Bit manipulation in C - builtin function

Question

Is there any builtin function in C that returns me the first bit equals to zero in a 32-bit integer?

I know I can check all 32-bit using a for-loop:

value <--- parameter (uint32_t)
for (int i=0; i<32; i++){
  uint32_t pos = 1 << i;
  if (pos ^ value) return i;  // xor
}
return -1;

In gcc you can use: [__builtin_ctz](http://gcc.gnu.org/onlinedocs/gcc-4.5.4/gcc/Other-Builtins.html) — Piotr Praszmo, Oct 27 '14 at 17:51
There are no built-ins in C. Can you specify a compiler/platform? — mafso, Oct 27 '14 at 18:05
You can use http://aggregate.org/MAGIC/#Leading%20Zero%20Count — barak manos, Oct 27 '14 at 18:07

score 2 · Accepted Answer · answered Oct 27 '14 at 18:22

Unfortunately there's no standardized C function to do this, but many C compilers offer one of their own. When you use GCC it's the _builtin_ctz function. When you use Microsoft Visual C it's the _BitScanForward function.

If you want to write programs that compile on multiple different compilers then you end up using a lot of #ifdef statements to provide the same thing in different ways. (I keep hoping the C committee will mandate one of them already!)

Bit manipulation in C - builtin function

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