The need for parentheses in macros in C [duplicate]

Answer 1

After preprocessing, SQR(b+5) will be expanded to (b+5*b+5). This is obviously not correct.

There are two common errors in the definition of SQR:

1. do not enclose arguments of macro in parentheses in the macro body, so if those arguments are expressions, operators with different precedences in those expressions may cause problem. Here is a version that fixed this problem

   #define SQR(x) ((x)*(x))
   

2. evaluate arguments of macro more than once, so if those arguments are expressions that have side effect, those side effect could be taken more than once. For example, consider the result of SQR(++x).

   By using GCC typeof extension, this problem can be fixed like this

   #define SQR(x) ({ typeof (x) _x = (x); _x * _x; })
   

Both of these problems could be fixed by replacing that macro with an inline function

   inline int SQR(x) { return x * x; }

This requires GCC inline extension or C99, See 6.40 An Inline Function is As Fast As a Macro.