How does one compare values in GAS assembly architecture?

Question

I'm having trouble finding an answer to this seemingly innocuous and simple question. I wish to use cmp or one of its derivatives (cmpl, cmpb, etc...) to compare two values in a GAS assembly program. The problem is, when I run multiple comparisons that should come out differently, they come out the same. I believe it involves my misunderstanding of how data is compared with the cmp operation.

Here is the situation:

I have a fill variable to take input like this, with a equ to hold the size:

buff:    .fill    20
         .equ     bufLen, .-buff

Then I put the bufLen variable in a register, and the comparison value in another:

         movl     $bufLen, %eax
         movl     $0x03, %ebx

Finally, I compare and if compare equal, jump to another line:

         cmpl     %eax, %ebx
         je       anotherplace

However, when I compare inputs of lengths 2 and 4, they both come out less than (I changed the je to jl for a quick debug). Can anybody tell me what I'm doing wrong or point me to a question I missed that might tell me how I messed up?

Just as a reminder, this is GAS assembly architecture.

All help is much appreciated.

Answer 1

According to the comments, I show you some ways to get the length of a zero-terminated string (.asciz) in Linux:

witch.s:

.data

witches: .asciz "Double, double toil and trouble; Fire burn, and cauldron bubble"
format0: .asciz "%s\n"
format1: .asciz "Return of printf: %u\n"
format2: .asciz "Return of strlen: %u\n"
format3: .asciz "Return of repne scasb: %u\n"

.text
.global main
main:

    push $witches
    push $format0
    call printf         # returns in EAX the amount of printed chars (+ \n!)
    add $8, %esp

    push %eax
    push $format1
    call printf
    add $8, %esp

    push $witches
    call strlen         # returns in EAX the length of the string
    add $4, %esp

    push %eax
    push $format2
    call printf
    add $8, %esp

    mov $witches, %edi
    xor %al, %al
    xor %ecx, %ecx
    dec %ecx
    repne scasb
    neg %ecx            # returns in ECX the length of the string + 2

    push %ecx
    push $format3
    call printf
    add $8, %esp

    mov $0, %eax        # return 0;
    ret

Compile & run:

gcc -m32 witch.s
./a.out

There are more ways