calling api functions

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MSBob

Joined: 17 Jun 2004
Posts: 2

MSBob 17 Jun 2004, 15:27

Hi

I'm pretty new to assembly and decided to use Flat Assembler to learn with.

I'm having a bit of difficulty understanding how api functions are actually called, as all the examples I can find simply use the invoke macro. As I'm trying to learn how things are done without macros I want to do it manually.

My understanding was that the arguments are pushed on to the stack in reverse order, and then the api function is called. When I do this however, the program just crashes. Using invoke and it works fine.

Could someone please explain where I'm going wrong? Here is the test code.

Code:

format PE GUI 4.0
include "%fasminc%\win32a.inc"

section ".data" data readable writeable

Caption db "Hello!",0
Text db "Hello assembly world!",0

section ".code" code readable executable
entry start
start:

push MB_OK or MB_ICONEXCLAMATION
push Caption
push Text
push NULL
call MessageBox
;invoke MessageBox,NULL,Text,Caption,MB_OK or MB_ICONEXCLAMATION

push NULL
call ExitProcess
;invoke ExitProcess,NULL

section ".import" import data readable writeable

library kernel,"KERNEL32.DLL",\
        user,"USER32.DLL"

import kernel,\
       ExitProcess,"ExitProcess"

import user,\
       MessageBox,"MessageBoxA"

17 Jun 2004, 15:27

Tomasz Grysztar

Joined: 16 Jun 2003
Posts: 8351
Location: Kraków, Poland

Tomasz Grysztar 17 Jun 2004, 15:41

The labels for API calls are actually labels to pointers to the functions, not to the functions themselves. Therefore you have to call them this way:

Code:

push MB_OK or MB_ICONEXCLAMATION 
push Caption 
push Text 
push NULL 
call [MessageBox] 
;invoke MessageBox,NULL,Text,Caption,MB_OK or MB_ICONEXCLAMATION 

push NULL 
call [ExitProcess]
;invoke ExitProcess,NULL

Note use of the square brackets inside the invoke macro.

17 Jun 2004, 15:41

MSBob

Joined: 17 Jun 2004
Posts: 2

MSBob 17 Jun 2004, 18:26

Privalov wrote:

The labels for API calls are actually labels to pointers to the functions, not to the functions themselves. Therefore you have to call them this way:

Thanks for the explanation, it works now. I obviously still have a lot to learn Smile

17 Jun 2004, 18:26

vid
Verbosity in development

Joined: 05 Sep 2003
Posts: 7105
Location: Slovakia

vid 17 Jun 2004, 19:47

i am sure i explained this question (+somewhat more) very deeply in some post, but forgot in which, maybe you could browse forum and find it

17 Jun 2004, 19:47

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