Reason for the difference between (macro 1) and (1 macro)-CodePudding

A macro (see below) to return the greater of two numbers is added by 1. But, the effect of the addition is absent.

PS: I have also checked by adding excessive brackets to the macro.

#define max(a,b) (a >= b ? a : b);
void main(){
    int result = max(5,10)   1; //result = 10
}

Solutions: CASE 1:

By manually replacing the macro in the expression (just as the pre-processor does, I think), the addition is performed correctly.

result = (5 >= 10 ? 5 : 10)   1;  // Evaluates correctly to 11.

CASE 2: The expression also evaluates correctly when the order of the operands are reversed.

result = 1   max(5,10); //Evaluates correctly to 11.

Why is the original expression not working as intended while the two solution cases do?

CodePudding user response：

You have erroneously added ; to your macro. So the expansion is actually

void main(){
    int result = (5 >= 10 ? 5 : 10);   1;
}

Here your 1 became a separate statement which does nothing, and result is initialized to 10.

By the way, if you use gcc or Clang, there is an -E flag which will run the actual preprocessor on your source code. The code above was obtained by running gcc -E a.c on your code.

I also recommend compiling with the maximal possible level of warnings. E.g. gcc -Wall -Wextra -Werror a.c hints that there is something wrong:

a.c:2:6: error: return type of 'main' is not 'int' [-Werror=main]
    2 | void main(){
      |      ^~~~
a.c: In function 'main':
a.c:3:28: error: statement with no effect [-Werror=unused-value]
    3 |     int result = max(5,10)   1; //result = 10
      |                            ^
a.c:3:9: error: unused variable 'result' [-Werror=unused-variable]
    3 |     int result = max(5,10)   1; //result = 10
      |         ^~~~~~
cc1.exe: all warnings being treated as errors

CodePudding user response：

It is because you placed a semicolon after the macro

#define max(a,b) (a >= b ? a : b);
                               ^^^

Remove it.

The macro should be written like

#define max(a,b) ( ( a ) >= ( b ) ? ( a ) : ( b ) )

So in the first case you in fact has

int result = ( 5 >= 10 ? 5 : 10 );
  1;

If you will place the declaration

int result = max(5,10)   1;

in the file scope before the function main then the compiler will issue an error because there will be statement

1;

and you may not place statements in the file scope.

As for this statement

result = 1   max(5,10);

then it expands to two statements

result = 1   ( 5 >= 10 ? 5 : 10 );
;

that is there is also a null statement that does not have an effect.

Pay attention to that according to the C Standard the function main without parameters shall be declared like

int main( void )