Trying to use libC functions

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OzzY

Joined: 19 Sep 2003
Posts: 1029
Location: Everywhere

OzzY 17 Mar 2005, 18:57

Hi guys!

I was thinking about using C function on ASM with FASM. C is so easy and asm so powerful, so libC+ASM=Great code!
I've been successful on using functions like fopen, fprintf, printf, scanf, etc...
I just import them from msvcrt.dll or crtdll.dll that MSVC++ uses and comes with Windows.

But this little proggie is working. The problem is that it should copy 1536 bytes of itself (copy it totally) to an txt file. But it does it and puts lots of trash data in there. And what should have 1536 bytes goes to 20KB.

Any helps?

Code:


format PE console
include '%fasminc%\win32a.inc'
start:
invoke GetModuleFileName,0,buf,256
invoke fopen,buf,rmode
mov [fp],eax
invoke fopen,dest,wmode
mov [fp2],eax
mov ecx,1536
next:
invoke fgetc,[fp]
invoke fputc,eax,[fp2]
loop next
done:
invoke fclose,[fp]
invoke fclose,[fp2]
ret
;data
dest db 'test.txt',0
buf rb 256
rmode db 'rb',0
wmode db 'wb',0
fp dd 0
fp2 dd 0
;idata
data import
library crt,'msvcrt.dll',\
kernel32,'kernel32.dll'
import kernel32,GetModuleFileName,\
'GetModuleFileNameA'
import crt,fopen,'fopen',\
fclose,'fclose',\
fgetc,'fgetc',\
fputc,'fputc'
end data

Thanks a lot!
And sorry for my bad english... although code is self-explanatory... The universal language of ASM... heheh Wink

17 Mar 2005, 18:57

OzzY

Joined: 19 Sep 2003
Posts: 1029
Location: Everywhere

OzzY 17 Mar 2005, 19:15

Oh sorry... I've just discovered it right now...
I just read the libC reference and realized that it returns -1 on EAX if error or EOF. Wink

So, here is the code fixed:

Code:

format PE console
include '%fasminc%\win32a.inc'
start:
invoke GetModuleFileName,0,buf,256
invoke fopen,buf,rmode
mov [fp],eax
invoke fopen,dest,wmode
mov [fp2],eax
mov ecx,1536
next:
invoke fgetc,[fp]
cmp eax,-1
jz done
invoke fputc,eax,[fp2]
loop next
done:
invoke fclose,[fp]
invoke fclose,[fp2]
ret
dest db 'test2.txt',0
buf rb 256
rmode db 'rb',0
wmode db 'wb',0
fp dd 0
fp2 dd 0
;idata
data import
library crt,'msvcrt.dll',\
kernel32,'kernel32.dll'
import kernel32,GetModuleFileName,\
'GetModuleFileNameA'
import crt,fopen,'fopen',\
fclose,'fclose',\
fgetc,'fgetc',\
fputc,'fputc'
end data

Anyway thanks!
And FASM is just great!
Soon I'll be writing macros that will allow people to use libC just like a normal c program, like:
printf "Hi!"
printf "What is your name?"
scanf "%s",buf
.
.
.
and so on...

Bye!

17 Mar 2005, 19:15

Nikolay Petrov

Joined: 22 Apr 2004
Posts: 101
Location: Bulgaria

Nikolay Petrov 17 Mar 2005, 20:14

the functions and procedures are not stdcall. That's why you MUST "invoke" with "cinvoke" to return the stack - like wsprintf.

Code:

;save as test.asm and run it...
format PE console
entry start
include '%fasminc%\win32a.inc'

proc start
enter
        invoke GetModuleFileName,0,buf,256
        cinvoke fopen,buf,rmode
        mov [fp],eax
        cinvoke fopen,dest,wmode
        mov [fp2],eax
        mov ecx,1536
next:
        cinvoke fgetc,[fp]
        cmp eax,-1
        jz done
        cinvoke fputc,eax,[fp2]
        loop next
done:
        cinvoke fclose,[fp]
        cinvoke fclose,[fp2]
        return
endp

dest db 'test2.exe',0
buf rb 256
rmode db 'rb',0
wmode db 'wb',0
fp dd ?
fp2 dd ?

data import

library crt,'msvcrt.dll',kernel32,'kernel32.dll'

import kernel32,GetModuleFileName,'GetModuleFileNameA'

import crt, fopen,'fopen',\
            fclose,'fclose',\
            fgetc,'fgetc',\
            fputc,'fputc'
end data

17 Mar 2005, 20:14

f0dder

Joined: 19 Feb 2004
Posts: 3175
Location: Denmark

f0dder 23 Mar 2005, 10:30

Keep the calling convention in mind - when you call external code (libc, the API, ...) those routines are allowed to trash eax,ecx,edx and must preserve ebx,esi,edi,ebp - this means you can't depend on the values of eax,ecx,edx after a call to libc (except that eax is returncode, of course).

23 Mar 2005, 10:30

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