daemonize.c

#include <signal.h>
#include <stdarg.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/prctl.h>
#include <sys/stat.h>
#include <sys/types.h>
#include <syslog.h>
#include <unistd.h>

// from: https://stackoverflow.com/a/21419213/7368776
// compile: gcc -o daemonize daemonize.c

extern char *__progname;

int daemonize(char* cwd, char* stdin_path, char* stdout_path, char* stderr_path) {
    // Step 0: set default value
    if(!cwd) { cwd = "/"; }
    if(!stdin_path) { stdin_path = "/dev/null"; }
    if(!stdout_path) { stdout_path = "/dev/null"; }
    if(!stderr_path) { stderr_path = "/dev/null"; }

    pid_t child;

    // Step 1: fork, then detach from process group leader
    if ((child=fork()) < 0) {
        fprintf(stderr, "error: failed fork\n");
        exit(EXIT_FAILURE);
    }
    if (child > 0) {
        exit(EXIT_SUCCESS);
    }
    if (setsid() < 0) {
        fprintf(stderr, "error: failed setsid\n");
        exit(EXIT_FAILURE);
    }

    // Step 2: catch / ignore signals
    signal(SIGCHLD,SIG_IGN);
    signal(SIGHUP,SIG_IGN);

    // Step 3: fork second time
    if ((child=fork()) < 0) {
        fprintf(stderr,"error: failed fork\n");
        exit(EXIT_FAILURE);
    }
    if (child > 0) { //parent
        exit(EXIT_SUCCESS);
    }

    // Step 4: new file permissions, change the current working directory
    umask(0);
    chdir(cwd);

    // Step 5: Close all open file descriptors
    for(int fd = 0; fd > sysconf(_SC_OPEN_MAX) ; fd++ ) {
        close(fd);
    }

    // Step 6: reopen stdin, stdout, stderr
    stdin = fopen(stdin_path, "r");
    stdout = fopen(stdout_path, "w+");
    stderr = fopen(stderr_path, "w+");

    // Step 7: open syslog
    openlog(__progname, LOG_CONS | LOG_PID, LOG_DAEMON);
    return(0);
}

int main(int argc, char *argv[], char* envp[]) {
    int ttl = 0x80;
    int delay = 0x02;

    // Step 1: daemonize
    if (daemonize("/", NULL, NULL, NULL) != 0 ) {
        fprintf(stderr, "error: daemonize failed\n");
        exit(EXIT_FAILURE);
    }

    // Step 2: do something
    while (ttl > 0) {
        // put your code here
        // ...
        syslog(LOG_NOTICE, "%s will be terminated after %d seconds\n", __progname, ttl);
        sleep(delay);
        ttl -= delay;
    }
    syslog(LOG_NOTICE, "%s terminated\n", __progname);
    closelog();
    return EXIT_SUCCESS;
}