struct (C programming language)

A struct in the C programming language (and many derivatives) is a complex data type declaration that defines a physically grouped 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, or the struct declared name which returns the same address. The struct can contain many other complex and simple data types in an association, so is a natural organizing type for records like the mixed data types in lists of directory entries reading a hard drive (file length, name, extension, physical (cylinder, disk, head indexes) address, etc.), or other mixed record type (patient names, address, telephone... insurance codes, balance, etc.).

The C struct directly corresponds to the assembly language data type of the same use, and both reference a contiguous block of physical memory, usually delimited (sized) by word-length boundaries. Language implementations which could utilize half-word or byte boundaries (giving denser packing, using less memory) were considered advanced in the mid-eighties. Being a block of contiguous memory, each variable within is located at a fixed offset from the index zero reference, the pointer. As an illustration, many BASIC interpreters once fielded a string data struct organization with one value recording string length, one indexing (cursor value of) the previous line, one pointing to the string data.

Because the contents of a struct are stored in contiguous memory, the sizeof operator must be used to get the number of bytes needed to store a particular type of struct, just as it can be used for primitives. The alignment of particular fields in the struct (with respect to word boundaries) is implementation-specific and may include padding, although modern compilers typically support the #pragma pack directive, which changes the size in bytes used for alignment.^[1]

In the C++ language, a struct is identical to a C++ class but a difference in the default visibility exists: class members are by default private, whereas struct members are by default public.

In other languages

Like its C counterpart, the struct data type in C# (Structure in Visual Basic .NET) is similar to a class. The biggest difference between a struct and a class in these languages is that when a struct is passed as an argument to a function, any modifications to the struct in that function will not be reflected in the original variable (unless pass-by-reference is used).^[2]

This distinction differs from C++, where classes or structs can be allocated either on the stack (similar to C#) or on the heap, with an explicit pointer. In C++, the only difference between a struct and a class is that the members and base classes of a struct are public by default. (A class defined with the class keyword has private members and base classes by default.)

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.

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.^[3]

/* 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^[4] 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.^[5]

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 will copy as one might expect. Depending on the contents, a compiler might use memcpy() in order to perform this operation.

#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);

/* Define a variable r of type point. Members are uninitialized. */
    point r;

/* Assign values using compound literal (ISO C99/supported by GCC > 2.95) */
    r = (point) {1,2};

    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 */

References

↑ C struct memory layout? - Stack Overflow
↑ Parameter passing in C#
↑ Kelley, Al; Pohl, Ira (2004). A Book On C: Programming in C (Fourth ed.). p. 418. ISBN 0-201-18399-4.
↑ "IBM Linux compilers. Initialization of structures and unions".
↑ "The New C Standard, §6.7.8 Initialization".

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