Does it? On my computer it's a builtin (__builtin_frexp). But even if it did use type-punning, that doesn't make it legal for your program to do it. I don't believe there's any requirements on the C standard library to follow the standard themselves.

To answer the second part of your response, the correct way to do this yourself would be to memcpy the double and then inspect the result directly. Any compiler worth its salt will end up generating the assembly code you want (namely, no call to memcpy, just a couple of arithmetic shifts and masks).

> To answer the second part of your response, the correct way to do this yourself would be to memcpy the double and then inspect the result directly. Any compiler worth its salt will end up generating the assembly code you want (namely, no call to memcpy, just a couple of arithmetic shifts and masks).

What does a compiler "worth its salt" do with the memcpy at "-O0"?

See for yourself: https://godbolt.org/z/nsg9nt