sizeof() for Floating Constant in C

In C language, we have three floating data types i.e. float, double and long double. And the exact size of each of these 3 types depends on the C compiler implementation/platform. The following program can be used to find out the size of each floating data type on your machine.

#include "stdio.h"
int main()
{
 printf("%d %d %d",sizeof(float), sizeof(double), sizeof(long double));
 return 0;
}

But what about the size of a floating point constant (e.g. 31.4 or 2.718)? For example if we have PI macro defined as follows, what would be the sizeof(3.14).

#define PI 3.14

Now if we do sizeof(PI), what will be its size? Is is equal to sizeof(float) ? Or is it also compiler implementation dependent. Well, for floating constants, C standard (C11 i.e. ISO/IEC 9899:2011) has given guideline. As per C11 clause 6.4.4.2, “An unsuffixed floating constant has type double. If suffixed by the letter f or F, it has type float. If suffixed by the letter l or L, it has type long double.

It means the type of a floating constant is same as that of double data type. So if size of a double on a machine is 8 bytes, the size of floating constant would be 8 bytes. One can find out this using the below program

#include "stdio.h"
#define PI 3.14
int main()
{
 printf("%d",sizeof(PI));
 return 0;
}

As per the above mentioned C standard clause, a floating constant can be converted to float type by using f or F. Similarly, a floating constant can be converted to long double by using l or L. So it shouldn’t take much thought on guessing the output of the following:

#include "stdio.h"
int main()
{
 printf("%d %d",sizeof(3.14F), sizeof(3.14L));
 return 0;
}

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