The need for parentheses in macros in C [duplicate]

Answer 1

The macro expands to

 a = b+5*b+5;

i.e.

 a = b + (5*b) + 5;

So 23.

Answer 2

Because Macros are just string replacement and it is happens before the completion process. The compiler will not have the chance to see the Macro variable and its value. For example: If a macro is defined as

#define BAD_SQUARE(x)  x * x 

and called like this

BAD_SQUARE(2+1) 

the compiler will see this

2 + 1 * 2 + 1

which will result in, maybe, unexpected result of

5

To correct this behavior, you should always surround the macro-variables with parenthesis, such as

#define GOOD_SQUARE(x)  (x) * (x) 

when this macro is called, for example ,like this

GOOD_SQUARE(2+1)

the compiler will see this

(2 + 1) * (2 + 1)

which will result in

9

Additionally, Here is a full example to further illustrate the point

#include <stdio.h>

#define BAD_SQUARE(x)  x * x 
// In macros alsways srround the variables with parenthesis
#define GOOD_SQUARE(x)  (x) * (x) 

int main(int argc, char const *argv[])
{
    printf("BAD_SQUARE(2) = : %d \n", BAD_SQUARE(2) ); 
    printf("GOOD_SQUARE(2) = : %d \n", GOOD_SQUARE(2) ); 
    printf("BAD_SQUARE(2+1) = : %d ; because the macro will be \
subsituted as 2 + 1 * 2 + 1 \n", BAD_SQUARE(2+1) ); 
    printf("GOOD_SQUARE(2+1) = : %d ; because the macro will be \
subsituted as (2 + 1) * (2 + 1) \n", GOOD_SQUARE(2+1) ); 

    return 0;
}