Declaration

The general syntax for a struct declaration in C is:

struct tag_name {
   type member1;
   type member2;
   /* declare as many members as desired, but the entire structure size must be known to the compiler. */
};

Here tag_name is optional in some contexts.

Such a struct declaration may also appear in the context of a typedef declaration of a type alias or the declaration or definition of a variable:

typedef struct tag_name {
   type member1;
   type member2;
} struct_alias;

Often, such entities are better declared separately, as in:

typedef struct tag_name struct_alias;

// These two statements now have the same meaning:
// struct tag_name struct_instance;
// struct_alias struct_instance;

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.

A pointer to an instance of the "account" structure will point to the memory address of the first variable, "account_number". The total storage required for a struct object is the sum of the storage requirements of all the fields, plus any internal padding.

Pointers to struct

Pointers can be used to refer to a struct by its address. This is particularly useful for passing structs to a function by reference or to refer to another instance of the struct type as a field. The pointer can be dereferenced just like any other pointer in C, using the * operator. There is also a -> operator in C which dereferences the pointer to struct (left operand) and then accesses the value of a member of the struct (right operand).

struct point {
   int x;
   int y;
};
struct point my_point = { 3, 7 };
struct point *p = &my_point;  /* To declare and define p as a pointer of type struct point,
                                 and initialize it with the address of my_point. */

(*p).x = 8;                   /* To access the first member of the struct */
p->x = 8;                     /* Another way to access the first member of the struct */

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