struct (C programming language)

A struct in the C programming language is a declaration that defines a list of variables to be placed under one name in a block of memory, allowing the different variables to be accessed via a single pointer.

Basic syntax

The best way to describe it is via example:

struct Point {
   int x;
   int y;
};

declares a structure called "Point" and states that this structure contains two pieces of information. The first is an integer called "x", the second is an integer called "y". A structure is a singular object and all of the members of the structure may be treated as one unit. A pointer to structure "Point" will point to the first integer "x" which is immediately followed by the second integer "y".

Once a structure is declared, variables may be declared using it:

struct Point vPoint;

declares a variable called "vPoint" which is a "Point". "vPoint.x" accesses the integer member "x" of the structure while "vPoint.y" accesses the integer member "y" of the structure. It is quite common to declare a structure as follows:

typedef struct Point {
   int x;
   int y;
} Point;

so that "Point" may be used as well as "struct Point" as in the following:

Point vPoint;

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, but such entities are better declared separately as in

typedef struct tag_name struct_alias;
struct tag_name struct_instance_1;
struct_alias struct_instance_2;

A struct declaration consists of a list of fields, each of which can have almost any object 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 data types, 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.

Struct initialization

There are three ways to initialize a structure. For the struct type

/* Forward declare a type "point" to be a struct. */
typedef struct point point;
/* Declare the struct with integer members x, y */
struct point {
   int    x;
   int    y;
};

C89-style initializers are used when contiguous members may be given.

/* Define a variable p of type point, and initialize its first two members in place */
point p = {1,2};

For non contiguous or out of order members list, designated initializer style may be used

/* Define a variable p of type point, and set members using designated  initializers*/
point p = {.y = 2, .x = 1};

If an initializer is given or if the object is statically allocated, omitted elements are initialized to 0.

A third way of initializing a structure is to copy the value of an existing object of the same type

/* Define a variable q of type point, and set members to the same values as those of p */
point q = p;

Assignment

The following assignment of a struct to another struct does what one might expect. It is not necessary to use memcpy() to make a duplicate of a struct type. The memory is already given and zeroed by just declaring a variable of that type regardless of member initialization. This should not be confused with the requirement of memory management when dealing with a pointer to a struct.

#include <stdio.h>
 
/* Define a type point to be a struct with integer members x, y */
typedef struct {
   int    x;
   int    y;
} point;
 
int main(void) {
 
/* Define a variable p of type point, and initialize all its members inline! */
    point p = {1,3};
 
/* Define a variable q of type point. Members are uninitialized. */
    point q;
 
/* Assign the value of p to q, copies the member values from p into q. */
    q = p;
 
/* Change the member x of q to have the value of 3 */
    q.x = 3;
 
/* Demonstrate we have a copy and that they are now different. */
    if (p.x != q.x) printf("The members are not equal! %d != %d", p.x, q.x);
 
    return 0;
}

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 */

C does not allow recursive declaration of struct; a struct can not contain a field that has the type of the struct itself. But pointers can be used to refer to an instance of it:

typedef struct list_element list_element;
struct list_element {
   point p;
   list_element * next;
};
list_element el = { .p = { .x = 3, .y =7 }, };
list_element le = { .p = { .x = 4, .y =5 }, .next = &el };

Here the instance el would contain a point with coordinates 3 and 7. Its next pointer would be a null pointer since the initializer for that field is omitted. The instance le in turn would have its own point and its next pointer would refer to el.

typedef

Main article: typedef

Typedefs can be used as shortcuts, for example:

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

Different users have differing preferences; proponents usually claim:

• shorter to write
• can simplify more complex type definitions
• can be used to forward declare a struct type

As an example, consider a type that defines a pointer to a function that accepts pointers to struct types and returns a pointer to struct:

Without typedef:

struct point {
   int    x;
   int    y;
};
struct point *(*point_compare_func) (struct point *a, struct point *b);

With typedef:

typedef struct point point_type;
struct point {
   int    x;
   int    y;
};
point_type *(*point_compare_func) (point_type *a, point_type *b);

A common naming convention for such a typedef is to append a "_t" (here point_t) to the struct tag name, but such names are reserved by POSIX so such a practice should be avoided. A much easier convention is to use just the same identifier for the tag name and the type name:

typedef struct point point;
struct point {
   int    x;
   int    y;
};
point *(*point_compare_func) (point *a, point *b);

Without typedef a function that takes function pointer the following code would have to be used. Although valid, it becomes increasingly hard to read.

/* Using the struct point type from before */
 
/* Define a function that returns a pointer to the biggest point,
   using a function to do the comparison. */
struct point *
biggest_point (size_t size, struct point *points,
               struct point *(*point_compare) (struct point *a, struct point *b))
{
    int i;
    struct point *biggest = NULL;
 
    for (i=0; i < size; i++) {
        biggest = point_compare(biggest, points + i);
    }
    return biggest;
}

Here a second typedef for a function pointer type can be useful

typedef point *(*point_compare_func_type) (point *a, point *b);

Now with the two typedefs being used the complexity of the function signature is drastically reduced.

/* Using the struct point type from before and the typedef for the function pointer */
 
/* Define a function that returns a pointer to the biggest point,
   using a function to do the comparison. */
point *
biggest_point (size_t size, point * points, point_compare_func_type point_compare)
{
    int i;
    point * biggest = NULL;
 
    for (i=0; i < size; i++) {
        biggest = point_compare(biggest, points + i);
    }
    return biggest;
}

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

• They pollute the main namespace (see below), however this is easily overcome with prefixing a library name to the type name.
• Harder to figure out the aliased type (having to scan/grep through code), though most IDEs provide this lookup automatically.
• 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.

/* Example for namespace clash */
 
typedef struct account { float balance; } account;
struct account account; /* possible */
account account; /* error */

See also

• GGWP
• Composite data type
• Union type