Detect 64-bits in C using size_t

Is sizeof(size_t)==8 the equivalent of saying that the platform is 64-bits ? Conversely, is sizeof(size_t)==4 the equivalent of saying that the platform is 32-bits ?

More importantly, is this test safe and reliable under all circumstances, keeping in mind OS and compilers portability ? Are there some weird corner-cases, including potential situations where size_t might be missing ?

I'm a bit worried that size_t might only be guaranteed for C99 environments.