What is OpenSSL API?
OpenSSL is more than just SSL. It is capable of message digests, encryption and decryption of files, digital certificates, digital signatures, and random numbers. There is quite a bit to the OpenSSL library, much more than can be put into one article. OpenSSL is more than just the API, it is also a command-line tool.
Is OpenSSL C or C++?
You must use a C compiler to build the OpenSSL library. You cannot use a C++ compiler. Later, once the library is built, it is OK to create user programs with a C++ compiler. But the library proper must be built with a C compiler.