Sizeof Operator in C

The sizeof operator in C is used to determine the memory size (in bytes) of a variable, data type, or object. It is a compile-time operator.

Syntax

sizeof(expression_or_data_type);

Examples

• int a; printf("%zu", sizeof(a)); // Size of integer variable
• printf("%zu", sizeof(int)); // Size of int data type
• float f; printf("%zu", sizeof(f)); // Size of float variable
• double d; printf("%zu", sizeof(d)); // Size of double variable
• char c; printf("%zu", sizeof(c)); // Size of char variable

Key Points

• Returns the size in bytes of the operand
• Can be used with variables, data types, arrays, and structures
• Evaluated at compile-time, so it doesn’t affect runtime performance
• For arrays, returns the total size (number of elements × size of each element)

Common Mistakes

• Using sizeof on pointers to get array size (returns pointer size, not array size)
• Assuming sizeof always returns the same value across different systems (depends on architecture)
• For structures, not considering padding bytes
• Using sizeof with dynamically allocated memory incorrectly