How can I verify the signature for server?

[DazeHolic]

Hello, I want to verify the signature wit c++ for server. But I am confuse by the param type.
How can I convert from std::string to secp256k1_ecdsa_recoverable_signature?
How can I convert the private key(18E14A7B6A307F426A94F8114701E7C8E774E7F9A47E2C2035DB29A206321725) to unsigned char privkey[32]?
How can I convert public key to address?
Thanks in advance for all.
here is my code：

int main()
{
	std::string strMessage = "login.xyz wants you to sign in with your Ethereum account:\n0xfB6CD42a8Ec306A62b014Fe299d5854389379296\n\n\n\nURI: https://login.xyz/demo#login\nVersion: 1\nChain ID: 1\nNonce: ae058a6b-784d-4de3-90e3-850644d1b35d";
	std::string strSignature = "0x6a0107a2a409270d58a9bed473800b7443090e5004e3a50f5249f5798678e4a36b13d19ac399a96345b83cfa45be8b09e6b30b2f11109904ab2f33594736f2921b";
	std::string strAddress = "0xfB6CD42a8Ec306A62b014Fe299d5854389379296";

	if (strSignature.length() != 132)
	{
		return -1;
	}

	// how to convert std::string to secp256k1_ecdsa_recoverable_signature?
	secp256k1_ecdsa_recoverable_signature sigRecover;
	for (int i = 0; i < strSignature.length(); i += 2) {
		std::string byteString = strSignature.substr(i, 2);
		if ((i - 2) >= 0)
		{
			sigRecover.data[(i - 2) / 2] = std::strtol(byteString.c_str(), nullptr, 16);
		}
	}

	std::vector<unsigned char> vecHash(picosha2::k_digest_size);
	picosha2::hash256(strMessage.begin(), strMessage.end(), vecHash.begin(), vecHash.end());
	unsigned char msgHash[32] = "";
	for (size_t i = 0; i < 32; i++)
	{
		msgHash[i] = vecHash[i];
	}

	secp256k1_pubkey publicKey;
	auto ec = secp256k1_ecdsa_recover(secp256k1_context_static, &publicKey, &sigRecover, msgHash);
	if (ec != 1)
	{
		return ec;
	}

	secp256k1_ecdsa_signature sig;
	ec = secp256k1_ecdsa_recoverable_signature_convert(secp256k1_context_static, &sig, &sigRecover);
	if (ec != 1)
	{
		return ec;
	}

	ec = secp256k1_ecdsa_verify(secp256k1_context_static, &sig, msgHash, &publicKey);
	if (ec != 1)
	{
		return ec;
	}

	// how to convert public key to address?
	std::string addressConvert; 
	if (addressConvert != strAddress)
	{
		return -1;
	}
	return 0;
}