Indeed, you're breaking the rules. If you made I_test a global variable, your program would probably segfault as it attempted to write to protected memory! But since it's a stack variable it can't protect it from roundabout tampering like this.

What's probably happening is the compiler is going "Hmm, since it's a constant, I don't have to actually *read* it every time -- it won't change. I'll just keep it in a CPU register to save time." So when you change the memory, the copy in the register doesn't change with it.

The const specifier is more a reminder to the programmer, to tell them that they're really not supposed to be writing to some memory from this function. The truly determined might get around that with pointers and typecasting, but if you made it const in the first place you probably had a reason.