The sizeof is a keyword, but it is a compile-time operator that determines the size, in bytes, of a variable or data type. The sizeof operator can be used to get the size of classes, structures, unions and any other user defined data type.Click to see full answer. Consequently, what is sizeof () in C?The sizeof operator is the most common operator in C. It is a compile-time unary operator and used to compute the size of its operand. It returns the size of a variable. It can be applied to any data type, float type, pointer type variables.Likewise, what does sizeof return? Answer: sizeof returns the size of the type in bytes. Example: sizeof(char) is 100% guaranteed to be 1 , but this does not mean, that it’s one octet (8 bits). The sizeof operator yields the size (in bytes) of its operand, which may be an expression or the parenthesized name of a type. Furthermore, how does sizeof work? The sizeof keyword refers to an operator that works at compile time to report on the size of the storage occupied by a type of the argument passed to it (equivalently, by a variable of that type). That size is returned as a multiple of the size of a char, which on many personal computers is 1 byte (or 8 bits).Is sizeof an operator or function?In C language, sizeof( ) is an operator. Though it looks like a function, it is an unary operator. For example in the following program, when we pass a++ to sizeof, the expression “a++” is not evaluated. However in case of functions, parameters are first evaluated, then passed to function.