watch_files.c

#include <sys/event.h>
#include <sys/time.h>
#include <fcntl.h>
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>

// Watches files for changes and prints which file changed
int main(int argc, char *argv[]) {
    if (argc < 2) {
        fprintf(stderr, "Usage: %s <file1> [file2] ...\n", argv[0]);
        exit(1);
    }

    int nfiles = argc - 1;

    // Create the kqueue
    int kq = kqueue();
    if (kq == -1) {
        perror("kqueue");
        exit(1);
    }

    // Open all files and set up change events
    int *fds = malloc(nfiles * sizeof(int));
    struct kevent *changes = malloc(nfiles * sizeof(struct kevent));
    if (!fds || !changes) {
        perror("malloc");
        exit(1);
    }
    for (int i = 0; i < nfiles; i++) {
        fds[i] = open(argv[i + 1], O_RDONLY | O_EVTONLY);
        if (fds[i] == -1) {
            fprintf(stderr, "open(%s): ", argv[i + 1]);
            perror(NULL);
            exit(1);
        }
        printf("Monitoring '%s' (fd %d)\n", argv[i + 1], fds[i]);
        EV_SET(
            &changes[i],
            fds[i],
            EVFILT_VNODE,
            EV_ADD | EV_CLEAR,
            NOTE_WRITE,
            0,
            // Store the filename as udata so we can identify it later
            (void *)argv[i + 1]
        );
    }

    // Register all events at once
    if (kevent(kq, changes, nfiles, NULL, 0, NULL) == -1) {
        perror("kevent register");
        exit(1);
    }

    printf("Waiting for changes...\n");

    struct kevent event;
    while (1) {
        // Wait for one event at a time
        int n = kevent(kq, NULL, 0, &event, 1, NULL);
        if (n == -1) {
            perror("kevent wait");
            break;
        }
        if (event.fflags & NOTE_WRITE) {
            const char *name = (const char *)event.udata;
            printf("[%s] written\n", name);
        }
    }

    // Cleanup
    for (int i = 0; i < nfiles; i++)
        close(fds[i]);
    free(fds);
    free(changes);
    close(kq);
    return 0;
}