Daniel Lemire's blog

, 1 min read

For case-insensitive string comparisons, avoid char-by-char functions

Sometimes we need to compare strings in a case-insensitive manner. For example, you might want ‘abc’ and ‘ABC’ to be considered. It is a well-defined problem for ASCII strings. In C/C++, there are basically two common approaches. You can do whole string comparisons:

bool isequal = (strncasecmp(string1, string2, N) == 0);

Or you can do character-by-character comparisons, mapping each and every character to a lower-case version and comparing that:

bool isequal{true};
for (size_t i = 0; i < N; i++) {
      if (tolower(string1[i]) != tolower(string2[i])) {
        is_the_same = false;

Intuitively, the second version is worse because it requires more code. We might also expect it to be slower. How much slower? I wrote a quick benchmark to test it out:

strncasecmp Linux/GNU GCC macOS/LLVM
strncasecmp 0.15 ns/byte 1 ns/byte
tolower 4.5 ns/byte 4.0 ns/byte

I got these results with GNU GCC under Linux. And on a different machine running macOS.

So for sizeable strings, the character-by-character approach might be 4 to 40 times slower! Results will vary depending on your standard library and of the time of the day. However, in all my tests, strncasecmp is always substantially faster. (Note: Microsoft provides similar functions under different names, see _strnicmp for example.)

Could you go faster? I tried rolling my own and it runs at about 0.3 ns/byte. So it is faster than the competition under macOS, but slower under Linux. I suspect that the standard library under Linux must rely on a vectorized implementation which might explain how it beats me by a factor of two.

I bet that if we use vectorization, we can beat the standard librairies.


My code is available.