Multiline macros in C
In this article, we will discuss how to write a multi-line macro. We can write multi-line macro same like function, but each statement ends with “\”. Let us see with example. Below is simple macro, which accepts input number from user, and prints whether entered number is even or odd.
#include
#define MACRO(num, str) {\
printf(“%d”, num);\
printf(” is”);\
printf(” %s number”, str);\
printf(“\n”);\
}
int main(void)
{
int num;
printf(“Enter a number: “);
scanf(“%d”, &num);
if (num & 1)
MACRO(num, “Odd”);
else
MACRO(num, “Even”);
return 0;
}
At first look, the code looks OK, but when we try to compile this code, it gives compilation error.
[shubham@/media/partition/GFG]$ make macro
cc macro.c -o macro
macro.c: In function ‘main’:
macro.c:19:2: error: ‘else’ without a previous ‘if’
make: *** [macro] Error 1
[shubham@/media/partition/GFG]$
Let us see what mistake we did while writing macro. We have enclosed macro in curly braces. According to C-language rule, each C-statement should end with semicolon. That’s why we have ended MACRO with semicolon. Here is a mistake. Let us see how compile expands this macro.
if (num & 1)
{
————————-
—- Macro expansion —-
————————-
}; /* Semicolon at the end of MACRO, and here is ERROR */
else
{
————————-
—- Macro expansion —-
————————-
};
We have ended macro with semicolon. When compiler expands macro, it puts semicolon after “if” statement. Because of semicolon between “if and else statement” compiler gives compilation error. Above program will work fine, if we ignore “else” part.
To overcome this limitation, we can enclose our macro in “do-while(0)” statement. Our modified macro will look like this.
#include
#define MACRO(num, str) do {\
printf(“%d”, num);\
printf(” is”);\
printf(” %s number”, str);\
printf(“\n”);\
} while(0)
int main(void)
{
int num;
printf(“Enter a number: “);
scanf(“%d”, &num);
if (num & 1)
MACRO(num, “Odd”);
else
MACRO(num, “Even”);
return 0;
}
Compile and run above code, now this code will work fine.
[shubham@/media/partition/GFG]$ make macro
cc macro.c -o macro
[shubham@/media/partition/GFG]$ ./macro
Enter a number: 9
9 is Odd number
[shubham@/media/partition/GFG]$ ./macro
Enter a number: 10
10 is Even number
[shubham@/media/partition/GFG]$
We have enclosed macro in “do – while(0)” loop and at the end of while, we have put condition as “while(0)”, that’s why this loop will execute only one time.
Similarly, instead of “do – while(0)” loop we can enclose multi-line macro in parenthesis. We can achieve the same result by using this trick. Let us see example.
#include
#define MACRO(num, str) ({\
printf(“%d”, num);\
printf(” is”);\
printf(” %s number”, str);\
printf(“\n”);\
})
int main(void)
{
int num;
printf(“Enter a number: “);
scanf(“%d”, &num);
if (num & 1)
MACRO(num, “Odd”);
else
MACRO(num, “Even”);
return 0;
}
Run on IDE
[shubham@/media/partition/GFG]$ make macro
cc macro.c -o macro
[shubham@/media/partition/GFG]$ ./macro
Enter a number: 10
10 is Even number
[shubham@/media/partition/GFG]$ ./macro
Enter a number: 15
15 is Odd number
[shubham@/media/partition/GFG]$
