How to Find Size of Structure in C: Simple Guide

In C, you can find the size of a structure using the sizeof operator with the structure's name or a variable of that structure type. For example, sizeof(struct MyStruct) returns the size in bytes of the structure MyStruct.

Syntax

The sizeof operator returns the size in bytes of a data type or variable. To find the size of a structure, use:

• sizeof(struct StructName) - size of the structure type
• sizeof(variable) - size of a structure variable

This size includes all members and any padding added by the compiler.

c

sizeof(struct StructName);

// or

struct StructName var;
sizeof(var);

Example

This example shows how to define a structure and find its size using sizeof. It prints the size in bytes.

c

#include <stdio.h>

struct Person {
    char name[50];
    int age;
    double height;
};

int main() {
    struct Person p;
    printf("Size of struct Person: %zu bytes\n", sizeof(struct Person));
    printf("Size of variable p: %zu bytes\n", sizeof(p));
    return 0;
}

Output

Size of struct Person: 64 bytes
Size of variable p: 64 bytes

Common Pitfalls

Some common mistakes when finding structure size include:

• Assuming the size is just the sum of member sizes; padding may increase size.
• Using sizeof(pointer_to_struct) instead of the structure itself, which returns pointer size, not structure size.
• Not considering alignment and padding added by the compiler.

Always use sizeof(struct StructName) or sizeof(variable) to get the correct size.

c

#include <stdio.h>

struct Data {
    char c;
    int i;
};

int main() {
    struct Data d;
    printf("Wrong size (pointer): %zu bytes\n", sizeof(&d)); // size of pointer, usually 4 or 8
    printf("Correct size (struct): %zu bytes\n", sizeof(d));
    return 0;
}

Output

Wrong size (pointer): 8 bytes
Correct size (struct): 8 bytes

Quick Reference

Remember these tips when finding structure size:

• Use sizeof(struct StructName) or sizeof(variable).
• Do not use sizeof(pointer) to get structure size.
• Size includes padding for alignment.
• Use %zu format specifier with printf for sizeof results.

Key Takeaways

Use the sizeof operator with the structure type or variable to get its size in bytes.

Do not use sizeof on a pointer to a structure to find the structure's size.

Structure size includes padding added by the compiler for alignment.

Use %zu in printf to correctly print the size returned by sizeof.

Always verify structure size especially when working with binary data or memory layouts.