Structure in C

Structure is the collection of variables of different types under a single name for better handling. For example: You want to store the information about person about his/her name, citizenship number and salary. You can create these information separately but, better approach will be collection of these information under single name because all these information are related to person.
Structure Definition in C : Keyword struct is used for creating a structure.

Syntax of structure :

struct structure_name 
{
    data_type member1;
    data_type member2;
    .
    .
    data_type memeber;
};

We can create the structure for a person as mentioned above as:

struct person
{
    char name[50];
    int cit_no;
    float salary;
};

This declaration above creates the derived data type struct person.
Structure variable declaration
When a structure is defined, it creates a user-defined type but, no storage is allocated. For the above structure of person, variable can be declared as:

This declaration above creates the derived data type struct person.

Structure variable declaration
When a structure is defined, it creates a user-defined type but, no storage is allocated. For the above structure of person, variable can be declared as:
struct person
{
    char name[50];
    int cit_no;
    float salary;
};

Inside main function:
struct person p1, p2, p[20];
Another way of creating sturcture variable is:
struct person
{
    char name[50];
    int cit_no;
    float salary;
}p1 ,p2 ,p[20];
In both cases, 2 variables p1, p2 and array p having 20 elements of type struct person are created.

Accessing members of a structure :
There are two types of operators used for accessing members of a structure.
Member operator(.)
Structure pointer operator(->)
Any member of a structure can be accessed as: structure_variable_name.member_name
Suppose, we want to access salary for variable p2. Then, it can be accessed as:
p2.salary

Example code:

C program to add two distances entered by user. Measurement of distance should be in inch and feet.(Note: 12 inches = 1 foot)

#include <stdio.h>
struct Distance{
    int feet;
    float inch;
}d1,d2,sum;
int main(){
    printf("1st distance\n");
    printf("Enter feet: ");
    scanf("%d",&d1.feet);  /* input of feet for structure variable d1 */
    printf("Enter inch: ");
    scanf("%f",&d1.inch);  /* input of inch for structure variable d1 */
    printf("2nd distance\n");
    printf("Enter feet: ");
    scanf("%d",&d2.feet);  /* input of feet for structure variable d2 */
    printf("Enter inch: ");
    scanf("%f",&d2.inch);  /* input of inch for structure variable d2 */
    sum.feet=d1.feet+d2.feet;
    sum.inch=d1.inch+d2.inch;
    if (sum.inch>12){  //If inch is greater than 12, changing it to feet.
        ++sum.feet;
        sum.inch=sum.inch-12;
    }
    printf("Sum of distances=%d\'-%.1f\"",sum.feet,sum.inch); 
/* printing sum of distance d1 and d2 */
    return 0;
}

Output

1st distance
Enter feet: 12
Enter inch: 7.9
2nd distance
Enter feet: 2
Enter inch: 9.8
Sum of distances= 15'-5.7"

Keyword typedef while using structure
Programmer generally use typedef while using structure in C language. For example:
typedef struct complex{
  int imag;
  float real;
}comp;

Inside main:
comp c1,c2;
Here, typedef keyword is used in creating a type comp(which is of type as struct complex). Then, two structure variables c1 and c2 are created by this comp type.
Structures within structures
Structures can be nested within other structures in C programming.

struct complex
{
 int imag_value;
 float real_value;
};
struct number{
   struct complex c1;
   int real;
}n1,n2;

Suppose you want to access imag_value for n2 structure variable then, structure member n1.c1.imag_value is used.
Structure and Pointer
Pointers can be accessed along with structures. A pointer variable of structure can be created as below:

1st distance
struct name {
    member1;
    member2;
    .
    .
};

Inside function 
struct name *ptr;
Here, the pointer variable of type struct name is created.
Structure's member through pointer can be used in two ways:
Referencing pointer to another address to access memory
Using dynamic memory allocation

Consider an example to access structure's member through pointer.

#include <stdio.h>
struct name{
   int a;
   float b;
};
int main(){
    struct name *ptr,p;
    ptr=&p;            /* Referencing pointer to memory address of p */
    printf("Enter integer: ");
    scanf("%d",&(*ptr).a);
    printf("Enter number: ");
    scanf("%f",&(*ptr).b);
    printf("Displaying: ");
    printf("%d%f",(*ptr).a,(*ptr).b);
    return 0;
}

In this example, the pointer variable of type struct name is referenced to the address of p. Then, only the structure member through pointer can can accessed.
Structure pointer member can also be accessed using ->operator.
(*ptr).a is same as ptr->a
(*ptr).b is same as ptr->b

Accessing structure member through pointer using dynamic memory allocation
To access structure member using pointers, memory can be allocated dynamically using malloc() function defined under "stdlib.h" header file.
Syntax to use malloc()
ptr=(cast-type*)malloc(byte-size)

Example to use structure's member through pointer using malloc() function

#include <stdio.h>
#include<stdlib.h>
struct name {
   int a;
   float b;
   char c[30];
};
int main(){
   struct name *ptr;
   int i,n;
   printf("Enter n: ");
   scanf("%d",&n);
   ptr=(struct name*)malloc(n*sizeof(struct name));
/* Above statement allocates the memory for n structures with pointer ptr pointing to base address */
   for(i=0;i<n;++i){
       printf("Enter string, integer and floating number  respectively:\n");
       scanf("%s%d%f",&(ptr+i)->c,&(ptr+i)->a,&(ptr+i)->b);
   }
   printf("Displaying Infromation:\n");
   for(i=0;i<n;++i)
       printf("%s\t%d\t%.2f\n",(ptr+i)->c,(ptr+i)->a,(ptr+i)->b);
   return 0;
}

Output

Enter n: 2
Enter string, integer and floating number  respectively:
Programming
2
3.2
Enter string, integer and floating number  respectively:
Structure
6
2.3
Displaying Information
Programming      2      3.20
Structure      6      2.30

Structure and Function
In C, structure can be passed to functions by two methods:

1. Passing by value (passing actual value as argument)
2. Passing by reference (passing address of an argument)

Passing structure by value
A structure variable can be passed to the function as an argument as normal variable. If structure is passed by value, change made in structure variable in function definition does not reflect in original structure variable in calling function.
Write a C program to create a structure student, containing name and roll. Ask user the name and roll of a student in main function. Pass this structure to a function and display the information in that function.

#include <stdio.h>
struct student{
    char name[50];
    int roll;
};
void Display(struct student stu);
/* function prototype should be below to the structure 
declaration otherwise compiler shows error */
int main(){
    struct student s1;
    printf("Enter student's name: ");
    scanf("%s",&s1.name);
    printf("Enter roll number:");
    scanf("%d",&s1.roll);
    Display(s1);   // passing structure variable s1 as argument
    return 0;
}
void Display(struct student stu){
  printf("Output\nName: %s",stu.name);
  printf("\nRoll: %d",stu.roll);
}

Output

Enter student's name: Kevin Amla
Enter roll number: 149
Output
Name: Kevin Amla
Roll: 149

Passing structure by reference
The address location of structure variable is passed to function while passing it by reference. If structure is passed by reference, change made in structure variable in function definition reflects in original structure variable in the calling function.
example:
C program to add two distances(feet-inch system) entered by user. To solve this program, make a structure. Pass two structure variable (containing distance in feet and inch) to add function by reference and display the result in main function without returning it.

#include <stdio.h>
struct distance{
    int feet;
    float inch;
};
void Add(struct distance d1,struct distance d2, struct distance *d3); 
int main()
{
    struct distance dist1, dist2, dist3;
    printf("First distance\n");
    printf("Enter feet: ");
    scanf("%d",&dist1.feet);
    printf("Enter inch: ");
    scanf("%f",&dist1.inch);
    printf("Second distance\n");
    printf("Enter feet: ");
    scanf("%d",&dist2.feet);
    printf("Enter inch: ");
    scanf("%f",&dist2.inch);
    Add(dist1, dist2, &dist3); 

/*passing structure variables dist1 and dist2 by value whereas passing structure variable dist3 by reference */
    printf("\nSum of distances = %d\'-%.1f\"",dist3.feet, dist3.inch);
    return 0;
}
void Add(struct distance d1,struct distance d2, struct distance *d3) 
{
/* Adding distances d1 and d2 and storing it in d3 */
     d3->feet=d1.feet+d2.feet; 
     d3->inch=d1.inch+d2.inch;
     if (d3->inch>=12) {     /* if inch is greater or equal to 12, converting it to feet. */
         d3->inch-=12;
         ++d3->feet;
    }
}

Output

First distance
Enter feet: 12
Enter inch: 6.8
Second distance
Enter feet: 5
Enter inch: 7.5

Sum of distances = 18'-2.3"

Explanation :
In this program, structure variables dist1 and dist2 are passed by value (because value of dist1 and dist2 does not need to be displayed in main function) and dist3 is passed by reference ,i.e, address of dist3 (&dist3) is passed as an argument. Thus, the structure pointer variable d3 points to the address of dist3. If any change is made in d3 variable, effect of it is seed in dist3 variable in main function.

Union :
Union is user defined data type used to stored data under unique variable name at single memory location.
Union is similar to that of stucture. Syntax of union is similar to stucture. But the major difference between structure and union is 'storage.' In structures, each member has its own storage location, whereas all the members of union use the same location. Union contains many members of different types, it can handle only one member at a time.
To declare union data type, 'union' keyword is used.
Union holds value for one data type which requires larger storage among their members.
Syntax:

union union_name
	{
		<data-type>element 1;
		<data-type>element 2;
		<data-type>element 3;
	}union_variable;

Difference between union and structure
Though unions are similar to structure in so many ways, the difference between them is crucial to understand. This can be demonstrated by this
Example :

#include <stdio.h>
union job {    //defining a union
   char name[32];
   float salary;
   int worker_no;
}u;
struct job1 {
   char name[32];
   float salary;
   int worker_no;
}s;
int main(){
   printf("size of union = %d",sizeof(u));
   printf("\nsize of structure = %d", sizeof(s));
   return 0;
}

Output

size of union = 32
size of structure = 40

There is difference in memory allocation between union and structure as suggested in above example. The amount of memory required to store a structure variables is the sum of memory size of all members.

But, the memory required to store a union variable is the memory required for largest element of an union

Important questions:

Q. What the advantages of using Unions?
Ans: When the C compiler is allocating memory for unions it will always reserve enough room for the largest member.

Q. What are bit fields? What is the use of bit fields in a Structure declaration?
Ans:A bit field is a set of adjacent bits within a single implementation based storage unit that we
will call a “word”.
The syntax of field definition and access is based on structure.
Struct
{
unsigned int k :1;
unsigned int l :1;
unsigned int m :1;
}
flags;
the number following the colon represents the field width in bits.Flag is a variable that contains three bit fields.