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.
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return 0;
}
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