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Re: How can I do calling for external address in my "C" programm by inline GAS?
Presuming you're building this for Linux, I highly recommend you read the 64-bit ABI.
Chapter 3 is your friend. There's a lot of details there, hand-rolling 64-bit assembly is more complicated than 32-bit. There are things like red-zone, which you're writing into with the pushq. And since you probably won't read the ABI, here's fixed source:
void *p_printf = (void *)&printf;
const char *str = "Hello\n";
printf("addr of printf=%p\n", &printf); // 0x400490
asm ("movq %0, %%rdi;" /* str into RDI */
"leaq (, %1), %%rcx;" /* address of PRINTF into RCX */
"call * %%rcx;" /* call PRINTF */
: /* output */
:"r"(str),"r"(p_printf) /* input */
: "rdi", "rcx"
Nota Bene: The Windows 64-bit ABI is completely different, so while this source may compile and work on Linux... it will not work for Windows.