DOS ain't dead - Watcom 1.9 stack corruption after return from function

bencollver

05.07.2026, 01:12

Watcom 1.9 stack corruption after return from function (Developers)

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

jadoxa Queensland, Australia, 05.07.2026, 04:18 @ bencollver	Watcom 1.9 stack corruption after return from function Post reply
	> The stack is different than it was before calling awka_insertop(), which is > bad. SP was FCE4 and now it is FCF0. No, that's right, as the retf 0x0c pops the six pushes of the arguments (other functions don't have as many arguments, so they're wholly passed in registers). If you're getting stack corruption, it's not because of that.

bencollver

05.07.2026, 05:09

@ jadoxa

Watcom 1.9 stack corruption after return from function

Post reply

> No, that's right, as the retf 0x0c pops the six pushes of the arguments
> (other functions don't have as many arguments, so they're wholly passed in
> registers). If you're getting stack corruption, it's not because of that.

Thanks for the hint, i think it will help.

The awka_insertop() declaration in da.c has 6 arguments:

void awka_insertop(int op, char *cval, char *carg, int minst, char *file, int line) {

The awka_insertop() prototype in code.c has 5 arguments:

void awka_insertop(int, char *, char *, int, int);

And dump_code() indeed calls it with 5 arguments instead of 6.

I guess it's GIGO and it's on me to fix it.

Rugxulo Usono, 05.07.2026, 07:10 @ bencollver	Watcom 1.9 stack corruption after return from function Post reply
	Don't forget that common optimizations like -ox are the same as -obmiler -s (and turns off stack checking). Plus, being large model (and not flat), the stack is much smaller (4 kb?).

jadoxa Queensland, Australia, 05.07.2026, 08:21 @ bencollver	Watcom 1.9 stack corruption after return from function Post reply
	Have you seen https://github.com/noyesno/awka/ ?

bencollver 05.07.2026, 15:24 @ jadoxa	Watcom 1.9 stack corruption after return from function Post reply
	> Have you seen https://github.com/noyesno/awka/ ? Yes, that's the source code i am in the middle of porting to Watcom.