Difference between Structure and Union in C

Structures in C is a user-defined data type available in C that allows to combining of data items of different kinds. Structures are used to represent a record. 

Defining a structure: To define a structure, you must use the struct statement. The struct statement defines a new data type, with more than or equal to one member. The format of the struct statement is as follows:

   struct [structure name]
   {
       member definition;
       member definition;
       ...
       member definition;
   };
   
   (OR)

   struct [structure name]
   {
       member definition;
       member definition;
       ...
       member definition;
   }structure variable declaration;

Union in C is a special data type available in C that allows storing different data types in the same memory location. You can define a union with many members, but only one member can contain a value at any given time. Unions provide an efficient way of using the same memory location for multiple purposes. 

Defining a Union: To define a union, you must use the union statement in the same way as you did while defining a structure. The union statement defines a new data type with more than one member for your program. The format of the union statement is as follows: 

    union [union name]
    {
       member definition;
       member definition;
       ...
       member definition;
    };
   
    (OR) 

    union [union name]
    {
       member definition;
       member definition;
       ...
       member definition;
    }union variable declaration;

Similarities between Structure and Union

1. Both are user-defined data types used to store data of different types as a single unit.
2. Their members can be objects of any type, including other structures and unions or arrays. A member can also consist of a bit field.
3. Both structures and unions support only assignment = and sizeof operators. The two structures or unions in the assignment must have the same members and member types.
4. A structure or a union can be passed by value to functions and returned by value by functions. The argument must have the same type as the function parameter. A structure or union is passed by value just like a scalar variable as a corresponding parameter.
5. ‘.’ operator or selection operator, which has one of the highest precedences, is used for accessing member variables inside both the user-defined datatypes.

Differences between Structure and Union are as shown below in tabular format as shown below as follows:

C

// C program to illustrate differences
// between structure and Union

#include <stdio.h>
#include <string.h>

// declaring structure
struct struct_example
{
    int integer;
    float decimal;
    char name[20];
};

// declaring union

union union_example
{
    int integer;
    float decimal;
    char name[20];
};

void main()
{
    // creating variable for structure
    // and initializing values difference
    // six
    struct struct_example s={18,38,"geeksforgeeks"};

    // creating variable for union
    // and initializing values
    union union_example u={18,38,"geeksforgeeks"};


    printf("structure data:\n integer: %d\n"
                "decimal: %.2f\n name: %s\n",
                s.integer, s.decimal, s.name);
    printf("\nunion data:\n integer: %d\n"
                 "decimal: %.2f\n name: %s\n",
                u.integer, u.decimal, u.name);


    // difference two and three
    printf("\nsizeof structure : %d\n", sizeof(s));
    printf("sizeof union : %d\n", sizeof(u));

    // difference five
    printf("\n Accessing all members at a time:");
    s.integer = 183;
    s.decimal = 90;
    strcpy(s.name, "geeksforgeeks");

    printf("structure data:\n integer: %d\n "
                "decimal: %.2f\n name: %s\n",
            s.integer, s.decimal, s.name);

    u.integer = 183;
    u.decimal = 90;
    strcpy(u.name, "geeksforgeeks");

    printf("\nunion data:\n integer: %d\n "
                "decimal: %.2f\n name: %s\n",
            u.integer, u.decimal, u.name);

    printf("\n Accessing one member at time:");

    printf("\nstructure data:");
    s.integer = 240;
    printf("\ninteger: %d", s.integer);

    s.decimal = 120;
    printf("\ndecimal: %f", s.decimal);

    strcpy(s.name, "C programming");
    printf("\nname: %s\n", s.name);

    printf("\n union data:");
    u.integer = 240;
    printf("\ninteger: %d", u.integer);

    u.decimal = 120;
    printf("\ndecimal: %f", u.decimal);

    strcpy(u.name, "C programming");
    printf("\nname: %s\n", u.name);

    //difference four
    printf("\nAltering a member value:\n");
    s.integer = 1218;
    printf("structure data:\n integer: %d\n "
                " decimal: %.2f\n name: %s\n",
                s.integer, s.decimal, s.name);

    u.integer = 1218;
    printf("union data:\n integer: %d\n"
           " decimal: %.2f\n name: %s\n",
            u.integer, u.decimal, u.name);
}

Output:

structure data:
 integer: 18
 decimal: 38.00
 name: geeksforgeeks

union data:
 integer: 18
 decimal: 0.00
 name: ?

sizeof structure: 28
sizeof union: 20

 Accessing all members at a time: structure data:
 integer: 183
 decimal: 90.00
 name: geeksforgeeks

union data:
 integer: 1801807207
 decimal: 277322871721159510000000000.00
 name: geeksforgeeks

 Accessing one member at a time:
structure data:
integer: 240
decimal: 120.000000
name: C programming

 union data:
integer: 240
decimal: 120.000000
name: C programming

Altering a member value:
structure data:
 integer: 1218
 decimal: 120.00
 name: C programming
union data:
 integer: 1218
 decimal: 0.00
 name: ?

Note: structures are better than unions since memory is shared in a union which results in a bit of ambiguity. But technically speaking, unions are better in that they help save a lot of memory, resulting in the overall advantage over structures in the long run.
Quiz on structures and Union. 

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