unclear c code: double underscore in c macro

Question

I have to interpret some c-code as a kind of homework and I don't really understand this macro:

#define rdtscll(val)__asm__ __volatile__("rdtsc":"=A"(val))

I know it's some kind of static function named rdtscll which receives one parameter val. But what about the rest?

What are __asm__ and __volatile__ and why those underscores?
are those 2 different constants separated by a space?

did you try searching for any info? check [this](http://stackoverflow.com/q/14950614/2173917) — Sourav Ghosh, Oct 30 '14 at 11:58
This is not C, but an extension to integrate assembler. Are you sure that you are supposed to understand this part of the example that your teacher gave to you? — Jens Gustedt, Oct 30 '14 at 12:06
Also see https://stackoverflow.com/questions/26456510/what-does-asm-volatile-do-in-c — Deduplicator, Oct 30 '14 at 12:06
@Deduplicator modified the question itself as the second part is removed. — Sourav Ghosh, Oct 30 '14 at 12:08

score 2 · Accepted Answer · answered Oct 30 '14 at 14:04

In this case __asm__ and __volatile__ are compiler extensions for embedding assembly statements into a C file. The two leading underscores are to prevent clashes with identifiers in user code, and the trailing are pure choice. __volatile__ is just an "anti-optimization" directive.

rdtsc is an x86 instruction for reading the Time-Stamp Counter, a very high-speed counter based on clock cycles. "=A" is part of the GCC inline assembly syntax - there are plenty of references explaining more about it.

unclear c code: double underscore in c macro

1 Answers1