EVP_PKEY_new_mac_key(EVP_PKEY_HMAC, NULL, (const unsigned char*)"", 0);

#include <string.h>
#include <stdio.h>

#include "openssl/evp.h"

int test(const char* key_str)
{
  EVP_PKEY* key = EVP_PKEY_new_mac_key(EVP_PKEY_HMAC, NULL,
                                       (const unsigned char*) key_str, strlen(key_str));
  if (!key) return 1;

  EVP_MD_CTX* ctx = EVP_MD_CTX_new();
  if (!ctx) return 2;
  const EVP_MD* md = EVP_sha512();
  if (!md) return 3;

  EVP_PKEY_CTX* evpPkeyCtxPtr;
  if (!EVP_DigestSignInit(ctx, &evpPkeyCtxPtr, md, NULL, key)) return 4;

  if (!EVP_DigestSignUpdate(ctx, "message", 7)) return 5;

  unsigned char sign[1024];
  size_t sign_len = 1024;
  if (!EVP_DigestSignFinal(ctx, sign, &sign_len)) return 6;

  EVP_PKEY_free(key);
  EVP_MD_CTX_free(ctx);
  return 0;
}

int main()
{
  printf("key: %d\n", test("key"));
  printf("empty key: %d\n", test(""));
  return 0;
}