__asm__ and return value problem?

[Devil Panther]

The next function should return the stack pointer's value.
I think I'm losing my mind, but how??? Am I missing something?
I don't see a return... how could it return a value then?

unsigned long get_sp(void)
{
        __asm__("movl %esp, %eax");
}

Thank you.

[Speedy5]

Returning values is completely platform/compiler independent. On most systems (x86 and stuff, you know), returning integer values (anything under sizeof int bytes), you return the value in eax. For floating points values, you return it on st0 (top of the stack). So it would be like this in VC++.

__declspec(naked) unsigned int sp() {
   __asm mov eax, esp;
}

The naked specifier tells the compiler not to place any supplemental code to the function. I guess you can call it inline but I bet VC++ will inline it in the Release build with certain options on. And btw, I haven't tested/compiled that.

EDIT: Oh this is Linux. I bet its the same anyways haha! But eax is still the return value. Just look up your compiler's documentation for naked fuctions and inline assembly.

[Salem]

You figure out the asm, but like this

unsigned long get_sp(void)
{
        unsigned long result;
        /* mov %esp result */
        __asm__("movl %esp, %eax");
        return result;
}

Then you only have to worry about getting the esp value, and not how the result is returned.

[Devil Panther]

It funny since I don't recall reading it in a book, any book.

So AX is always used to return a value from a function.

Just one last thing...

movl %esp, %eax

why is %eax (the source parameter) on the right, and not on the left?



Thank you.

[Fordy]

It funny since I don't recall reading it in a book, any book.

So AX is always used to return a value from a function.

Just one last thing...

movl %esp, %eax

why is %eax (the source parameter) on the right, and not on the left?



Thank you.

It's AT&T syntax. For some reason they do it the other way around to the intel syntax...

Just to make life that little bit more complex for you

[Devil Panther]

damn at&t... if it weren't for C and unix, i would have burned them long time ago

[Thantos]

We are usingi the AT&T syntax in my ASM class right now and

movl %esp, %eax

is copying the stack pointer into eax, not the other way around.