hmac-sha256.c

#include <openssl/hmac.h>
#include <string.h>
#include <stdio.h>

int main()
{
  unsigned char *key = (unsigned char*)"This is your secret";
  unsigned char *data = (unsigned char*) "hoge";
  unsigned char *expected = (unsigned char*) "4a7bc6c59ebc1a83dc38ec4fd537f98994a9210bf09ad9fc8c60c2ae83746d82";
  unsigned char *result;
  int result_len = 32;
  int i;
  static char res_hexstring[64];

  result = HMAC(EVP_sha256(), key, strlen((char *)key), data, strlen((char *)data), NULL, NULL);
  for (i = 0; i < result_len; i++) {
    sprintf(&(res_hexstring[i * 2]), "%02x", result[i]);
  }

  if (strcmp((char *) res_hexstring, (char *) expected) == 0) {
    printf("Got %s\nTest ok, result length %d\n", res_hexstring, result_len);
  } else {
    printf("Got %s instead of %s\n", res_hexstring, expected);
  }
}

@yoshiki, thank you for the good code.
However, the size of the res_hexstring array is incorrectly declared.
Here is the revised code, so please refer to it.

#include <openssl/hmac.h>
#include <string.h>
#include <stdio.h>

int main()
{
  unsigned char *key = (unsigned char*)"This is your secret";
  unsigned char *data = (unsigned char*) "hoge";
  unsigned char *expected = (unsigned char*) "4a7bc6c59ebc1a83dc38ec4fd537f98994a9210bf09ad9fc8c60c2ae83746d82";
  unsigned char *result;
  int result_len = 32;
  int i;
  static char res_hexstring[64];

  result = HMAC(EVP_sha256(), key, strlen((char *)key), data, strlen((char *)data), NULL, NULL);
  for (i = 0; i < result_len; i++) {
    sprintf(&(res_hexstring[i * 2]), "%02x", result[i]);
  }

  if (strcmp((char *) res_hexstring, (char *) expected) == 0) {
    printf("Got %s\nTest ok, result length %d\n", res_hexstring, result_len);
  } else {
    printf("Got %s instead of %s\n", res_hexstring, expected);
  }
}

@ddolsoon
Thank you for your comment!
I have incorporated your changes.

@yoshiki Hi I'm using Openssl version 3.0.0. And I get an error, when I try to compile the code. The error is as follows.

kdf_ck.c: In function ‘main’:
kdf_ck.c:15:3: warning: ‘HMAC’ is deprecated: Since OpenSSL 3.0 [-Wdeprecated-declarations]
result = HMAC(EVP_sha256(), key, strlen((char *)key), data, strlen((char *)data), NULL, NULL);
^~~~~~
In file included from kdf_ck.c:1:0:
/usr/local/include/openssl/hmac.h:48:38: note: declared here
OSSL_DEPRECATEDIN_3_0 unsigned char *HMAC(const EVP_MD *evp_md, const void *key,