Why does pushing and printing the value in a register affect later comparisons of that register to another?

Question

I'm attempting to write a short assembly program to compare 2 strings. I'm using AT&T syntax and assembling my program with gcc and the -m32 flag.

When I run the code, I always enter "rectangle" when prompted.

I have written the following code. With the questionable block of code (enclosed in comments) active, if I enter "rectangle" when prompted, the program does not print "You entered rectangle." In fact, the program jumps out of the loop on the first iteration. If I comment out said block of code, the program does print "You entered rectangle." Since this block of code only prints the values in %eax and %ebx, why does it affect the jump condition?

.section .data
format:
    .asciz "%s"
test_str:
    .asciz "\n%c%c"
equal_str:
    .asciz "You entered rectangle"
rectangle:
    .asciz "rectangle"

.section .bss
.lcomm element, 100

.section .text
.globl main

main:
    pushl $element
    pushl $format
    call scanf
    addl $8, %esp

    movl $0, %edi
    loop:
    movb element(,%edi,1), %al
    movb rectangle(,%edi,1), %bl

    #BEGIN QUESTIONABLE BLOCK
    pushl %eax
    pushl %ebx
    pushl $test_str
    call printf
    addl $8, %esp
    #END QUESTIONABLE BLOCK

    cmpb %al, %bl
    jne end
    incl %edi
    cmpb $0, %al
    jne loop

pushl $equal_str
call printf
addl $4, %esp
jmp end

end:
    call exit

I suppose none of the following information matters, but I really have no idea what could or couldn't affect assembly, so my CPU is an Intel Xeon E5-2650 and I'm using CentOS 6.5. Thanks in advance!

Answer 1

printf returns its result in %eax, thereby trashing your character. You do: addl $8, %esp. That isn't enough - you've pushed three parameters. As it happens, this leaves your old %eax on the stack, where a popl %eax might solve your problem.