# <stddef.h>

**[<stddef.h>](https://en.cppreference.com/w/c/header/stddef)** provides fundamental types and macros: `size_t` (for sizes), `ptrdiff_t` (pointer differences), `wchar_t`, `NULL`, and `offsetof()`. Most headers include it transitively, but it's essential to include directly for portable code.

Use `size_t` for array indices and loop counters, never `int`.

## Example

This example queries structure member offsets and demonstrates `size_t` usage.

```c
// compile: gcc -o stddefexample stddefexample.c
// run: ./stddefexample
// description: query struct member offsets and sizes

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

struct packet {
    char type;
    int length;
    unsigned int checksum;
};

int main() {
    printf("sizeof(packet) = %zu\n", sizeof(struct packet));
    printf("offset(type) = %zu\n", offsetof(struct packet, type));
    printf("offset(length) = %zu\n", offsetof(struct packet, length));
    printf("offset(checksum) = %zu\n", offsetof(struct packet, checksum));
    
    return 0;
}
```