// Ivan Novick
// Aug 26, 2009
// Advisory Locking on a file
#include <fcntl.h>
#include <sys/stat.h>
#include <stdio.h>

#define BUFSIZE 64

int main(int argc, char** argv){

    int fd = open("data_file", O_RDWR|O_CREAT, S_IRUSR|S_IWUSR);
    if (fd < 0) {
        fprintf(stderr, "failed opening file with error code %d\n", fd);
        return 1;
    }

    pid_t pid = getpid();
    char buf[BUFSIZE] = {0}; // initialize to all zeros
    sprintf(buf, "%u\n", pid);

    struct flock lock;
    lock.l_whence = SEEK_SET;
    lock.l_start = 0;
    lock.l_len = 0;
    lock.l_pid = 0;

    int i;
    for (i = 0; i < 10; ++i){

        // lock the file
        lock.l_type = F_WRLCK;
        if (fcntl(fd, F_SETLKW, &lock) == -1){
            fprintf(stderr, "error taking write lock on the file\n");
            return 1;
        }
    
        fprintf(stdout, "file is locked for writing by %u\n", pid);
        fflush(stdout);

        // just write the pid id in the beginning of the file
        lseek(fd, 0, SEEK_SET);
        write(fd, buf, BUFSIZE);
    
        sleep(5); // sleep for demo purpose to show the file is locked by this process
    
        // unlock the file
        lock.l_type = F_UNLCK;
        if (fcntl(fd, F_SETLKW, &lock) == -1){
            fprintf(stderr, "error unlocking the file\n");
            return 1;
        }
    
        fprintf(stdout, "file is unlocked by %u\n", pid);
        fflush(stdout);

        sleep(1);
    }

    return 0;
}