Assembler Error for GCC inline assembly for x86_64 machine(at&t syntax)

Question

I am trying to read a value from the stack using the gcc inline assembly with the default at&t syntax. My code is

unsigned int ret_val;
__asm volatile(
            "movq %%rbp %0;\n"
            :"=r"(ret_val)
            );

I am getting an error:

Assembler messages:
Error: junk `%eax' after register

What is the meaning of this error and how do I get rid of this. I already read through some similar questions on SO question but no help from them. I am more interested in knowing the cause behind the error and the correct way to do it.

The code is written for x86_64 ubuntu machine.

Thanks

Answer 1

There are two problems. First - you need a comma between registers in mov command:

"movq %%rbp, %0;\n"

Second, you need 64-bit variable to hold value of %rbp. int is 32-bit on x86_64 linux. You can use long or preferably uint64_t.