In UNIX and other multitasking operating systems, a daemon is a computer program that runs in the background, rather than under the direct control of a user; they are usually initiated as background processes. Typically daemons have names that end with the letter "d": for example,httpd, the daemon that handles the HTTP requirements, or sshd, which handles incoming SSH connections.

In a Unix environment, the parent process of a daemon is often (but not always) the init process (pid=1). Processes usually become daemons by forking a child process and then having their parent process immediately exit, thus causing init to adopt the child process. This is a somewhat simplified view of the process as other operations are generally performed, such as dissociating the daemon process from any controlling tty.

/*daemon.c*/

#include <stdio.h>
#include <time.h>
#include <unistd.h>
#include <signal.h>
#include <sys/param.h>
#include <sys/types.h>
#include <sys/stat.h>

void initDaemon(void)
{
    int pid;
    int i;

    /* terminate parent process */
    if(pid=fork())
        exit(0);
   
    /* exit when fork failed */
    else if(pid< 0)
        exit(1);

    /* continue only when it it the first child process
    ** run in a new session
    */
    setsid();

    /* seperate from terminator,kill the first child process */
    if(pid=fork())
        exit(0);
    else if(pid< 0)
        exit(1);

    /* close all file descriptors */
    for(i=0;i< NOFILE;++i)
        close(i);
    chdir("/home/djstava");

    /* reset file umask */
    umask(0);
    return;
}

void signalUSR1(int signal)
{
    /*handle SIGUSR1*/
    FILE *fp;
    time_t t;
    if((fp=fopen("log","a")) >=0)
    {
        t=time(0);
        fprintf(fp,"Received signal(%d),at %s\n", signal, asctime(localtime(&t)) );
        fclose(fp);
    }
}


int main(int argc, char ** argv)
{
    FILE *fp;
    time_t t;
    initDaemon();

    /* ignore signal */
    signal(SIGCHLD, SIG_IGN);

    /* handle SIGUSR1 */
    signal(SIGUSR1, signalUSR1);

    while(1)
    {
        sleep(60);
        if((fp=fopen("log","a")) >=0)
        {
            t=time(0);
            fprintf(fp,"djstava at %s\n",asctime(localtime(&t)) );
            fclose(fp);
        }
    }
}

Now compile the example and execute it like this.

gcc -o daemon daemon.c

./daemon

ps ax

sudo kill -s SIGUSR1 6942

cat /home/djstava/log

Finally,add a line to the file /etc/rc.local,and cp the executable file to the dir /bin.That's down,daemon will run when the system boots.

su - djstava -c "/bin/daemon"

References:

Unix Signals

djstava