Struct (C Programming Language)

Struct (C Programming Language)

A struct in C programming language is a structured (record) type that aggregates a fixed set of labelled objects, possibly of different types, into a single object.

A struct declaration consists of a list of fields, each of which can have any type. The total storage required for a struct object is the sum of the storage requirements of all the fields, plus any internal padding.

For example:

struct account { int account_number; char *first_name; char *last_name; float balance; };

defines a type, referred to as struct account. To create a new variable of this type, we can write

struct account s;

which has an integer component, accessed by s.account_number, and a floating-point component, accessed by s.balance, as well as the first_name and last_name components. The structure s contains all four values, and all four fields may be changed independently.

The primary use of a struct is for the construction of complex datatypes, but in practice they are sometimes used to circumvent standard C conventions to create a kind of primitive subtyping. For example, common Internet protocols rely on the fact that C compilers insert padding between struct fields in predictable ways; thus the code

struct ifoo_version_42 { long x, y, z; char *name; long a, b, c; }; struct ifoo_old_stub { long x, y; }; void operate_on_ifoo(struct ifoo_version_42 *); struct ifoo_old_stub s; . . . operate_on_ifoo(&s);

is often assumed to work as expected, if the operate_on_ifoo function only accesses fields x and y of its argument.