title: How to unlock using lockf()?
body: <p>I want to read from txt file and append the next number in it, I want to use fork as well to work as a second process. In following code, I need help to unlock the file. I am unable to unlock the file.</p>
  
  <pre><code>int main() {
    int x;
    pid_t child = fork();
    FILE *file;
    //flock(fileno(file),LOCK_EX);
    file = fopen ("list.txt", "r");
    //printf("file is locked");
    int fdSource = (int)file;
    if (fdSource &gt; 0){
      if (lockf(fdSource, F_LOCK, 0) == -1)
         x = readValue(file);
         return 0; /* FAILURE */
      }
      else {
        return 1;
      }
      if (lockf(fdSource, F_ULOCK, 0) == -1){
         printf("file is not lock");
         appendValue(x);
      }
      else {
         return 1;
      }
      appendValue(x);
  }
  </code></pre>