Operator sizeof() in C

Consider the program

main()  
{  
 printf("%d %d %d",sizeof('3'开发者_StackOverflow),sizeof("3"),sizeof(3));  
}

output from a gcc compiler is:

4 2 4

Why is it so?

Assuming you are running on a 32-bit system:

sizeof a character literal '3' is 4 because character literals are ints in C language (but not C++).

sizeof "3" is 2 because it is an array literal with length 2 (numeral 3 plus NULL terminator).

sizeof literal 3 is 4 because it is an int.

A few points to keep in mind:

1. sizeof isn't a function, it's an operator. It returns the size of a type in units of sizeof char. In other words sizeof char is always 1.
2. '3' is an int
3. "3" is a char[2], the character 3 then the null terminator.
4. 3 is an int

With these the differences are easily explained:

1. an int requires 4 chars of space to hold it
2. char[2] naturally only requires 2 chars

To quote K & R,

Each compiler is free to choose appropriate sizes for its own hardware, subject only to the the restriction that shorts and ints are at least 16 bits, longs are at least 32 bits, and short is no longer than int, which is no longer than long.

sizeof() Output depends on compiler you are using