sizeof and alignment in C

sizeof measures storage, and a struct is usually bigger than the sum of its fields.

#include <stddef.h>
#include <stdio.h>

struct Padded {
    char flag;
    int count;
    char label;
};

void layout(void) {
    printf("char %zu int %zu double %zu\n",
           sizeof(char), sizeof(int), sizeof(double));
    printf("struct %zu, fields %zu\n",
           sizeof(struct Padded), sizeof(char) * 2 + sizeof(int));
    printf("offsets %zu %zu %zu\n",
           offsetof(struct Padded, flag),
           offsetof(struct Padded, count),
           offsetof(struct Padded, label));
    printf("alignment %zu\n", _Alignof(struct Padded));
}

How it works

1. sizeof is evaluated at compile time and yields a size_t.
2. Padding aligns each member to its own requirement.
3. offsetof shows you exactly where the holes are.

Keywords and builtins used here

• Padded
• _Alignof
• char
• double
• int
• layout
• sizeof
• struct
• void

The run, in numbers

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Step 1 of 4 in Values & storage, step 27 of 35 in Language basics.

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