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: <source lang="c"> struct account {

  int account_number;
  char *first_name;
  char *last_name;
  float balance;

}; </source>

defines a type, referred to as struct account. To create a new variable of this type, we can write <source lang="c"> struct account s; </source> 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 <source lang="c"> 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); </source>

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

typedef

Typedefs can be (mis)used as shortcuts, for example:

<source lang="c"> typedef struct account_ {

  int    account_number;
  char   *first_name;
  char   *last_name;
  float  balance;

} account; </source>

Different users have differing preferences; proponents usually claim:

• shorter to write

However, there are a handful of disadvantages in using them:

• they pollute the main namespace (see below)
• harder to figure out the aliased type (having to scan/grep through code)
• typedefs do not really "hide" anything in a struct or union — members are still accessible (account.balance)

  (To really hide struct members, one needs to use 'incompletely-declared' structs.)

<source lang="c"> /* Example for namespace clash */

typedef struct account { float balance; } account; struct account account; /* possible */ account account; /* error */ </source>

See also

• C language union