C macro compiler warnings

Question

I've defined a macro, using input from a previous question I asked here. The macro is intended to either set, clear, or check a GPIO pins state. The macro works as expected however a problem shows up when compiling. I get compiler warnings anywhere it's used:

Warning right-hand operand of comma expression has no effect

when I use the macro like this:

#define ON  1
#define OFF 2
#define ENA 3
#define OUT_3(x) (x==ON) ? (PORTJ.OUTSET=PIN2_bm) : (x==OFF) ? (PORTJ.OUTCLR=PIN2_bm) : (x==ENA) ? (PORTJ.DIRSET=PIN2_bm) : (PORTJ.DIRCLR=PIN2_bm)
#include <avr/io.h>

if (something) OUT_3(ENA);

However, if I do this:

if (something) {OUT_3(ENA);}

I no longer get the warnings.

Why is there a difference? How should I alter the macro to prevent this scenario?

Additionally this invokes the warning:

int i=0;
if (something) i=1, OUT_3(ENA);

However this does not:

int i=0;
if (something) OUT_3(ENA), i=1;

My understanding of comma separated expressions is clearly a bit off. How is the compiler seeing this? I looked at several other questions similar to this but still do not fully understand the difference.

Answer 1

该宏令人讨厌，原因如下：

1. 宏参数应始终用括号括起来，以避免潜在的运算符优先级问题。更改(x==ON)为((x)==ON)。
2. 嵌套三元运算应使用括号括起来，以使执行顺序显而易见。更改a ? b : c ? d : e为a ? b : (c ? d : e)。
3. 完整的宏应该用括号#define MACRO (...)或“零时执行”循环包围，#define MACRO do {...} while(0)以避免运算符优先级出现问题。下面更多。

4. 三元运算符在这里并不是真正有用，因为您没有使用它的返回值。您应该使用常规的if或switch语句。这是前面提到的“做零时循环”变得很方便的地方：

   #define OUT_3(x) \
       do { \
           if((x) == ON)       { PORTJ.OUTSET = PIN2_bm; } \
           else if((x) == OFF) { PORTJ.OUTCLR = PIN2_bm; } \
           else if((x) == ENA) { PORTJ.DIRSET = PIN2_bm; } \
           else                { PORTJ.DIRCLR = PIN2_bm; } \
       } while(0)
   

5. 但是，宏真的必要吗？您可以改用内联函数，并消除所有宏怪异：

   static inline void OUT_3(x_type x) {
       if(x == ON)       { PORTJ.OUTSET = PIN2_bm; }
       else if(x == OFF) { PORTJ.OUTCLR = PIN2_bm; }
       else if(x == ENA) { PORTJ.DIRSET = PIN2_bm; }
       else              { PORTJ.DIRCLR = PIN2_bm; }
   }
   

通过这些更改，您的错误可能会消失，并且您的代码更容易阅读。