watching_files.cpp

// Example of how to watch files on BSD/OSX, with a timer thrown in for fun
#define MAIN_WATCH "file1"
#define ALTERNATING_WATCH "file2"

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

struct MyWatchContext {
    char* path;
    int fd;
};

void add_watch(int queue_fd, MyWatchContext* ctx) {
    ctx->fd = open(ctx->path, O_EVTONLY);
    assert(ctx->fd > 0);
    struct kevent queue_change;
    printf("add %d\n", ctx->fd);
    EV_SET(&queue_change, ctx->fd, 
           EVFILT_VNODE, EV_ADD | EV_CLEAR | EV_RECEIPT, 
           NOTE_DELETE | NOTE_WRITE  | NOTE_RENAME | NOTE_REVOKE,
           0, ctx);
    int kr = kevent(queue_fd, &queue_change, 1, NULL, NULL, NULL);
    assert(kr==0);
}
void del_watch(MyWatchContext* ctx) {
    close(ctx->fd);
}
void add_timer(int queue_fd, int id, int ms) {
    struct kevent queue_change;
    EV_SET(&queue_change, id, 
           EVFILT_TIMER, EV_ADD | EV_ONESHOT | EV_RECEIPT, 0, ms, NULL);
    int kr = kevent(queue_fd, &queue_change, 1, NULL, NULL, NULL);
    assert(kr==0);
}

int main(int argc, char *argv[]) {
    int queue_fd = kqueue();
    assert(queue_fd > 0);
    
    MyWatchContext main_watch_ctx = { MAIN_WATCH        };
    MyWatchContext alt_watch_ctx  = { ALTERNATING_WATCH };
    bool alt_status = false;
    
    add_watch(queue_fd, &main_watch_ctx);
    add_watch(queue_fd, &alt_watch_ctx);
    add_timer(queue_fd, 10, 10000);
    
    for (;;) {
        struct kevent event_to_handle = {};
        int r1 = kevent(queue_fd, NULL, 0, &event_to_handle, 1, NULL);
        assert(r1 == 1);
        if (event_to_handle.udata == NULL) {
            assert(event_to_handle.ident == 10);
            printf("got_timer\n");
            if (alt_status) {
                add_watch(queue_fd, &alt_watch_ctx);
            } else {
                del_watch(&alt_watch_ctx);
            }
            alt_status = !alt_status;
            add_timer(queue_fd, 10, 10000);
        } else {
            MyWatchContext*ctx = (MyWatchContext *)event_to_handle.udata;
            printf("Got event for file: %s\n", ctx->path);
        }
    }
    return 0;
}