Calling Convention Confusion

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moveax41h

Joined: 18 Feb 2018
Posts: 59

moveax41h 07 Mar 2018, 09:17

I'm trying to determine which calling convention to use in my simple programs which are using msvcrt functions. For example:

Code:

format PE console
entry main
include 'macro/import32.inc'

section '.rdata' data readable
msg db 'Hello world.', 0
p db 'pause>nul', 0
section '.code' code readable executable

main:
push msg
call [printf]
add esp, 4

push p
call [system]
add esp, 4
push 0
call [exit]

section '.idata' import data readable
library msvcrt, 'msvcrt.dll'
import msvcrt, \
printf, 'printf', \
system, 'system', \
exit, 'exit'

You can see here how I attempt to clean up the stack with add esp, 4 and I typically do this after any of the msvcrt function calls. Is this necessary? I ask because I know stdcall is usually used in Microsoft libs and I believe the callee is supposed to clean it up but there have been times I built programs where it didn't seem to do so...

So is this actually cdecl or stdcall? Thanks.

_________________
-moveax41h

07 Mar 2018, 09:17

revolution
When all else fails, read the source

Joined: 24 Aug 2004
Posts: 20451
Location: In your JS exploiting you and your system

revolution 07 Mar 2018, 09:22

MSVCRT is 100% C-call. You must restore the stack in your code.

Most of the Windows 32-bit API is std-call. The API will restore the stack. The only exception here is the vararg functions (wsprintf), which are c-call.

Note that MSVCRT is not part of the official Windows API.

07 Mar 2018, 09:22

DimonSoft

Joined: 03 Mar 2010
Posts: 1228
Location: Belarus

DimonSoft 07 Mar 2018, 10:49

Also remember that there are handy macros for calling functions of different calling conventions in the FASM package. MACRO\PROC32.inc is your choice. Saves a few lines of code and makes manual parameter counting unnecessary.

07 Mar 2018, 10:49

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