structures in c programming size of structure

the size of a structure is expected is the total of all data members in the structure.

but in reality the sizeof the structure will show additional bytes.

In C, structures can sometimes occupy more memory than the sum of their individual members due to a concept called padding. This is done to ensure proper alignment of data members in memory for efficient access by the CPU.

Reasons for padding:

Alignment requirements:

Most CPUs prefer to access data that is aligned to specific memory addresses. For example, a 4-byte integer might be required to start at an address that is a multiple of 4.

Performance optimization:

Accessing unaligned data can cause performance penalties, as the CPU might need to perform multiple memory accesses to retrieve the data.

#include <stdio.h>

struct myStruct {
char c; // 1 byte
int i; // 4 bytes
};

int main() {
printf("Size of char: %zu\n", sizeof(char));
printf("Size of int: %zu\n", sizeof(int));
printf("Size of myStruct: %zu\n", sizeof(struct myStruct));

return 0;
}

Output

Size of char: 1
Size of int: 4
Size of myStruct: 8

Explanation:

• The char member occupies 1 byte.
• The int member occupies 4 bytes.
• However, the total size of the structure is 8 bytes, not 5 bytes as you might expect. This is because the compiler adds 3 bytes of padding after the char member to ensure that the int member is aligned to a 4-byte boundary.