Generate HMAC SHA256 in C - c

Question

I am creating an HMAC digest in java application and want to verify it into the C program. I have a hardcoded secret key in hex format.
I'm getting Segmentation fault while trying to calculate HmacSHA256 in C. I couldn't figure out what I am messing up.
The java program
byte[] decodedKey = hexStringToByteArray("d44d4435c5eea8791456f2e20d7e176a");
SecretKey key = new SecretKeySpec(decodedKey, 0, decodedKey.length, "AES");
try {
Mac mac = Mac.getInstance("HmacSHA256"); //Creating a Mac object
mac.init(key); //Initializing the Mac object
byte[] bytes = challenge.getBytes();
byte[] macResult = mac.doFinal(bytes);
return macResult;
} catch (NoSuchAlgorithmException e) {
System.out.println("Not valid algorithm"+ e);
} catch (InvalidKeyException e) {
System.out.println("Invalid key"+ e);
}
C program
const char* key = hexstr_to_char("d44d4435c5eea8791456f2e20d7e176a");
unsigned char *result;
unsigned int* resultlen;
hmac_sha256(key, strlen(key),
challenge, strlen("d44d4435c5eea8791456f2e20d7e176a"),
result, resultlen);
unsigned char* hmac_sha256(const void *key, int keylen,
const unsigned char *data, int datalen,
unsigned char *result, unsigned int* resultlen)
{
return HMAC(EVP_sha256(), key, keylen, data, datalen, result, resultlen);
}
unsigned char* hexstr_to_char(const char* hexstr)
{
size_t len = strlen(hexstr);
if (len % 2 != 0)
return NULL;
size_t final_len = len / 2;
unsigned char* chrs = (unsigned char*)malloc((final_len+1) * sizeof(*chrs));
for (size_t i=0, j=0; j<final_len; i+=2, j++)
chrs[j] = (hexstr[i] % 32 + 9) % 25 * 16 + (hexstr[i+1] % 32 + 9) % 25;
chrs[final_len] = '\0';
return chrs;
}

Answer 1

If you read the documentation for HMAC() more carefully you'll see that there are a few ways to call the method to determine size of the output buffer, or to let it return a static array buffer.
You can pass NULL for the result and get the length with a first call, then allocate the result buffer and call again.
Or you can pass NULL for the result buffer, and it will return a static array buffer (which you don't own), however that is documented as not thread-safe.
The easiest way may be to rely on EVP_MAX_MD_SIZE to statically declare your buffer. You should also declare the length variable as int not int*, and pass its address with the & operation - this is very basic C.
Try this:
unsigned int resultlen = 0;
unsigned char resultbuf[EVP_MAX_MD_SIZE];
hmac_sha256(key, strlen(key), challenge, strlen("d44d4435c5eea8791456f2e20d7e176a"),
resultbuf, &resultlen);
Documentation:
https://www.openssl.org/docs/man1.1.1/man3/HMAC.html
There are many C language tutorials out there.