C Tutorial – structures, unions, typedef

In the C language structures are used to group together different types of variables under the same name. For example you could create a structure “telephone”: which is made up of a string (that is used to hold the name of the person) and an integer (that is used to hold the telephone number).
Take a look at the example:

	struct telephone
	{
		char *name;
		int number;
	};

Note: the ; behind the last curly bracket.

With the declaration of the structure you have created a new type, called telephone. Before you can use the type telephone you have to create a variable of the type telephone. Take a look at the following example:

	#include<stdio.h>

	struct telephone
	{
		char *name;
		int number;
	};

	int main()
	{
		struct telephone index;

		return 0;
	}

Note: index is now a variable of the type telephone.

To access the members of the structure telephone, you must use a dot between the structure name and the variable name(variables:name or number.) Take a look at the next example:

	#include<stdio.h>

	struct telephone
	{
		char *name;
		int number;
	};

	int main()
	{
		struct telephone index;

		index.name = "Jane Doe";
		index.number = 12345;
		printf("Name: %s\n", index.name);
		printf("Telephone number: %d\n", index.number);

		return 0;
	}

Type definitions and structures

Type definitions make it possible to create your own variable types. In the following example we will create a type definition called “intpointer” (a pointer to an integer):

	#include<stdio.h>

	typedef int *int_ptr;

	int main()
	{
		int_ptr myvar;
		return 0;
	}

It is also possible to use type definitions with structures. The name of the type definition of a structure is usually in uppercase letters. Take a look at the example:

	#include<stdio.h>

	typedef struct telephone
	{
		char *name;
		int number;
	}TELEPHONE;

	int main()
	{
		TELEPHONE index;

		index.name = "Jane Doe";
		index.number = 12345;
		printf("Name: %s\n", index.name);
		printf("Telephone number: %d\n", index.number);

		return 0;
	}

Note: The word struct is not needed before TELEPHONE index;

Pointer to structures

If you want a pointer to a structure you have to use the -> (infix operator) instead of a dot.
Take a look at the following example:

	#include<stdio.h>

	typedef struct telephone
	{
		char *name;
		int number;
	}TELEPHONE;

	int main()
	{
		TELEPHONE index;
		TELEPHONE *ptr_myindex;

		ptr_myindex = &index;

		ptr_myindex->name = "Jane Doe";
		ptr_myindex->number = 12345;
		printf("Name: %s\n", ptr_myindex->name);
		printf("Telephone number: %d\n", ptr_myindex->number);

		return 0;
	}

Note: The -> (infix operator) is also used in the printf statement.

Unions

A union is like a structure in which all members are stored at the same address. Members of a union can only be accessed one at a time. The union data type was invented to prevent memory fragmentation. The union data type prevents fragmentation by creating a standard size for certain data. Just like with structures, the members of unions can be accessed with the . and -> operators. Take a look at the example:

	#include<stdio.h>

	typedef union myunion
	{
		double PI;
		int B;
	}MYUNION;

	int main()
	{
		MYUNION numbers;
		numbers.PI = 3.14;
		numbers.B = 50;

	return 0;
	}

That’s all for this tutorial.