Why structure padding in C Language

Categories: C language

struct student  

{  

  char a; // 1 byte  

  char b; // 1 byte  

  int c; // 4 bytes   

}  

If we have a 32-bit processor (4 bytes at a time), then the pictorial representation of the memory for the above structure would be:

As we know that structure occupies the contiguous block of memory as shown in the above diagram, i.e., 1 byte for char a, 1 byte for char b, and 4 bytes for int c, then what problem do we face in this case.


What's the problem?

The 4-bytes can be accessed at a time as we are considering the 32-bit architecture. The problem is that in one CPU cycle, one byte of char a, one byte of char b, and 2 bytes of int c can be accessed. We will not face any problem while accessing the char a and char b as both the variables can be accessed in one CPU cycle, but we will face the problem when we access the int c variable as 2 CPU cycles are required to access the value of the 'c' variable. In the first CPU cycle, the first two bytes are accessed, and in the second cycle, the other two bytes are accessed.

Suppose we do not want to access the 'a' and 'b' variable, we only want to access the variable 'c', which requires two cycles. The variable 'c' is of 4 bytes, so it can be accessed in one cycle also, but in this scenario, it is utilizing 2 cycles. This is an unnecessary wastage of CPU cycles. Due to this reason, the structure padding concept was introduced to save the number of CPU cycles. The structure padding is done automatically by the compiler. Now, we will see how structure padding is done.


How is structure padding done?

In order to achieve the structure padding, an empty row is created on the left, as shown in the above diagram, and the two bytes which are occupied by the 'c' variable on the left are shifted to the right. So, all the four bytes of 'c' variable are on the right. Now, the 'c' variable can be accessed in a single CPU cycle. After structure padding, the total memory occupied by the structure is 8 bytes (1 byte+1 byte+2 bytes+4 bytes), which is greater than the previous one. Although the memory is wasted in this case, the variable can be accessed within a single cycle.


#include <stdio.h>  

struct student  

{  

   char a;  

   char b;  

   int c;  

};  

int main()  

{  

   struct student stud1; // variable declaration of the student type..  

   // Displaying the size of the structure student.  

   printf("The size of the student structure is %d", sizeof(stud1));  

   return 0;  

}  

In the above code, we have created a structure named 'student'. Inside the main() method, we declare a variable of student type, i.e., stud1, and then we calculate the size of the student by using the sizeof() operator. The output would be 8 bytes due to the concept of structure padding, which we have already discussed in the above.

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