System Calls Fundamentals Questions

#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <fcntl.h>
#include <sys/stat.h>
#include <sys/types.h>
#include <sys/file.h>
#include <signal.h>
#include <string.h>
#include <errno.h>

static int lockfd = -1;

int lock_pidfile(const char *path) {
    int fd = open(path, O_RDWR | O_CREAT, 0640);
    if (fd < 0) return -1;
    struct flock fl = {0};
    fl.l_type = F_WRLCK;
    fl.l_whence = SEEK_SET;
    fl.l_start = 0;
    fl.l_len = 0; // lock whole file
    if (fcntl(fd, F_SETLK, &fl) < 0) {
        close(fd);
        return -1; // already locked or error
    }
    // truncate and write pid
    if (ftruncate(fd, 0) == 0) {
        char buf[32];
        snprintf(buf, sizeof(buf), "%ld\n", (long)getpid());
        write(fd, buf, strlen(buf));
        lseek(fd, 0, SEEK_SET);
    }
    lockfd = fd; // keep open to maintain lock
    return 0;
}

void daemonize(const char *pidfile) {
    pid_t pid;

    // Optional: catch SIGCHLD, SIGHUP appropriately in caller

    // First fork
    pid = fork();
    if (pid < 0) exit(EXIT_FAILURE);
    if (pid > 0) _exit(EXIT_SUCCESS); // parent exits

    // Become session leader and detach from terminal
    if (setsid() < 0) exit(EXIT_FAILURE);

    // Second fork to prevent reacquiring a controlling terminal
    pid = fork();
    if (pid < 0) exit(EXIT_FAILURE);
    if (pid > 0) _exit(EXIT_SUCCESS);

    // Clear file mode creation mask and change working directory
    umask(0);
    if (chdir("/") < 0) exit(EXIT_FAILURE);

    // Close inherited file descriptors and redirect std fds to /dev/null
    for (int fd = sysconf(_SC_OPEN_MAX); fd >= 0; fd--) {
        if (fd == lockfd) continue; // preserve lock if already open (unlikely here)
        close(fd);
    }
    int devnull = open("/dev/null", O_RDWR);
    if (devnull >= 0) {
        dup2(devnull, STDIN_FILENO);
        dup2(devnull, STDOUT_FILENO);
        dup2(devnull, STDERR_FILENO);
        if (devnull > 2) close(devnull);
    }

    // Acquire single-instance lock after becoming daemon to avoid race with parent exit
    if (lock_pidfile(pidfile) < 0) {
        // cannot obtain lock — another instance is running
        exit(EXIT_FAILURE);
    }
    // Daemon is now running; continue with service initialization...
}