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Watcom 1.9 stack corruption after return from function (Developers)

posted by bencollver , 05.07.2026, 01:12

I am debugging the AWKA compiler with Watcom 1.9. The stack appears to be corrupted when returning from the awka_insertop() function in da.c.

I compiled da.obj like so:

wcc -q -os -okz -s -bt=dos -ml -fpi87 -i=\watcom\h -i=. -dSTDC_HEADERS -wcd=118 da.c

Here are code listings:

da.c
https://termbin.com/adti

da.lis (wdis da.obj >da.lis)
https://termbin.com/ccvp

code.c:dump_code():

    void
    dump_code()
    {
       if (begin_start)
       { awka_insertop(_BEGIN, NULL, NULL, 0, 0);
       ...

ASM code to call awka_insertop():

    30D7:DC15  dump_code_+00000016
    DC15  xor        ax,ax
    DC17  push       ax
    DC18  push       ax
    DC19  push       ax
    DC1A  push       ax
    DC1B  xor        bx,bx
    DC1D  xor        cx,cx
    DC1F  mov        ax,00B6
    DC22  call       far ptr awka_insertop_

Stack after tracing to the last line of ASM code:

    SS: 4465
    BP: 0000
    SP: FCE4

Stack after entery to da.c:awka_insertop():

    SS: 4465
    BP: 0000
    SP: FCE0

ASM code on entry:

    1F4C  push       dx
    1F4D  push       si
    1F4E  push       di
    1F4F  push       bp
    1F50  mov        bp,sp
    1F52  sub        sp,0008

Stack after tracing through this last line of ASM code:

    SS: 4465
    BP: FCD8
    SP: FCD0

ASM code before return:

    2397  mov       sp,bp
    2399  pop       bp
    239A  pop       di
    239B  pop       si
    239C  pop       dx
    239D  retf      000C

Stack at the first line of ASM code:

    SS: 4465
    BP: FCD8
    SP: FCD0

Stack after tracing to the last line of ASM code:

    SS: 4465
    BP: 0000
    SP: FCE0

The stack is the same as it was on entering awka_insertop(), which is good!

Stack after returning to dump_code():

    SS: 4465
    BP: 0000
    SP: FCF0

The stack is different than it was before calling awka_insertop(), which is bad. SP was FCE4 and now it is FCF0.

I think it would have been OK if Watcom had emitted CB retf, which it did for all the *other* returns in da.c. For some reason, Watcom generated CA 0C 00 retf 0x00c here, and this seems to break the stack.

I skimmed through the code generation options in the Watcom 1.9 cguide.pdf. I tried changing the compiler flag from -okz to -oz. That didn't help.

Any other recommendations?

Thank you!

-Ben

Complete thread:

• Watcom 1.9 stack corruption after return from function - bencollver, 05.07.2026, 01:12 (Developers)
  • Watcom 1.9 stack corruption after return from function - jadoxa, 05.07.2026, 04:18
    • Watcom 1.9 stack corruption after return from function - bencollver, 05.07.2026, 05:09
      • Watcom 1.9 stack corruption after return from function - Rugxulo, 05.07.2026, 07:10
      • Watcom 1.9 stack corruption after return from function - jadoxa, 05.07.2026, 08:21
        • Watcom 1.9 stack corruption after return from function - bencollver, 05.07.2026, 15:24