Facundo Ciccioli writes:
> 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).
Something in the library code, probably. I thin the ABI specifies
that the stack poiunter will be aligned. But you can't just randomly
mess with the stack pointer inside compiler-generated code and expect
reasonable results, and if you do trash a hard register, you must tell
the compiler you've done so:
asm("sub $2, %%esp":::"esp");
Andrew.
