c++ HMAC sha512 bad coding

  c++, hmac, openssl

I’m trying code a openssl HMAC SHA512 encryption but i compare my result with some online pages such HMAC-SHA256 Online Generator Tool and the result is not the same I can not figure out what i’m doing wrong, its my first time coding c++ with openssl encryption.

this is my code until now:

char *key = "apiPrivateKey";
const unsigned char data[512] = "message";
int data_len = sha512HashData.size();
unsigned char hmac[128];
for (int i = 0; i < 128; i++)
    hmac[i]= 0;
char outputBuffer[128];
for (int i = 0; i < 128; i++)
    outputBuffer[i]= 0;

unsigned int len = EVP_MAX_MD_SIZE;
std::vector<unsigned char> digest(len);

HMAC_CTX *ctx;
unsigned int res_len;

ctx = HMAC_CTX_new();
HMAC_Init_ex(ctx, key, strlen((const char*)key), EVP_sha512(), NULL);
HMAC_Update(ctx, data, data_len);
HMAC_Final(ctx, hmac, &res_len);

int i = 0;
for(i = 0; i < 128; i++)
    sprintf(outputBuffer + (i * 2), "%02x", hmac[i]);

std::string myString;
myString.assign(outputBuffer, 128);
std::cout << "ComputeHMAC512Hash: " << myString << std::endl;

return myString;

Any suggestions? thanks

Source: Windows Questions C++