Introduction to Structure Padding in C

Structure padding mainly talks about memory for variables which are aligned based on the size of the variable. Let suppose a “char” of 1 byte memory can be assigned anywhere in between like 0x5000 to 0x5001. Same way if we have an “int” of 4 bytes memory can be assigned anywhere in between like 0x5004 to 0x5008. This structure padding concept is automatic for its member are byte aligned by the compiler.

Pre-requisites: Padding, Structure member alignment and data packing.

How Does Structure Padding Work in C?

Structure padding is said to be in order to align the data in memory 1 or more un-occupied bytes (empty bytes) are kept between any memory addresses which are actually assigned for other data structure members at the time of memory allocation.
If we observe the architecture of the computer processor can be read 1 word means bytes in 32 bit processor from memory at a time.
Utilize this advantage of processor then data is always inserted as 4 bytes package which will becomes insert empty address spaces in between other existing members address.
After introducing this structure padding concept in we got to know that size of the structure is not always same.

Syntax:

Struct member{
Char character;
Int number;
Double salary;
}

Explanation: “Char” data type takes only 1 byte after 3 byte padding(Char, Int and Double) then the number will starts at 4 bytes boundary and rest “Int” and “Double” will takes 4 and 8 bytes respectively.

Examples of Structure Padding in C

Below are the different eamples of Structure Padding in C.

Example #1

Code:

//include basic C library files
#include <stdio.h>
//including string data member in C
#include <string.h>
//creating first structure
struct first_structure
{
int rollNo1, rollNo2;
char firstName;
char character;
float salary;
};
//creating second structure
struct second_structure
{
int rollNo1;
char firstName;
int rollNo2;
char character;
float salary;
};
//main method to run the C application
int main()
{
//taking first structure reference
struct first_structure s1;
//taking second structure reference
struct second_structure s2;
//displaying first_structure and second_structure output
printf("===================FIRST STRUCTURE===============================\n");
printf("size of first_structure in bytes : %d\n",sizeof(s1));
printf ( "\n   Address of rollNo1        = %u",&s1.rollNo1 );
printf ( "\n   Address of rollNo2        = %u",&s1.rollNo2 );
printf ( "\n   Address of firstName       = %u",&s1.firstName );
printf ( "\n   Address of character          = %u",&s1.character);
printf ( "\n   Address of salary = %u",&s1.salary);
printf("\n===================SECOND STRUCTURE===============================\n");
printf("size of second_structure in bytes : %d\n",sizeof(s2));
printf ( "\n   Address of rollNo1        = %u",&s2.rollNo1 );
printf ( "\n   Address of rollNo2        = %u",&s2.rollNo2 );
printf ( "\n   Address of firstName       = %u",&s2.firstName );
printf ( "\n   Address of character          = %u",&s2.character);
printf ( "\n   Address of salary = %u",&s2.salary);
getchar();
return 0;
}

Output:

Example #2

Code:

//include basic C library files
#include<stdio.h>
//including string data member in C
#include <string.h>
//creating first structure
struct employee
{
char first_name[40];
char last_name[30];
};
//main method to run the C application
int main()
{
//taking first structure reference
struct employee e;
printf("Enter your first name:");
scanf("%s", &e.first_name);
printf("Enter your last name:");
scanf("%s",&e.last_name);
printf("First Name of Employee is :%s\n",e.first_name);
printf("Last Name of Employee is :%s\n", e.last_name);
//displaying output
printf("==================1ST STRUCTURE=========================\n");
printf("size of employee in bytes : %d\n",sizeof(e));
printf ( "\n   Address of first_name        = %u",&e.first_name);
printf ( "\n   Address of last_name        = %u",&e.last_name );
return 0;
}

Output:

Example #3

Overcome Structure padding problem

Code:

//include basic C library files
#include <stdio.h>
//including string data member in C
#include <string.h>
#pragma pack(1)
//creating first structure
struct first_structure
{
int rollNo1, rollNo2;
char firstName;
char character;
float salary;
};
//creating second structure
struct second_structure
{
int rollNo1;
char firstName;
int rollNo2;
char character;
float salary;
};
//main method to run the C application
int main()
{
//taking first structure reference
struct first_structure s1;
//taking second structure reference
struct second_structure s2;
//displaying first_structure and second_structure output
printf("===================FIRST STRUCTURE===============================\n");
printf("size of first_structure in bytes : %d\n",sizeof(s1));
printf ( "\n   Address of rollNo1        = %u",&s1.rollNo1 );
printf ( "\n   Address of rollNo2        = %u",&s1.rollNo2 );
printf ( "\n   Address of firstName       = %u",&s1.firstName );
printf ( "\n   Address of character          = %u",&s1.character);
printf ( "\n   Address of salary = %u",&s1.salary);
printf("\n===================SECOND STRUCTURE===============================\n");
printf("size of second_structure in bytes : %d\n",sizeof(s2));
printf ( "\n   Address of rollNo1        = %u",&s2.rollNo1 );
printf ( "\n   Address of rollNo2        = %u",&s2.rollNo2 );
printf ( "\n   Address of firstName       = %u",&s2.firstName );
printf ( "\n   Address of character          = %u",&s2.character);
printf ( "\n   Address of salary = %u",&s2.salary);
getchar();
return 0;
}

Output:

Conclusion

Structure padding is said to be in order to align the data in memory 1 or more un-occupied bytes (empty bytes) are kept between any memory addresses which are actually assigned for other data structure members at the time of memory allocation.