If I get this right, you want to compute a MAC (specifically HMAC-SHA256) of a file using a given key.
(I hope the key you posted here is only a "demo" key!)
The C/C++ standard library does
not contain such highly specific crypto functions. So, you'll either have to code it
all yourself, or you have to use a suitable
third-party library. But, unless this is for your own education, you certainly do
not want to implement crypto functions yourself! Not only because it would be a whole lot of work to implement it all yourself, but also because it is
way to easy to shoot yourself in the foot!
(Most "homebrew" crypto code is
insecure for one reason or another)
Therefore, it is
highly recommended to use an existing crypto library. Since OpenSSL in fact consists of two C libraries (
libcrypto and
libssl) plus a command-line application that uses these libraries, you can simply write your own C/C++ application that calls the relevant functions from
libcrypto (OpenSSL crypto library).
See
libcrypto API documentation on OpenSSL web-site for details:
https://wiki.openssl.org/index.php/Libcrypto_API
Specifically for HMAC see:
*
https://www.openssl.org/docs/manmaster/man3/EVP_DigestSignInit.html
*
https://wiki.openssl.org/index.php/EVP_Signing_and_Verifying#HMAC
(Warning: OpenSSL API documentation is often confusing and/or incomplete, in my experience, so if you are not used to calling third-party libraries from your own C/C++ code, this will be a rough start)