Q43166: C: How Stack Checking Is Done

Article: Q43166
Product(s): See article
Version(s): 5.10   | 5.10
Operating System(s): MS-DOS | OS/2
Keyword(s): ENDUSER | | mspl13_c
Last Modified: 22-MAY-1989

Microsoft C does stack checking to check for stack overflow. The stack
for an application program grows from high memory to low memory. The
lowest memory location to which the stack can grow is determined in
Microsoft C as follows:

&end + STACKSLOP      /* in assembly: OFFSET _end + STACKSLOP */

The _end symbol is created by Microsoft LINK which assigns it the
address of the lowest memory location of the STACK segment.

STACKSLOP is a manifest constant. For DOS, STACKSLOP = 256 (decimal);
for OS/2, STACKSLOP = 512 (decimal).

At runtime, the function _chkstk is called, on entry to each function,
to check the stack. If the current value of the SP register minus the
total size of the local variables is less than the sum of &end plus
STACKLOP, _chkstk prints the following error message and terminates
the program:

R6000   stack overflow

Otherwise, _chkstk will update the value of SP according to the total
size of the local variables.

The source code for the function _chkstk is included in the startup
source code in the Microsoft C Optimizing Compiler Version 5.10.

The value of the SP register is not updated if the stack overflow
condition is detected.

Stack checking can be manually disabled with either the compile line
option /Gs, or with the pragma; #pragma check_stack(off). In that
case, SP will be updated even if a stack overflow occurs.

For more information on _end, query on _edata.

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