Common Mistakes in C : Preprocessor Errors

Errors / Mistakes / Misuse of C Preprocessor in C
1 . Macro expansion should be enclosed within parenthesis

#include<stdio.h>
#include<conio.h>
#define SQU(x) x*x
void main()
{
int num ;
clrscr();
num = SQU(2)/SQU(2);
printf("Answer : %d",num);
getch();
}

Output :

Answer : 4

How ? Answer should be 1 not 4 then how it happens ?

• SQU(x) / SQU(x) is expanded as [ x*x / x*x ] = [ x * x ]
• As the macro #define only substitutes the SQU(2) by just 2*2 not by 4 .
• So SQU(2)*SQU(2) = 2*2/2*2 = 4

How to avoid this problem ?

#define SQU(x)((x)*(x))

2 .Do not Leave blank between macro-template and its argument

#define SQU(x)((x)*(x))//Write like this

• Do not give space between SQU and (x) , as it becomes the part of macro expansion
• It results into Improper result OR Compile time Error

3 . Do not Give semicolon at the end of #define statement .

#define SQU(x)((x)*(x));//Do not give semicolon at the end

4 . Allot one line for each macro preprocessor

#include<stdio.h>
#include<conio.h>// Write on separate line

Do not write like this

#include<stdio.h>#include<conio.h>