Q61969: Changes to malloc() and the Rover Pointer Behavior in C 6.00

Article: Q61969
Product(s): See article
Version(s): 6.00   | 6.00
Operating System(s): MS-DOS | OS/2
Keyword(s): ENDUSER | s_quickc | mspl13_c
Last Modified: 11-JUL-1990

Many changes and additions have been made in C version 6.00 concerning
the heap and heap management. The heap code is completely rewritten
and, in addition to the new based heap capabilities and new heap
minimization routines, the malloc() function has undergone several
important changes. These changes include new malloc() rover pointer
behavior in the near heap and a different malloc() allocation scheme
in the far heap.

The internal data layout for the heap is basically the same in C 6.00
as it is in C 5.10, but all code and control structures are new. In
the near heap, the malloc() rover pointer is now reset after each
allocation, so freed memory is always allocated first if enough is
available to meet a new request. In C 5.10, the rover pointer only
moves forward, so freed memory is used only after the pointer wraps
around to the beginning of the heap after it reaches the end of the
current memory block.

The near heap (in DGROUP) is where programs are usually limited in
space, so it is considered a viable trade-off to change the rover
logic to conserve space, rather than maximize speed. For the far and
based heaps, we still do not reset the rover pointer, since resetting
it there would take considerably more code than in the near heap,
resulting in bigger programs and slower execution speed.

A change concerning the far heap is that an attempt is no longer made
to allocate space on the near heap when far memory is not available.
In C 5.10, if the far heap does not have enough free memory to satisfy
a memory request, then the near heap is checked to see if it could
satisfy the request. In C 6.00, the near heap is not checked, so a
null pointer is now returned if far memory is not available, even
though enough near memory may be free.

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