sizeof

// Returns the size of a type in bytes.
sizeof(type)

// Returns the size of a variable in bytes.
sizeof(variable)
sizeof variable  // Parentheses can be omitted for variables.

// Returns the total size of an array in bytes.
sizeof(array)

// Common idiom to get the number of elements in an array.
sizeof(array) / sizeof(array[0])

#include <stdio.h>

int main(void) {
    // Print the size of each type.
    printf("char:      %zu bytes\n", sizeof(char));
    printf("short:     %zu bytes\n", sizeof(short));
    printf("int:       %zu bytes\n", sizeof(int));
    printf("long:      %zu bytes\n", sizeof(long));
    printf("long long: %zu bytes\n", sizeof(long long));
    printf("float:     %zu bytes\n", sizeof(float));
    printf("double:    %zu bytes\n", sizeof(double));

    // Check the size of variables.
    int n = 42;
    double x = 3.14;
    printf("size of n: %zu bytes\n", sizeof n);
    printf("size of x: %zu bytes\n", sizeof x);

    // Get the total size and element count of an array.
    int scores[10];
    printf("array size:    %zu bytes\n", sizeof(scores)); // 40 bytes (10 × 4)
    printf("element count: %zu\n", sizeof(scores) / sizeof(scores[0])); // 10

    // Pointer size depends on the address width.
    int *p = &n;
    printf("pointer size: %zu bytes\n", sizeof(p)); // 8 on a 64-bit environment

    // Using sizeof with dynamic memory allocation (combined with malloc).
    // int *arr = malloc(sizeof(int) * 100);

    return 0;
}

sizeof is evaluated at compile time (except when used with variable-length arrays). Its return type is size_t, an unsigned integer type. Use %zu with printf to print it.

If you assign an array to a pointer or pass it to a function, sizeof returns the size of the pointer itself, not the array. You can only get the full array size when passing the array directly. This behavior is known as "array-to-pointer decay."

When using dynamic memory allocation with malloc(), always use sizeof(type) instead of hardcoding the size — this improves portability. You can also use sizeof with structs, but be aware that the compiler may insert padding bytes for alignment purposes.