How can i get the last 5 chars from a buffer

[gavin]

Hi guys.

I ran into a slight problem in my code ,i'm making a udp socket program.

My problem is i have a string in a buffer and i need to get the last 5 chars of that string.

Any ideas would be great.

Thanks.
Gavin.

[PBrennick]

gavin,

.data
SomeString  db  255 dup (0)

.code

...
        mov  esi, offset MRUDump    ; Point to the string
        add    esi, 250
...

Paul

[hutch--]

Gavin,

With MASM32, you can either use the macro,

Code Select Expand


mov lpString, right$(lpString,slen)


or do a direct call to the masm32 library procedure szRight,

Code Select Expand


szRight proc source:DWORD, dest:DWORD, lnth:DWORD


It is a good idea once you undersand the technique to have a good look at the code to see how it works.

[raymond]

"Assuming" you want the last 5 characters of a null-terminated string in a buffer of sufficient size declared as a global variable,

mov edi,offset buffer
mov ecx,SIZEOF buffer
xor eax,eax
repnz scasb
sub edi,6

EDI would then be pointing to the start of the last 5 characters regardless of the size of the string or the size of the buffer.

If your "buffer" is allocated memory, you would have to modify the first two instructions.

Raymond

[gavin]

Hi guys.
I'm learning as i go along so i went with Raymonds way.
It's working fine now but i had to make a few changes and in the end i didn't have to d it ,but i learned from that code.
Thanks all of you guys.

[AeroASM]

mov edi,offset buffer
mov ecx,SIZEOF buffer
xor eax,eax
repnz scasb
sub edi,6

What is wrong with:

Code Select Expand


mov edi,offset buffer + sizeof buffer - 6


or with an allocated buffer:

Code Select Expand


mov edi,pMem
add edi,ccbMem


[Jeff]

if there was a possibility of the string not taking up the entire buffer, it would take care of it.  ;)

[PBrennick]

Jeff,
That may be true but it is not what he asked for.  That is why my answer is so similar to Aeros.  He is asking about a string that fills the buffer so testing the length is not necessary and I doubt he used it.  He said he modified Raymonds response and I think that is why.

Getting too fancy is never a good idea when you are trying to teach someone.  It just confuses the person.  and repnz scasb is an advanced instruction.

Paul

[raymond]

i have a string in a buffer

I've reread gavin's original post over and over again but I couldn't find anything indicating that his string was taking up the entire buffer. One assumption should be as good as the other. My assumption was to cover ALL possibilities.

Buffers are normally declared with a size sufficient to handle many possible strings of varying size (such as file names).

Raymond