/**
 * Compile: cc daemonize.c -o daemonize
 *
 * Execute: daemonize <path to lockfile> <path to logfile> <command>
 */ 
#include <stdio.h>
#include <fcntl.h>
#include <signal.h>
#include <unistd.h>
#include <string.h>
#include <stdlib.h>
#include <unistd.h>

char *logfile;
char *pidfile;
char *command;

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

void signal_handler(int sig) {
        switch(sig) {
                case SIGHUP:
                        log_message("hangup signal caught\n");
                        break;
                case SIGTERM:
                        log_message("terminate signal caught\n");
                        exit(0);
                        break;
        }
}

void daemonize() {
        int i;
        int lfp;
        char str[1024];

        // START: Fork
        if ( getppid() == 1 ) {
                return; /* already a daemon */
        }
        i = fork();
        if ( i < 0 ) {
                exit(1); /* fork error */
        }
        if ( i > 0 ) {
                exit(0); /* parent exits */
        }
        setsid(); /* obtain a new process group */
        // STOP: Fork

        // START: Detach
        for ( i = getdtablesize(); i >= 0; --i ) {
                close(i); /* close all descriptors */
        }
        i = open( "/dev/null", O_RDWR ); 
        dup(i); 
        dup(i); 
        // STOP: Detach

        // SART: locking, pidfile
        umask(027); /* set newly created file permissions */
        lfp = open( pidfile, 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 */
        // STOP: locking, pidfile

        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 */

        sprintf(str, "Daemon Started: %s (%d)\n", command, getpid());

        log_message(str);
}

int main( int argc, char *argv[] ) {
        if ( argc < 4 ) { 
                printf("Usage:\t%s pidfile logfile executable\n", argv[0]);
                return 1;
        }

        pidfile = argv[1];
        logfile = argv[2];
        command = argv[3];
        daemonize();

        FILE *fpipe;
        char str[1024];
        if ( !(fpipe = (FILE*)popen(command,"r")) ) {
                sprintf( str, "Unable to execute command via pipe: %s\n", command );
                log_message( str );
                return 1;
        }
        while ( fgets( str, sizeof str, fpipe)) {
                log_message( str );
        }
        pclose(fpipe);
        return 0;
}