sizeof

Sizeof is a unary operator in C that generates the size of a variable or datatype.

sizeof (char) // 1 Byte, type must be enclosed in parenthesis 

The actual number of bits of the types is specified by prepocessor macros in limits.h.

/* the following code fragment illustrates the use of sizeof 
 * with variable and expressions (no parens needed), 
 * and with type names (parens needed)
 */
 char c;
 printf("%zu, %zu", sizeof c, sizeof(int))

The output for this program should be 1,4

When applied to a fixed length data type or variables, expressions with the operator sizeof are evaluated during program compilation; they are replaces by a constant result.

Structure padding

Man structures include padding between the different datatypes in memory so it may be bigger than the sum of the sizeof the types in the structure.