Passing char array from C to assembly function

[cableguy31]

Here is what I need to do.

I have a very small C program that takes user input and passes argv to a function written in GNU assembly language that will then count the number of characters in the string.

The function seems to work fine when I enter a string as a variable directly in the program, but when I try to pass it from C, it gives me an incorrect count.

Here is my C code:

#include <stdio.h>
#include <stdlib.h>

extern int my_strlen(char*);

main(int argc, char *argv[])
{
        printf("\nThe number of characters is %i\n\n.", my_strlen(argv[1]));

        return 0;
}

Here is my assembly code. Please note that I need to keep this as a loop.

.globl my_strlen          
.type my_strlen, @function

my_strlen:             

pushl %ebp             
movl %esp, %ebp        

movl 8(%ebp), %ebx
movl $0, %eax          

loop:                  

incl %ebx              

cmpl $0, (%ebx)        

je end_loop            

incl %eax              

jmp loop               

end_loop:              

cmpl $0, %eax          

je empty               

jmp not_empty          

not_empty:             
decl %eax              

empty:                 

movl %ebp, %esp       
popl %ebp              

ret

If I run the application "./test test", it gives me a character count of 1188. It should be 4...

Thanks.

[Ancient Dragon]

You don't need that loop at all -- just set up al, eax then call rep scasb to search for the string length

[untio]

Hi,
I give you an advice:
If your string is of type char (1 byte), why are you comparing double words (cmpl), may be you need cmpb (byte).
Another one:
You increment ebx before comparing the first character.
I hope that this can be useful.

[untio]

Hi once more,
Another suggestion is that you must restore ebx before exiting. It must return with the same value that it entered.
You are using the cdecl calling convention and its rules apply. Take a look at:
http://www.scalprum.eu/cdecl_and_stdcall_calling_conventions.html
Cheers.

[Ancient Dragon]

you mean esp not ebx. ebx is a general purpose register which does not need to be restored.

[untio]

Hi,
Look at:
http://www.scalprum.eu/cdecl_and_stdcall_calling_conventions.html
or look at:
http://en.wikipedia.org/wiki/X86_calling_conventions
or at:
http://www.hackcraft.net/cpp/MSCallingConventions/
Who is wrong?.
Another comment is that string opcodes are slower than a well optimized algoritm. Which is not the reference code.
The problem of restoring ebx can be solved using another register, maybe edx.

Cheers.

[untio]

Hi,
I doubt between two options, but I put the shortest one. Try the next code:

.global _my_strlen

_my_strlen:

pushl %ebp
movl %esp, %ebp

movl 8(%ebp), %edx
movl $0xFFFFFFFF, %eax

loop:

inc %eax
cmpb $0, (%edx , %eax)

jne loop

movl %ebp, %esp
popl %ebp
ret

The other solution is increment edx and not to use eax in the comparison. It can be timed, but I think this code is not so important.

Cheers.

[untio]

Hi,
A correction to my code:
You can erase:
movl %ebp, %esp
We are not using stack local space. esp is not modified.

Cheers.