g++ inline assembly mismatch in brackets

user_14260799 0
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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

inline-assembly g++ x86 assembly c++
1 Answers

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.

user_2241320
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