Parse the 20-byte header of an IQ-samples capture

Challenge

Parse the 20-byte header of an IQ-samples capture — the same alignment-safe binary-parse discipline as pcap and ELF, on a different fixture format.

Task

Implement int parse_iq_header(const uint8_t *buf, size_t n, iq_hdr_t *out) where:

typedef struct {
    uint32_t sample_rate_hz;
    uint64_t center_freq_hz;
    uint32_t sample_count;
} iq_hdr_t;

Input

• buf, n: a fixture byte buffer and its length, baked into the harness. Layout (20 bytes):
  • 0..3 magic "IQHD"
  • 4..7 sample_rate_hz (u32 LE)
  • 8..15 center_freq_hz (u64 LE)
  • 16..19 sample_count (u32 LE)
• out: the struct to fill on success.

Output

Returns int: 0 on valid magic and n >= 20 (filling out), or -1 on NULL inputs, n < 20, or wrong magic.

Example

20-byte buffer with magic "IQHD"   ->   0, fields filled
n < 20, or wrong magic, or NULL    ->   -1

Edge cases

• n < 20 returns -1.

Rules

• Build the u64 with shifts (or two u32 reads combined via ((uint64_t)hi << 32) | lo); do not cast (uint64_t *)(buf + 8) (alignment-unsafe).

Why this matters

The same alignment-safe binary-parse discipline you used on pcap and ELF works here on a different fixture format.

Input format

A byte buffer of at least 20 bytes + the output struct pointer.

Output format

0 on success, -1 otherwise. Struct fields filled only on success.

Constraints

No pointer casts; build integers from bytes.

Starter code

#include <stdint.h>
#include <stddef.h>
typedef struct {
    uint32_t sample_rate_hz;
    uint64_t center_freq_hz;
    uint32_t sample_count;
} iq_hdr_t;
int parse_iq_header(const uint8_t *buf, size_t n, iq_hdr_t *out) {
    /* TODO */
    (void)buf; (void)n; (void)out;
    return -1;
}

Common mistakes

Using (uint64_t *) casts. Forgetting to memcmp the magic. n bound off-by-one.

Edge cases to handle

Maximum u64 value. Magic mismatch with otherwise valid sizes.

Complexity

O(1).

Background lessons

• Parse an IQ-samples capture header

Up next

• Count unique non-empty subdomains in a wordlist