HMAC on Mountain lion OSX 10.8.3 EXC_CRASH - c++

Question

Looking for a bit of help using OpenSSL's HMAC function. Currently this function is failing on the HMAC call. ONLY for OSX. Both linux and windows os's are working okay.
QString tradingDialog::HMAC_SHA512_SIGNER(QString UrlToSign, QString Secret){
QString retval = "";
QByteArray byteArray = UrlToSign.toUtf8();
const char* URL = byteArray.constData();
QByteArray byteArrayB = Secret.toUtf8();
const char* Secretkey = byteArrayB.constData();
const EVP_MD *md = EVP_sha512();
unsigned char* digest = NULL;
// Be careful of the length of string with the choosen hash engine. SHA1 produces a 20-byte hash value which rendered as 40 characters.
// Change the length accordingly with your choosen hash engine
char mdString[129] = { 0 };
// Using sha512 hash engine here.
digest = HMAC(md, Secretkey, strlen( Secretkey), (unsigned char*) URL, strlen( URL), NULL, NULL);
for(int i = 0; i < 64; i++){
sprintf(&mdString[i*2], "%02x", (unsigned int)digest[i]);
}
retval = mdString;
return retval;
}

Answer 1

You don't say what the problem is on osx, but it looks like you're not nul terminating mdString, so try changing it to
char mdString[129] = { 0 };
The crashlog you linked to shows that your app is aborting because the stack has been corrupted (I assume this happens on exit).
I would say the final sprintf is causing this, as it is adding a nul byte after the end of your mdString array. Try the above modification and see if that helps.
This ought to crash on all platforms, but I guess you got "lucky".