alt Hacker NewsI read the text as saying the object size can be less or equal to returned memory size. Anyway, section 7 is library. As you say, replacing through loading is a common thing to do — surely compiler is not free to simply elide external library function at will? This is not C++ after all, it must be sensible
If the function is equivalent to a no-op, and not explicitly marked as volatile for side-effects, it absolutely can elide it. If there is a side-effect in hardware or wider systems like the OS, then it must be marked as volatile. If the code is just code, then a function call that does effectively nothing, will probably become nothing.
That was one of the first optimisations we had, back with Fortran and COBOL. Before C existed - and as B started life as a stripped down Fortran compiler, the history carried through.
The K&R book describes the buddy system for malloc, and how its design makes it suitable for compiler optimisations - including ignoring a write to a pointer that does nothing, because the pointer will no longer be valid.