Daemon Processes

Daemons are processes that live for a long time.They are often started when the system is bootstrapped and terminate only when the system is shut down,they run in the background.Unix systems have numerous daemons that perform day-to-day activities.

 

 

 

★ Characteristics

 

(1) run with superuser privilege(most of the daemons,a use ID of 0)

(2) without contronlling terminal (terminal name is set to a queston mask,tty=? )

(3) the terminal foreground process group is -1

(4) All the user-level daemons are process group leaders and session leaders and are the only processes in their process group and session.

(5) most of these daemons is the init process

 

★ Coding Rules

(1) call umask to set the file mode creation mask to 0

(2) call fork to create child process and have the parent exit

(3) call setsid to create a new session

(4) change the current directory to the root directory

(5) close the unneeded file descriptors

 

★ Example

#include<stdio.h>
#include<stdlib.h>
#include<fcntl.h>
#include<sys/wait.h>
#include<sys/types.h>
#include<unistd.h>
#include<sys/resource.h>

int main()
{
    pid_t pid;
    struct rlimit r1;
    int i;

    //clear the file creation mask
    umask(0);

    //get maximum number of file descriptors
    if(getrlimit(RLIMIT_NOFILE,&r1) < 0)
        printf("can't get file limit\n");

    pid = fork();
    if(pid < 0)
        printf("fork error\n");
    else if(pid > 0)
        exit(0);

    setsid();

    //change directory to root directory
    if(chdir("/") < 0)
        printf("can't change directory\n");

    //close all open file descriptors
    if(r1.rlim_max == RLIM_INFINITY)
        r1.rlim_max = 1024;
    for(i = 1;i < r1.rlim_max; i++)
        close(i);
}