> In fact, when I started with C, I did not use a C89 one, if we weren't certain what a certain construct did, we'd try it on a compiler and use the result as a

> In fact, when I started with C, I did not use a C89 one, if we weren't certain what a certain construct did, we'd try it on a compiler and use the result as a definition for what that construct did on that compiler! This still doesn't mean that the behavior was implementation defined. Implementation defined requires the compiler vendor to document the behavior. However implementers are allowed to define and even document the behavior for operations that the standard leaves undefined. These are called extensions. It's common practice to have some extensions even now, sometimes behind specific compiler flags, or just being documented. I think it's a mistake to infer the compiler's behavior through trying a few examples, even for simple compilers. If you want to have guarantees for the behavior for otherwise undefined constructs then get it in writing from the compiler vendors. Even if you check the source code, the behavior might change in a future version.