sizeof Operator

Syntax

sizeof exp;

sizeof (type_name)

Description

The sizeof unary operator finds the size of an object. The sizeof operator accepts two types of operands: an expression or a data type. If you use an expression operand, the expression itself is not evaluated — the compiler only determines what type the result would be. Any side effects in the expression, therefore, will not have an effect. The result type of the sizeof operator is unsigned int.

If the operand is an expression, sizeof returns the number of bytes that the result occupies in memory:

/*  Returns the size of an int (4 if ints are four
 *  bytes long)
 */
sizeof(3 + 5)
 
/*  Returns the size of a double (8 if doubles are
 *  eight bytes long)
 */
sizeof(3.0 + 5)
 
/*  Returns the size of a float (4 if floats are
 *  four bytes long)
 */
float x;
sizeof(x)

For expressions, the parentheses are optional, so the following is legal:

sizeof x

By convention, however, the parentheses are usually included.

The operand can also be a data type, in which case the result is the length in bytes of objects of that type:

sizeof(char)    /* 1 on all machines */
sizeof(short)   /* 2 on HP 9000 Series */
sizeof(float)   /* 4 on HP 9000 Series  */
sizeof(int *)   /* 4 on HP 9000 Series */

The parentheses are required if the operand is a data type.

	NOTE: The results of most sizeof expressions are implementation dependent. The only result that is guaranteed is the size of a char, which is always 1.

In general, the sizeof operator is used to find the size of aggregate data objects such as arrays and structures.

Example

You can use the sizeof operator to obtain information about the sizes of objects in your C environment. The following prints the sizes of the basic data types:

/* Program name is "sizeof_example". This program * demonstrates a few uses of the sizeof operator. */ #include <stdio.h> int main(void) { printf("TYPE\t\tSIZE\n\n"); printf("char\t\t%d\n", sizeof(char)); printf("short\t\t%d\n", sizeof(short)); printf("int\t\t%d\n", sizeof(int)); printf("float\t\t%d\n", sizeof(float)); printf("double\t\t%d\n", sizeof(double)); }

If you execute this program, you get the following output:

TYPE SIZE char 1 short 2 int 4 float 4 double 8