Typedef struct

Structures are used to group together different data elements (types of variables) under the same name . These data elements, known as members, can have different types and different lengths.

A union is a special data type available in C that allows to store different data types in the same memory location . You can define a union with many members, but only one member can contain a value at any given time. Unions provide an efficient way of using the same memory location for multiple-purpose.

typedef is a reserved keyword in the programming languages C and C++ . It is used to create an additional name (alias) for another data type, but does not create a new type, except in the obscure case of a qualified typedef of an array type where the typedef qualifiers are transferred to the array element type.

The use of a typedef for a union type is very similar . typedef union Float Float; union Float { float f; char b[sizeof(float)]; }; A structure similar to this can be used to analyze the bytes that make up a float value.