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Structures And Unions

Posted By: Lurleen Fischer     Category: C Programming     Views: 22855

This article explains about Structures and Functions, Unions, Structure of Bitfields etc...

Structures And Unions

  • The need to have heterogeneous data to be stored together like student’s data
  • It is analogous of record of certain kind
  • It can group logically related items to make the complex design more meaningful
  • Structure and unions are two different data types which hold other data types in turn

Structure Definition


 struct student { int r_no;
char name[20];
char address[20];
                        } stud1,stud2,stud3;
 struct     { int r_no;
char name[20];
char address[20];
                        } stud1,stud2,stud3;

Assigning Values

 stud1.r_no = 10;
 strcpy(stud1.name , “Ram” );
 strcpy (stud1.address , “Ayodhya”);
 static struct student { …} stud3 = {20, ”Bharat” ,”Ayodhya”}
 static struct student student4= {30, “Shatrughna”, “Ayodhya”}
 struct student{…} stud1 = {40, “Laxman”, “Ayodhya” } // No static!
 { struct student student2 = {…}

Note : C does not support initialization to the template variables, it must be done to actual variables.

Comparing Structures

Operation Meaning
person1 = person2 Assign person2 to person1
person1 == person2  Compare all members of person1 and person2 and return 1 if they are
equal and 0 otherwise
person1 != person2 Return 1 if all the members are not equal, zero otherwise
  • Array of Structures
  • Array in structures
  • Structures within structures
  • The hierarchy of nested structures and use of ‘.’ operator to address deepest member
  • The inner structure can have more than one variables!

Structures and Functions

  • All individual members of the structure can be passed as if individual variables
  • Whole structure can be passed to a function
  • The function, in the second case, must have the structure argument defined to accept 
  • A pointer to structure can be passed like an array, where a function can work on the elements directly


  • Unions are structures with storage for a single member only at a  time
  •  union item{ int m; float x; char c;} code;
  •  code.m, code.x, code.c (can’t co-exist!)

sizeof() Operator

  •  sizeof(variable) or sizeof(type of variable)
  •  sizeof(student) or sizeof stud1
  •  Summation of the size of all individual members is not always equal to  the size of structure
  •  Sizeof( <array name>)/sizeof(record) will give total actual elements in  the array


  • Integer fields are too big for one or two bit data which is common in networking 
  • That is also very useful in data handling for files
  • A bit field is a set of adjacent bits whose size can be 1 to 16 bits
  • A word can be divided in no of bit fields
  • The internal rep depends on the machine

Structure of Bitfields

  • The first field always starts from the first bit of the word
  • A bit field can not overlap integer boundaries, total length of all bitfields of a structure should be < word size
  • Otherwise the overlapping field is forced to start with beginning of the next word
  • Unnamed fields provides padding
  • There can be unused bits in the word
  • Address of a bit field variable can not be taken, i.e. scanf can not be used to read it
  • We can’t use pointers to access them
  • Bit fields can not be arrayed
  • If values are assigned outside the range, results are unpredictable 

Examples of Usage

 emp.sex = 1;
 emp.age = 50;
 scanf(“%d”,AGE); emp.age = AGE;
 sum = sum + emp.age
 if (emp.marital_status) …

 Normal structure elements can be combined with bit fields
 packing will be done for cons BFs only

 struct personal
{ char name[20];
struct addr address; // structure variable
   unsigned sex :1;
  …… } emp [100];
 struct pack
 { unsigned a:2;
    int count;
    unsigned b :3; }


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Lurleen Fischer
Lurleen Fischer author of Structures And Unions is from Frankfurt, Germany.
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