Get file extension

[Vortex]

Here is a quick example to get the extension of a filename :

Code: [Select]

.386
.model flat,stdcall
option casemap:none

StrLen PROTO :DWORD

.code

GetFileExtension PROC USES esi edi pFileName:DWORD

    mov     esi,pFileName
    mov     edi,1
    invoke  StrLen,esi
    mov     cl,BYTE PTR [esi]
    push    ecx
    mov     BYTE PTR [esi],'.'
    add     eax,esi
@@:
    sub     eax,edi
    cmp     BYTE PTR [eax],'.'
    jne     @b
    pop     ecx
    mov     BYTE PTR [esi],cl
    ret

GetFileExtension ENDP

END

The procedure returns the address of the string if there is no file extension.

Code: [Select]

include Demo.inc

BUFFER_SIZE equ 128

.data

file db 'C:\WINDOWS\system32\kernel32.dll',0
format1 db 'Extension = %s',0

.data?

buffer db BUFFER_SIZE dup(?)

.code

start:

    invoke  GetFileExtension,ADDR file
    invoke  wsprintf,ADDR buffer,ADDR format1,eax
    invoke  StdOut,ADDR buffer
    invoke  ExitProcess,0

END start


[Vortex]

- EndOfStr, a modified version of the StrLen function determines the end of a NULL terminated string.
- Removed unnecessary preservation of ecx

Code: [Select]

.386
.model flat,stdcall
option casemap:none

EndOfStr PROTO :DWORD

.code

GetFileExtension PROC USES esi edi pFileName:DWORD

    mov     esi,pFileName
    mov     edi,1
    invoke  EndOfStr,esi
    movzx   ecx,BYTE PTR [esi]
    mov     BYTE PTR [esi],'.'
@@:
    sub     eax,edi
    cmp     BYTE PTR [eax],'.'
    jne     @b
   
    mov     BYTE PTR [esi],cl
    ret

GetFileExtension ENDP

END