g++ inline assembly mismatch in brackets

Question

g++ is complaining to me about a missing bracket in the following code:

                                              1                   2 3
                                              v                   v v
__asm__ volatile("inb %1, %0" : : "=a" (result) : "Nd" (portnumber) );
                ^                      ^               ^
                1                      2               3

as you can see the brackets are matching and there are three open brackets and three close brackets.

also for more information i am following a youtube tutorial

Answer 1

You have an extra : before the output, so you ended up with your output declaration in the input part. And the reason for that error: your input operand declaration where the compiler expects the clobber list. The clobber list can only include string literals (register names and "memory" and/or "cc"¹), not ().

    __asm__ volatile("inb %1, %0" 
            : "=a" (result)       // output
            : "Nd" (portnumber)   // input
          //  : "memory"   // optional, clobber list
     );

You might want a "memory" clobber list to make sure this is ordered wrt. memory accesses. Or not, if you're sure it doesn't need to be.

Footnote 1: asm statements on x86 implicitly clobber the condition codes, "cc". You can use it for documentation if you like. But you don't want it here because inb doesn't touch EFLAGS.