Stack pointer alignment

[Facundo Ciccioli]

Hi. I'll save you all the details that I think are irrelevant and go
directly to my question.
I would like to know why the following code doesn't work:

int main() {
    asm("sub $2, %esp");
    printf("Hi, this is a test\n");
    asm("add $2, %esp");

    return 0;
}

By "doesn't work" I mean that the call to printf doesn't have any
effect. Of course if I replace the number 2 (that I substract and the
add to esp) by any multiple of 4, the program does what expected. I
wonder if any of you knows from where does the restriction that the
esp must by a multiple of 4 comes from. From printf? I know that it
doesn't come from the processor (x86).

Thanks in advance,
FaQ