Struct (C Programming Language)

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Struct (C programming language) 1

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 type 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 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 the string data.

Basic syntax

The best way to describe it is via example:

structPoint {

intx;

inty;

};

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:

structPoint 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:

typedefstructPoint {

intx;

inty;

intz;

char*point_name;

} 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:
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structtag_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

typedefstructtag_name struct_alias;

structtag_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:

structaccount {

intaccount_number;

char*first_name;

char*last_name;

floatbalance;

};

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

structaccount 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

protocolsWikipedia:Please clarify rely on the fact that C compilers insert padding between struct fields in predictable

ways; thus the code

structifoo_version_42 {

longx, y, z;

char*name;

longa, b, c;

};

structifoo_old_stub {

longx, y;

};

voidoperate_on_ifoo(structifoo_version_42 *);

structifoo_old_stub s;

. . .

operate_on_ifoo(&s);
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is often assumed to work as expected,Wikipedia:Please clarify if the operate_on_ifoo function only accesses

fields x and y of its argument because a C compiler will map the struct elements to memory space exactly as it is

written in the source code, thus the x and y will point to exactly the same memory space in both structs.

Struct initialization

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

/* Forward declare a type "point" to be a struct. */

typedefstructpoint point;

/* Declare the struct with integer members x, y */

structpoint {

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

/* Define a type point to be a struct with integer members x, y */

typedefstruct{

int x;

int y;

} point;

intmain(void) {
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/* 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);

return0;

}

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).

structpoint {

intx;

inty;

};

structpoint my_point ={ 3, 7};

structpoint *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:

typedefstructlist_element list_element;

structlist_element { point p;


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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:

typedefstruct{

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:

structpoint {

int x;

int y;

};

structpoint *(*point_compare_func) (structpoint *a, structpoint *b);

With typedef:

typedefstructpoint point_type;

structpoint {

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:

typedefstructpoint point;

structpoint {

int x;
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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. */

structpoint *

biggest_point(size_tsize, structpoint *points,

structpoint *(*point_compare) (structpoint *a, struct

point *b))

{

inti; structpoint *biggest =NULL;

for(i=0; i


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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 */

typedefstructaccount { floatbalance; } account;

structaccount account; /* possible */

account account; /* error */

References
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Article Sources and ContributorsStruct (C programming language) Source: http://en.wikipedia.org/w/index.php?oldid=608670192 Contributors: Abdull, Adamrice, Beland, Bgwhite, CarminPolitano, Chris the speller,

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