How to find characters in a string Assembly x86? - string

Question

I'm trying to rewrite the C code below in Assembly x86
int myFn( char * v, char c, int size ) {
int i;
for(i=0; i < size; i++ )
if( v[i] == c )
return i;
return -1;
}
I've tried to use this code in x86:
myFn:
mov esi, 0
mov ebx, [esp + 8]
mov ecx, [esp + 12]
FOR:
mov eax, -1
cmp esi, [esp + 4]
jge ENDFOR
cmp [ecx + esi], ebx
je EQUAL
inc esi
jmp FOR
EQUAL:
mov eax, [esi]
ENDFOR:
ret
I've also created this program to test the function:
section .data
fmt: db "strfind: %d", 10, 0
str: db "test", 0
section .text
global main
extern printf
main:
mov eax, 's'
mov ebx, 4
push str
push eax
push ebx
call myFn
add esp, 12
push eax
push fmt
call printf
add esp, 8
ret
myFn:
mov esi, 0
mov ebx, [esp + 8]
mov ecx, [esp + 12]
FOR:
mov eax, -1
cmp esi, [esp + 4]
jge ENDFOR
cmp [ecx + esi], ebx
je EQUAL
inc esi
jmp FOR
EQUAL:
mov eax, [esi]
ENDFOR:
ret
I'm getting Segmentation Fault error or the wrong result when trying to test it. I believe the problem is when comparing the character of the string with the character I want to find