A simple example of extended inline assembly syntax of GCC that outputs the integer representations of some of the 64-bit x86 general-purpose registers.

inspect_reg.c

#include <stdio.h>

int main() {
    long rax, rbx, rcx, rdx, rdi, rsi, rsp, rbp;

    asm (
        "movq %%rax, %0\n"
        "movq %%rbx, %1\n"
        "movq %%rcx, %2\n"
        "movq %%rdx, %3\n"
        "movq %%rdi, %4\n"
        "movq %%rsi, %5\n"
        "movq %%rsp, %6\n"
        "movq %%rbp, %7\n"
        : "=r" (rax),
        "=r" (rbx),
        "=r" (rcx),
        "=r" (rdx),
        "=r" (rdi),
        "=r" (rsi),
        "=r" (rsp),
        "=r" (rbp)
    );

    printf("rax=%ld\n"
    "rbx=%ld\n" 
    "rcx=%ld\n" 
    "rdx=%ld\n" 
    "rdi=%ld\n" 
    "rsi=%ld\n" 
    "rsp=%ld\n" 
    "rbp=%ld\n", 
    rax, rbx, rcx, rdx, rdi, rsi, rsp, rbp);
}