Compile-Time Static Assertions

Challenge

Binary protocol and serialization headers depend on exact struct layout and alignment. In C11, _Static_assert (or static_assert from <assert.h>) catches struct padding and size mismatches at compile time rather than crashing in production.

Your Task

Given the protocol requirement:

• A packet header must be packed so that sizeof(PacketHeader) == 8.

Define the struct:

typedef struct {
    uint32_t stream_id;
    uint16_t length;
    uint8_t flags;
    uint8_t version;
} PacketHeader;

And implement validator functions:

size_t get_packet_header_size(void);
int pack_packet_header(uint32_t stream_id, uint16_t len, uint8_t flags, uint8_t ver, PacketHeader *out);

Rules

1. In get_packet_header_size: return sizeof(PacketHeader).
2. In pack_packet_header: if out == NULL, return -1. Otherwise populate all four struct fields and return 0.
3. Include a compile-time static assertion verifying that sizeof(PacketHeader) == 8.

Example

PacketHeader hdr;
pack_packet_header(1, 100, 0x01, 1, &hdr); // returns 0
get_packet_header_size(); // returns 8

Input format

stream_id: 32-bit int; len: 16-bit int; flags: 8-bit int; ver: 8-bit int; out: pointer to PacketHeader.

Output format

Returns 0 on success, -1 on NULL pointer.

Constraints

sizeof(PacketHeader) must strictly equal 8. Verify with _Static_assert.

Starter code

#include <stddef.h>
#include <stdint.h>

typedef struct {
    uint32_t stream_id;
    uint16_t length;
    uint8_t flags;
    uint8_t version;
} PacketHeader;

size_t get_packet_header_size(void) {
    return 0;
}

int pack_packet_header(uint32_t stream_id, uint16_t len, uint8_t flags, uint8_t ver, PacketHeader *out) {
    (void)stream_id; (void)len; (void)flags; (void)ver; (void)out;
    return -1;
}

Common mistakes

Ordering struct fields with internal padding; forgetting _Static_assert.

Edge cases to handle

NULL destination pointer returns -1; exact size 8.

Background lessons

• Structs
• typedef: Giving Types Clearer Names