 0;
}

#include "helpguy.h"  // 含有daemon_init函数
#include <syslog.h>

int main(int argc, char** argv)
{
     // daemon process
     daemon_init(argv[0], 0);

     int count = 0;
     while(++count <= 2)
     {
          sleep(3);
         
          // notifice
          pid_t pid;
          if((pid = fork()) == 0)
          {
               // 子进程继承父进程的文件描述符
               // 在daemon_init中，父进程的stdout被重定向到/dev/null

               close(1);
               open("/dev/pts/0", O_WRONLY);
              
               // exec
               execl("/home/alex_my/Code/UNP/example_daemon_help", "example_daemon_help", "3 sec now", nullptr);
               // 也可以使用这个
               // execl("/bin/echo", "echo", "Clock 3", nullptr);
          }
         
          // parent
          waitpid(pid, nullptr, 0);
     }

     return 0;
}