C Language and Libraries Help (clang.hlp) (
Table of Contents;
Topic list)
Important Notice
The pages on this site contain documentation for very old MS-DOS software,
purely for historical purposes.
If you're looking for up-to-date documentation, particularly for programming,
you should not rely on the information found here, as it will be woefully
out of date.
Controlling Stack and Heap Allocation
◄Up► ◄Contents► ◄Index► ◄Back►
─────C/C++ Language─────────────────────────────────────────────────────────
You can let the heap allocate memory from unused stack space by
linking your program with VARSTCK.OBJ. This file allows the near-
memory allocation functions to allocate items in unused stack
space if they run out of other memory. The near memory allocation
functions are _nmalloc, _nrealloc, _nexpand, and _ncalloc in all
memory models, and malloc, realloc, _expand, and calloc in small
data models (small and medium memory models).
Programs compiled and linked under Microsoft C/C++ run with a fixed
stack size (default size is 2K). The stack resides above static
data, and the near heap uses space left above the stack. However,
for some programs a fixed-stack model may not be ideal. Having the
stack and heap compete for space may be more appropriate. Use
VARSTCK.OBJ to do this. When the heap runs out of memory, it uses
available stack space until it reaches the top of the stack.
Be aware of the following:
■ The stack cannot grow beyond the last heap item allocated in
the stack or, if no heap items are in the stack, beyond the
size set at link time.
■ Once any part of stack memory is incorporated into the near
heap, it can never be used as stack space again.
■ While the heap can employ unused stack space, the stack
cannot take unused heap space.
When using VARSTCK.OBJ, be wary of suppressing stack checking,
which is done with the #check_stack pragma or with the /Gs or /Ox
option. Stack overflow can occur more easily in programs that
suppress stack checking, possibly causing errors that are
difficult to detect.
The following command line compiles TEST.C, then links the
resulting object module with VARSTCK.OBJ. Use the /NOE option to
avoid multiple symbol definitions. For example:
CL TEST.C VARSTCK /LINK /NOE
-♦-