Structures

Structure Initialization

• While defining a structure variable we can directly store values into it.

  struct student
  { 
    int rollno;
    char name[20];
    char gender;
  };
  main()
  {
    struct student a={4117,"Amit Jain",'m'};
    printf("\n Rollno is %d",a.rollno);
    printf("\n Name is %s",a.name);
    printf("\n Gender is %c",a.gender);
  }

Anonymous Structures

• If a structure variable is defined while defining a structure then there is no need to name the. Such a structure is called as anonymous structure. structure

  struct 
  { 
    int rollno;
    int age;
    char name[20];
  }a;
  main()
  { 
    printf("Enter the rollno,age,name\n");
    scanf("%d %d %s",&a.rollno,&a.age,a.name);
    printf("Rollno = %d\nAge = %d\nName = %s",a.rollno,a.age,a.name);
  }

Structure Pointer

• A pointer variable in which we can store address of a structure variable is called as structure pointer.

Accessing members :

• If a structure pointer is pointing to a structure variable then members of such structure variable can be accessed by using member indirection ( -> ) operator.

  struct stud
  { 
    int rollno;
    char name[20];
    char gender;
  };
  main()
  {
    struct stud a={4117,"Amit Jain",'m'};
    struct stud *p;
    p=&a;

    printf("\n Rollno is %d",p->rollno);
    printf("\n Name is %s",p->name);
    printf("\n gender is %c",p->gender);

    printf("\n Rollno is %d",(*p).rollno);
    printf("\n Name is %s",(*p).name);
    printf("\n gender is %c",(*p).gender);
  }

Rollno is 4117
Name is Amit Jain
gender is m
Rollno is 4117
Name is Amit Jain
gender is m
  

Structures as Arguments/call by Value

• While calling a function we can pass a structure as argument. If a structure variable is pass as argument then copy of structure variables data is passed to the function.

  #include<stdio.h>
  struct stud
  { 	
    int rollno;
    char name[20];
    char gender;
  };
  void display(struct stud s)
  { 	
    printf("\n Rollno is %d",s.rollno);
    printf("\n Name is %s",s.name);
    printf("\n Gender is %c",s.gender);
  }
  main()
  {
    struct stud a = {4117,"Amit Jain" ,'m'};
    struct stud b = {3012,"Raj Pande",'m'};
    display(a);
    display(b);
  }

Rollno is 4117
Name is Amit Jain
Gender is m
Rollno is 3012
Name is Raj Pande
Gender is m
  

Structure Pointer as Arguments/Call By Reference

• Instead of passing structure variable as argument we can pass address of structure variable as argument to the function.
• Here only the address of structure is passed instead of copy of structure variables data.
• The function can perform operation on structure by using its address.

  #include<stdio.h>
  struct stud
  { 	
    int rollno;
    char name[20];
    char gender ;
  };
  void display(struct stud *p)
  { 	
    printf("\n Rollno is %d", p->rollno );
    printf("\n Name is %s", p->name );
    printf("\n Gender is %c",p->gender);
  }
  main()
  {     
    struct stud a = {4117, "Amit Jain",'m'};
    struct stud b = {3012, "Raj Pande",'m'};
    display(&a);
    display(&b);
  }

Rollno is 4117
Name is Amit Jain
Gender is m
Rollno is 3012
Name is Raj Pande
Gender is m
  

Bit Fields

• To use memory efficiently we can specify size in bits of structure and union members.
• Bit Fields are used when we know that the value of a field or group of fields will never exceed a limit or is within a small range.

• we cannot use & addressof operator with bit fields
• A bit field size cannot exceed its datatype size.

  #include<stdio.h>
  struct date 
  { 
    unsigned int d; 
    unsigned int m; 
    unsigned int y; 
  }; 
  main()
  {
    struct date dt = {31, 12, 2014};  
    printf("Size of date is %d bytes\n", sizeof(dt)); 
    printf("Date is %d/%d/%d", dt.d, dt.m, dt.y); 
  }

Size of date is 12 bytes
Date is 31/12/2014
  

  #include<stdio.h>
  struct date 
  { 
    unsigned int d : 5 ; 
    unsigned int m : 4 ; 
    unsigned int y : 23 ; 
  }; 
  int main() 
  { 
    struct date dt = {31, 12, 2014}; 
    printf("Size of date is %d bytes\n", sizeof(dt)); 
    printf("Date is %d/%d/%d", dt.d, dt.m, dt.y); 
  }

Size of date is 4 bytes
Date is 31/12/2014
  

Structure padding

• In order to align the data in memory, one or more empty bytes (addresses) are inserted (or left empty) between memory addresses which are allocated for other structure members while memory allocation. This concept is called structure padding.
• Architecture of a computer processor is such a way that it can read 1 word (4 byte in 32 bit processor) from memory at a time.
• To make use of this advantage of processor, data are always aligned as 4 bytes package which leads to insert empty addresses between other member's address.
• Because of this structure padding concept in C, size of the structure is always not same as what we think.

for eg:

  struct structure1 
  {
    int id1;
    int id2;
    char name;
    char c;
    float percentage;
 };

for eg:

  struct structure2 
  {
    int id1;
    char name;
    int id2;
    char c;
    float percentage; 
  };