C in a Nutshell, 2nd Edition

Chapter 10. Structures, Unions, and Bit-Fields

The pieces of information that describe the characteristics of objects, such as information on companies or customers, are generally grouped together in records. Records make it easy to organize, present, and store information about similar objects.

A record is composed of fields that contain the individual details, such as the name, address, and legal form of a company. In C, you determine the names and types of the fields in a record by defining a structure type. The fields are called the members of the structure.

A union is defined in the same way as a structure. Unlike the members of a structure, all the members of a union start at the same address. Hence you define a union type when you want to use the same location in memory for different types of objects.

In addition to the basic and derived types, the members of structures and unions can also include bit-fields. A bit-field is an integer variable composed of a specified number of bits. By defining bit-fields, you can break down an addressable memory unit into groups of individual bits that you can address by name.

Structures

A structure type is a type defined within the program that specifies the format of a record, including the names and types of its members, and the order in which they are stored. Once you have defined a structure type, you can use it like any other type in declaring objects, pointers to those objects, and arrays of such structure elements.

Defining Structure ...