How does getchar_unlocked() work?

Question

My question is based on a CodeChef problem called Lucky Four.

This is my code:

int count_four() {
  int count = 0;
  char c = getchar_unlocked();
  while (c < '0' || c > '9')
    c = getchar_unlocked();
  while (c >= '0' && c <= '9') {
    if (c == '4')
      ++count;
    c = getchar_unlocked();
  }
  return count;
}

int main() {
  int i, tc;
  scanf("%d", &tc);
  for (i = 0; i < tc; ++i) {
    printf("%d\n", count_four());
  }
  return 0;
}

Let's say I make a slight change to count_four():

int count_four() {
  int count = 0;
  char c = getchar_unlocked();
  while (c >= '0' && c <= '9') {
    if (c == '4')
      ++count;
    c = getchar_unlocked();
  }
  while (c < '0' || c > '9') // I moved this `while` loop
    c = getchar_unlocked();
  return count;
}

This is my output after moving the while loop below the other one:

instead of:

The input used to test the program:

Why is this happening? How do getchar() and getchar_unlocked() work?

@RSahu Its given in the question itself follow the link any way its Input: 5 447474 228 6664 40 81 — Moinak Debnath, Feb 22 '15 at 20:40
It is better to put the sample input here rather than expecting others to follow the link to find it out. — R Sahu, Feb 22 '15 at 20:41
It works fine at http://ideone.com/jy8BwY after changing `getchar_unlocked` to just `getchar`. I don't know whether that is significant. — R Sahu, Feb 22 '15 at 21:15

score 1 · Answer 1 · answered Feb 22 '15 at 20:57

getchar_unlocked is just a lower level function to read a byte from the stream without locking it. In a single thread program, it behaves exactly like getchar().

Your change in the count_four function changes its behavior completely.

The original function reads the standard input. It skips non digits, causing an infinite loop at end of file. It then counts digits until it gets a '4'. The count is returned.

Your version reads the input, it skips digits, counting occurrences of '4', it then skips non digits, with the same bug on EOF, and finally returns the count.

How does getchar_unlocked() work?

1 Answers1