sizeof

Learn type sizes with the sizeof operator.

The sizeof operator

The sizeof operator tells how many bytes a type or variable takes in memory. The result has type size_t:

printf("%zu", sizeof(int));
// Output (typically): 4

The result can vary by platform, so use %zu instead of %d. sizeof is computed at compile time, with no runtime cost.

What is sizeof for?

sizeof is used for three main purposes:

Portability: write code based on the type (e.g., malloc(sizeof(int) * n))
Error prevention: avoids copying with the wrong byte count
Documentation: shows the type's size to the reader

Writing sizeof(int) instead of hardcoding 4 or 8 keeps your code portable across platforms.

Exercise Print type sizes

Add two printf calls to the program below: one printing sizeof(int) and the other sizeof(double) with %zu. The output should be int: 4\ndouble: 8.

Show example solution

#include <stdio.h>

int main(void) {
    printf("int: %zu\n", sizeof(int));
    printf("double: %zu", sizeof(double));
    return 0;
}

Test yourself

Which specifier for sizeof result?
- %zu
- %s
- %c
- %d
Show answer

%zu — sizeof returns a size_t; print it with %zu.

sizeof usage

printf("%zu", ____);

Show answer

printf("%zu", sizeof(int));

Match sizeof concepts

sizeof

Size operator
size_t

Size type
%zu

Size specifier
4

int size

Next lesson: Type conversion

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