7. Daemon Process

I. Daemon programming model

1 . Create a child process, the parent process exits all work in the subprocess  formally out of control terminal 2. Create a new session in a child process the  setsid () function  makes the child process completely independent, out of control  3. Change the current directory to the root  chdir () function to  prevent the use of an unmounted file system  can also be replaced by another path 4. reset File Permission Mask  the Umask () function  prevents inherited file creation masks from denying certain permissions (eventually inherited file screen words are inherited from the shell process, preventing inherited file creation masks from denying certain permissions)  increasing daemon flexibility  5. Closing file Descriptor  inherited open files will not be used, wasting system resources, unable to uninstall 6. Start the daemon core work 7. Daemon Exit Processing

Two. Code Model

Writes the current time in/tmp/damon.log every 10s

#include <stdio.h>#include<stdlib.h>#include<unistd.h>#include<sys/stat.h>#include<sys/types.h>#include<fcntl.h>#include<errno.h>#include<time.h>#include<string.h>#include<dirent.h>voiddaemonsize () {pid_t pid; if(PID = fork ()) <0) {perror ("Fork"); Exit (1); } Else if(PID! =0) {exit (3);    } setsid (); Umask (0); if(ChDir ("/") <0) {perror ("chdir"); Exit (2); } Close (0); Open ("/dev/null", O_RDWR); Dup2 (0,1); Dup2 (0,2);}intMain () {daemonsize (); /*Write Current time To/tmp/dameon.log*/DIR*dir; if((Dir=opendir ("tmp")) ==NULL) {        if(errno = =ENOENT) {            if(MkDir ("./tmp",0777) <0) {perror ("mkdir"); Exit (4); }        } Else{perror ("Opendir"); Exit (6); }    }    intFD; Umask (0); FD= Open ("./tmp/daemon.log", O_creat | O_RDWR,0777); if(FD <0) {perror ("Open"); Exit (5); }    Charbuf[1024x768] = {0};  while(1) {time_t timep; if(Time (&AMP;TIMEP) <0) {perror (" Time"); Exit (6); }        structTM *Now ; now= LocalTime (&TIMEP); memset (BUF,0,sizeof(BUF)); Strftime (BUF,sizeof(BUF),"%y-%m-%d%h:%m:%s \ n", now);        Write (FD, buf, strlen (BUF)); Sleep (Ten); }    return 0;}

Open/tmp/damon.log File

2018-03-29 14:54:29
2018-03-29 14:54:39
2018-03-29 14:54:49
2018-03-29 14:54:59
2018-03-29 14:55:09
2018-03-29 14:55:19
2018-03-29 14:55:29
2018-03-29 14:55:39
2018-03-29 14:55:49
2018-03-29 14:55:59
2018-03-29 14:56:09
2018-03-29 14:56:19
2018-03-29 14:56:29
2018-03-29 14:56:39
2018-03-29 14:56:49
2018-03-29 14:56:59
2018-03-29 14:57:09
2018-03-29 14:57:19
2018-03-29 14:57:29
2018-03-29 14:57:39
2018-03-29 14:57:49
2018-03-29 14:57:59
2018-03-29 14:58:09
2018-03-29 14:58:19
2018-03-29 14:58:29
2018-03-29 14:58:39
2018-03-29 14:58:49
2018-03-29 14:58:59

7. Daemon Process