How to write a daemon process?

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
#include
#include
#include
#include
#include
#include

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

SIGHUP	1	Exit	Hangup
SIGINT	2	Exit	Interrupt
SIGQUIT	3	Core	Quit
SIGILL	4	Core	Illegal Instruction
SIGTRAP	5	Core	Trace/Breakpoint Trap
SIGABRT	6	Core	Abort
SIGEMT	7	Core	Emulation Trap
SIGFPE	8	Core	Arithmetic Exception
SIGKILL	9	Exit	Killed
SIGBUS	10	Core	Bus Error
SIGSEGV	11	Core	Segmentation Fault
SIGSYS	12	Core	Bad System Call
SIGPIPE	13	Exit	Broken Pipe
SIGALRM	14	Exit	Alarm Clock
SIGTERM	15	Exit	Terminated
SIGUSR1	16	Exit	User Signal 1
SIGUSR2	17	Exit	User Signal 2
SIGCHLD	18	Ignore	Child Status
SIGPWR	19	Ignore	Power Fail/Restart
SIGWINCH	20	Ignore	Window Size Change
SIGURG	21	Ignore	Urgent Socket Condition
SIGPOLL	22	Ignore	Socket I/O Possible
SIGSTOP	23	Stop	Stopped (signal)
SIGTSTP	24	Stop	Stopped (user)
SIGCONT	25	Ignore	Continued
SIGTTIN	26	Stop	Stopped (tty input)
SIGTTOU	27	Stop	Stopped (tty output)
SIGVTALRM	28	Exit	Virtual Timer Expired
SIGPROF	29	Exit	Profiling Timer Expired
SIGXCPU	30	Core	CPU time limit exceeded
SIGXFSZ	31	Core	File size limit exceeded
SIGWAITING	32	Ignore	All LWPs blocked
SIGLWP	33	Ignore	Virtual Interprocessor Interrupt for Threads Library
SIGAIO	34	Ignore	Asynchronous I/O

djstava