Q47766: _amblksiz Affects Memory Allocation in Near and Far Heaps

Article: Q47766
Product(s): See article
Version(s): 5.10   | 5.10
Operating System(s): MS-DOS | OS/2
Keyword(s): ENDUSER | docerr | mspl13_c
Last Modified: 16-AUG-1989

The documentation on Page 33 of the "Microsoft C for the MS-DOS
Operating System: Run-Time Library Reference" states that adjusting
the value of the global variable _amblksiz has no effect on halloc or
_nmalloc() in any memory model.

This statement in the manual is incorrect. In fact, _amblksiz is used
to determine how much to expand both the near heap and the far heap
segments, and is used by calls to _nmalloc(). The only difference
between calls to _nmalloc() and _fmalloc is that _nmalloc() doesn't
actually request memory from DOS, it takes it from the pre-allocated
heap (in DOS).

However, since the the memory allocated by a call to halloc() is
maintained independently of the near and far heaps, the value of
_amblksiz does not affect calls to halloc().

The default value of _amblksiz is 8K (8192) bytes. The variable is
declared in malloc.h. For more information about the use of _amblksiz,
refer to the "Microsoft C for the MS-DOS Operating System: Run-Time
Library Reference," Section 3.2.

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