Q48686: How to Find the Total Stack Size from within a Program

Article: Q48686
Product(s): See article
Version(s): 5.10   | 5.10
Operating System(s): MS-DOS | OS/2
Keyword(s): ENDUSER | SR# G890831-26299 | mspl13_c
Last Modified: 16-JAN-1990

Question:

How can I find the total size of the stack from within my program?

Response:

The program below calculates the total size of the stack. Note that
this works slightly differently for protected-mode and bound programs
than for real-mode programs. (Normally, the two-byte difference
shouldn't be important.)

The symbol "_end" is declared just below the lowest location in the
stack. At any given time, the amount of space left on the stack is SP
- offset _end. (The C program doesn't use the underscore because C
automatically prepends an underscore to all identifiers.)

The start-up code stores the maximum (initial) value of SP in the
variable __atopsp. (The program below uses only one underscore rather
than two for the reason described in the previous paragraph.)

Default stack checking on function entry fails when the amount left is
less than 256 bytes for DOS, or less than 512 bytes for OS/2. When
stack checking fails, you receive the following run-time error:

   run-time error R6000
   - stack overflow

The program to calculate the total stack size follows:

#include <stdio.h>

extern unsigned int end;

extern unsigned int _atopsp;

void main(void)
{
    unsigned stksize;

    stksize = _atopsp - (unsigned)&end + 2;
        /* don't add 2 for protected-mode or bound programs */

    printf("Total stack size is %u bytes (%x hex bytes)\n",
        stksize, stksize);
}

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