Error: bad register name `%rax'
Error: bad register name `%rbx'

#include <iostream>
#include <cstdlib>
#include <stdint.h>

void read(void)
{
    uint64_t rax = 0, rbx = 0;
    __asm__ __volatile__ (
        /* read value from rbx into rbx */
        "movq %%rbx, %0;n"
        /* read value from rax into rax*/
        "movq %%rax, %1;n"
        /* output args */
        : "=r" (rbx), "=r" (rax)
        : /* no input */
        /* clear both rdx and rax */
        : "%rbx", "%rax"
    );

    /* print out registers content */
    std::cout << "RAX = " << rax << "n";
    std::cout << "RBX = " << rbx << "n";

}

int main(int argc, char **argv)
{
    read();

    return 0;
}

: "%rbx", "%rax"

: "rbx", "rax"