Daemonizing processes on Linux

Process and peripheral concepts

TTY (virtual terminal) | -Session | -Process group | -Process

"TTY = session 1> N process group 1> N process"

One of the processes is a session leader, and several of them are process group leaders.

session

A login session associated with a TTY. It is a group of one or more process groups, and the process that becomes the session leader has the role of interacting with the control terminal and terminating all child processes when the terminal is disconnected. Process groups can only be created within the session to which they belong.

Process group

A collection of processes that can send signals to other processes. The member has the process ID of the process group leader as the group ID. Call setpgid () or setsid () to become the process group leader.

fork() Calling this function duplicates the process. If the return value is 0, it is processed by the child process, and if the return value is 0 or more, it is processed by the parent process. The two run at the same time with the same source code. Therefore, it is necessary to branch with an if statement and write two source codes, one for the parent process and one for the child process.

Implementation

#include <iostream>
#include <sys/types.h>
#include <sys/stat.h>

#include <fcntl.h>
#include <signal.h>
#include <stdlib.h>
#include <sysexits.h>
#include <unistd.h>
#include <syslog.h>

#define MAXFD 64

int demonize(int chdir_flg){

    int i = 0;
    int fd = 0;
    pid_t pid = 0;


    //------------------------------------------------------
    //Status.
    //Parent process when the program starts(shell)Have a TTY.
    //------------------------------------------------------
    
    
    //------------------------------------------------------
    //Create a child process to detach the parent and return control to the shell.
    //------------------------------------------------------
    if((pid = fork()) == -1){

        return -1;

    //Parent process terminates after creating child process.
    }else if(pid != 0){

        //Don't call user-defined cleanup functions_exit()I do.
        _exit(0);
    }
    
    
    //------------------------------------------------------
    //Status.
    //Only child processes, but still retain TTY.
    //------------------------------------------------------
    

    //------------------------------------------------------
    //Separate TTY to become session leader and process group leader.
    //------------------------------------------------------
    setsid();
    

    //------------------------------------------------------
    //Ignore HUP signal.
    //Because a HUP signal can be sent to the child when the parent dies.
    //------------------------------------------------------
    signal(SIGHUP, SIG_IGN);
    

    //------------------------------------------------------
    //Status.
    //Since it is a session leader at this rate, if you open a TTY, that TTY will be associated..
    //------------------------------------------------------
    
    
    //------------------------------------------------------
    //Parent process(Session group leader)To disconnect.
    //Becoming a parentless, tty, and session leader.
    //------------------------------------------------------
    if((pid = fork()) == 0){
        //End child process.
        _exit(0);
    }
    


    //------------------------------------------------------
    //Prepare to run as a daemon.
    //------------------------------------------------------

    //Change current directory.
    if(chdir_flg == 0){
        
        //Move to root directory.
        chdir("/");
    }
    

        
    //Close all file descriptors inherited from parent.
    for(i = 0; i < MAXFD; i++){
        close(i);
    }
        
    // stdin,stdout,stderr dev/Open with null.
    //If you just close the descriptor, these outputs will result in an error, so dev/Redirect to null.
    if((fd = open("/dev/null", O_RDWR, 0) != -1)){
        
        //File descriptor replication.
        //0 of this process,1,Dev pointed to 2 by fd/Point to null.
        dup2(fd, 0);
        dup2(fd, 1);
        dup2(fd, 2);
        if(fd < 2){
            close(fd);
        }
    }
    
    return (0);

}

int main(int argc, const char * argv[]) {

    char buf[256];
    demonize(1);
    
    //View current directory.
    syslog(LOG_USER | LOG_NOTICE, "demon:cwd=%s¥n", getcwd(buf, sizeof(buf)));
    
    return 0;
}

It can be daemonized just by calling demonstrate () with the main () function. If you write a command to start a daemonized program in "/etc/rc.local", you can start it in the background when the server starts.

reference

["Linux Network Programming Bible"](http://www.amazon.co.jp/Linux Network Programming Bible-Mitsuyuki Omata-ebook/dp/B00O8GIL62/ref=sr_1_1?ie=UTF8&qid=1429337305&sr=8-1&keywords= linux + network programming bible) Scyphus Draft — How to make a daemon