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Asked: 2026-06-17T17:36:06+00:00

I have a daemon in my Linux OS, and I want to pass parameters

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I have a daemon in my Linux OS, and I want to pass parameters to this daemon from another C file which is daemon controller while the daemon’s running.

I search it with google, but nothing in my hands until now. Could you give some tricks and may be an example to do that?

The daemon I have is copy/paste from this web site: http://www.enderunix.org/docs/eng/daemon.php

/*
UNIX Daemon Server Programming Sample Program
Levent Karakas <levent at mektup dot at> May 2001

To compile: cc -o exampled examped.c
To run:     ./exampled
To test daemon: ps -ef|grep exampled (or ps -aux on BSD systems)
To test log:    tail -f /tmp/exampled.log
To test signal: kill -HUP `cat /tmp/exampled.lock`
To terminate:   kill `cat /tmp/exampled.lock`
*/

#include <stdio.h>
#include <fcntl.h>
#include <signal.h>
#include <unistd.h>

#define RUNNING_DIR "/tmp"
#define LOCK_FILE   "exampled.lock"
#define LOG_FILE    "exampled.log"

void log_message(filename,message)
char *filename;
char *message;
{
FILE *logfile;
    logfile=fopen(filename,"a");
    if(!logfile) return;
    fprintf(logfile,"%s\n",message);
    fclose(logfile);
}

void signal_handler(sig)
int sig;
{
    switch(sig) {
    case SIGHUP:
        log_message(LOG_FILE,"hangup signal catched");
        break;
    case SIGTERM:
        log_message(LOG_FILE,"terminate signal catched");
        exit(0);
        break;
    }
}

void daemonize()
{
int i,lfp;
char str[10];
    if(getppid()==1) return; /* already a daemon */
    i=fork();
    if (i<0) exit(1); /* fork error */
    if (i>0) exit(0); /* parent exits */
    /* child (daemon) continues */
    setsid(); /* obtain a new process group */
    for (i=getdtablesize();i>=0;--i) close(i); /* close all descriptors */
    i=open("/dev/null",O_RDWR); dup(i); dup(i); /* handle standart I/O */
    umask(027); /* set newly created file permissions */
    chdir(RUNNING_DIR); /* change running directory */
    lfp=open(LOCK_FILE,O_RDWR|O_CREAT,0640);
    if (lfp<0) exit(1); /* can not open */
    if (lockf(lfp,F_TLOCK,0)<0) exit(0); /* can not lock */
    /* first instance continues */
    sprintf(str,"%d\n",getpid());
    write(lfp,str,strlen(str)); /* record pid to lockfile */
    signal(SIGCHLD,SIG_IGN); /* ignore child */
    signal(SIGTSTP,SIG_IGN); /* ignore tty signals */
    signal(SIGTTOU,SIG_IGN);
    signal(SIGTTIN,SIG_IGN);
    signal(SIGHUP,signal_handler); /* catch hangup signal */
    signal(SIGTERM,signal_handler); /* catch kill signal */
}

main()
{
    daemonize();
    while(1) sleep(1); /* run */
}

/* EOF */
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1 Answer

  1. Editorial Team

    You don’t state clearly whether the controller is a related process (started from a common parent) or an unrelated process. If it’s related, you could arrange to have a pipe between the controller and the daemon; if not, you could use a Unix domain socket (see manpage unix(7)) or a FIFO (mknod filename p). Either way, replace the “sleep” call with a block that includes a call to “select” or “poll”, looking for data on the control-stream and any other file or socket of interest, and if data is available read it, parse it, and act on it.

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