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Getting the length of a string is very important for my projects, thus I was looking for different ways of implementing functions which would allow me to determine the length of a string via my FASM code. After doing some research I found two ways which worked fine for me.

The first way was to include the C-library and use its functions, strlen in this case:

import msvcrt,strlen,'strlen'

I later used it like that:

invoke strlen,myString

And EAX would hold the length of the string. Ok, so far so good.
Anyway, I was looking for a better way of getting its length, so I found a small procedure which would search a string for the null-terminator and later return its length.

Here’s the procedure:

; Will return the length of a null-terminated string
; The result will be written to ECX
;
; Usage:
; hello_tmp db "test",0
; ...
; ccall   strlen, hello_tmp
proc  strlen, strInput
 mov ecx,-1
 mov al,0
 mov edi,[strInput]
 cld
 repne scasb
 not ecx
 dec ecx
 ret
endp

As stated in the comments already, the result will be written to ECX. If you would prefer using a different terminator, replace

mov al,0

With another char from the ASCII-table, like:

mov al,0x32

It’s a great way of getting the length of a string and it’s also faster than the standard C-libraries.