movl AT&T syntax with attributes

Question

Here is a snippet of assembly code in AT&T Syntax .

int foo_array[64*1024]  __attribute__ ((aligned (8192)));

void
foo()
{
    __asm__("movl %0,%%eax"::"r"(&foo_array));

I understood that movl copies the data from source operand to destination operand. that is, in my case, it is moving 0 to eax .. This is what I have understood from this link - slide 2

However, I dont understand the rest of it. Can someone please explain it so me.

Answer 1

You should read up on GCC inline assembly constraints .

In short, what that ::"r"(&foo_array) says is that you want one input for your assembly code, that the input should be placed in a register, and that its value should be the address of foo_array .
The %0 in the code is substituted with that input, so the code places the address of foo_array in eax .