Re: 80286 protected mode

Thomas Koenig <tkoenig@netcologne.de> writes:

MitchAlsup1 <mitchalsup@aol.com> schrieb:
>

The paragraaph with 3 >'s indicates malloc() cannot be written
in std.  C.  It used to be written in std.  K&R C.  I am not asking
if it is still in the std libraries, I am asking what happened
to make it impossible to write malloc() in std.  C ?!?

>
You need to reserve memory by some way from the operating system,
which is, by necessity, outside of the scope of C (via brk(),
GETMAIN, mmap() or whatever).

Right.  And that is why malloc(), or some essential internal component
of malloc(), has to be platform specific, and thus malloc() must be
supplied by the implementation (which means both the compiler and the
standard library).

But more problematic is the implementation of free() without knowing
how to compare pointers.

Once there is a way to get additional memory from whatever underlying
environment is there, malloc() and free() can be implemented (and I
believe most often are implemented) without needing to compare
pointers.  Note:  pointers can be tested for equality without having
to compare them relationally, and testing pointers for equality is
well-defined between any two pointers (which may need to be converted
to 'void *' to avoid a type mismatch).