Return a pointer at a specific position - Assembly

Question

I am a beginner in Assembly and i have a simple question. This is my code :

BITS 64                     ; 64−bit mode
global strchr               ; Export 'strchr'

SECTION .text           ; Code section
strchr:
    mov rcx, -1
.loop:
    inc rcx
    cmp byte [rdi+rcx], 0
    je exit_null
    cmp byte [rdi+rcx], sil
    jne .loop
    mov rax, [rdi+rcx]
    ret

exit_null:
    mov rax, 0
    ret

This compile but doesn't work. I want to reproduce the function strchr as you can see. When I test my function with a printf it crashed ( the problem isn't the test ). I know I can INC rdi directly to move into the rdi argument and return it at the position I want. But I just want to know if there is a way to return rdi at the position rcx to fix my code and probably improve it.

Answer 1

Your function strchr seems to expect two parameters:

1. pointer to a string in RDI, and
2. pointer to a character in RSI.

Register rcx is used as index inside the string? In this case you should use al instead of cl. Be aware that you don't limit the search size. When the character refered by RSI is not found in the string, it will probably trigger an exception. Perhaps you should test al loaded from [rdi+rcx] and quit further searching when al=0.

If you want it to return pointer to the first occurence of character inside the string, just
replace mov rax,[rdi+rcx] with lea rax,[rdi+rcx].