Assembly Language for Beginners

1.17. MORE ABOUT STRINGS

TheSUBinstruction just got executed:

Figure 1.61:OllyDbg:EAXto be decremented now

The difference of pointers is in theEAXregister now—7. Indeed, the length of the “hello!” string is 6, but
with the zero byte included—7. Butstrlen()must return the number of non-zero characters in the string.
So the decrement executes and then the function returns.

Optimizing GCC

Let’s check GCC 4.4.1 with optimizations turned on (-O3key):

public strlen
strlen proc near

arg_0 = dword ptr 8

push ebp

mov ebp, esp

mov ecx, [ebp+arg_0]

mov eax, ecx

loc_8048418:
movzx edx, byte ptr [eax]
add eax, 1
test dl, dl
jnz short loc_8048418
not ecx
add eax, ecx
pop ebp
retn
strlen endp

Here GCC is almost the same as MSVC, except for the presence ofMOVZX. However, hereMOVZXcould be
replaced with
mov dl, byte ptr [eax].