size_t // Returns the number of bytes written to `out`
hmac_sha256(
    // [in]: The key and its length.
    //      Should be at least 32 bytes long for optimal security.
    const void* key, const size_t keylen,

    // [in]: The data to hash alongside the key.
    const void* data, const size_t datalen,

    // [out]: The output hash.
    //      Should be 32 bytes long. If it's less than 32 bytes,
    //      the resulting hash will be truncated to the specified length.
    void* out, const size_t outlen
);