Confusion about strings

[UoSunsetter]

Hi! I was playing with the codes, than I noticed that I am confused about strings(see my previous posts on old forum. I keep making mistakes on strings  ).

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source db "uosunsetter",0  ; null terminated string

When I use "source", what I think is I'm using the address of first char in the string. With other words, "source" represents the beginning of the buffer as a reference, byte pointer. For example:

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mov esi, source

Now esi has the address of letter "u" right? To access the next letter I just increase esi and move [esi] to ah. However here I get confused:

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push MB_OKCANCEL
push offset source
push offset source
push 0
call MessageBox

What is this??? What am I pushing? Pointer of pointer? Why not only "source"? This example was the first example I saw, thus, I was using strings in this way which is not true I guess:

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mov esi, OFFSET source

I don't even know what it does now. :(

[TouEnMasm]

mov esi,source ; load a value in esi (4 bytes ,the size of esi) from the adress of source
lea esi, source  ; load the adresse of source in esi:The linker could change this one
mov esi,offset source
;same as lea except that it is the compiler who give the adress replacing offset source by a constant (the linker couldn't change this)

with functions:
invoke MessageBox,NULL,ADDR source,addr Titre,MB_OK

[dedndave]

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source db "uosunsetter",0

mov esi, source

ESI will hold 'usou', the first 4 bytes in little-endian order

what you want is...

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mov esi, offset source
now, ESI holds the address of source

[tuohai]

soure is not a byte pointer. there isn't  byte pointer in masm32. all the pointers is dword.

[jj2007]

soure is not a byte pointer. there isn't  byte pointer in masm32. all the pointers is dword.

include \masm32\include\masm32rt.inc

.code
source db "uosunsetter",0  ; null terminated string
start:
   mov esi, offset byte ptr source
   MsgBox 0, esi, 0, MB_OK
   exit
end start

;)

... and welcome to the forum :icon14:

[Vortex]

Hi tuohai,

There are also qword pointers.

[jj2007]

Erol, the assembler zoo is full of strange animals :P

[qWord]

While we're on it:

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mov esi,source
this won't assemble, because source is of the type BYTE whereas ESI is DWORD 
It should be mention that MASM automatically dereference data labels (unlike other assmblers)– to get the Pointer requires the OFFSET/ADDR operator or a instruction like LEA.

[UoSunsetter]

Thanks for making it clear. I really need to learn proper debugging to avoid that kind of noob questions :)

[hutch--]

Just to clarify something in a previous post.

> soure is not a byte pointer. there isn't  byte pointer in masm32. all the pointers is dword.

There IS in fact such a thing as a BYTE PTR in 32 bit MASM, it is a DWORD that points to a BYTE location, usually the start of a string sequence of bytes. In 16 bit DOS it was a BYTE PTR that was a WORD and if we ever get a fully long 64 bit mode, a BYTE PTR will be a 64 bit unsigned integer.

To the original question, its a distinction between WHERE data is located (its address) and WHAT the data is (its content).