Why is getchar_unlocked() faster in c/c++?

Question

I learnt recently that getchar_unlocked() is faster than the normal io operations such as scanf or cin, but what makes it this fast. What does thread unsafe mean in this context?

Answer 1

Comparison of getchar_unlocked (or plain getchar, for that matter) to cin or scanf is not an apples-to-apples: getchar function grabs a single character from the buffer, and returns it to you, while streams and format-based functions first need to decide what to return by interpreting the current state or the format string, and only then return it to you.

getchar_unlocked should be compared to getchar. It may indeed be faster if you call it multiple times, e.g. from a tight loop, because it lets you lock and unlock the stream buffer once per loop, rather than once per loop iteration.

In order to read from your input stream safely, you need to lock it before the read. When you call getchar, it does this:

• flockfile(fileptr)
• getc_unlocked(fileptr)
• funlockfile(fileptr)

If you call it in a loop ten times, you would see ten calls to flockfile, ten calls to getc_unlocked, and ten calls to funlockfile. If you know that you are reading ten characters, you could call flockfile once, make ten calls to getc_unlocked, and then call funlockfile. This would save you nine calls to each of the flock/funlock functions.