Variable size automatic variables are also useful in some cases. These were added i nC99 and have been supported in gcc for a long time.
void foo(uint32_t extraPadding) {
uint8_t commBuffer[sizeof(myProtocol_t) + extraPadding];
You end up with a buffer on the stack with room for the fixed-size protocol header plus variable size data. You can get the same effect with alloca(), but this syntax is more compact.
You have to make sure extraPadding is a reasonable value before calling this routine, or you end up blowing the stack. You'd have to sanity check the arguments before calling malloc or any other memory allocation technique, so this isn't really unusual.
