How to check if openssl or cryptopp is installed and use the library that actually exists in the system (is installed)?

I wrote function that encrypts/decrypts a buffer (2 versions of the same function - first, with cryptopp, second - with openssl).

I would like to make something like this:

#if defined OPENSSL
   run_aes_openssl(...);
#elif defined CRYPTOPP
   run_aes_crytopp(...);
#else
    error(...);
#end

Is it possible?

Solution

It's not quite that simple. In order to find that a macro is defined, you have to include the header that defines that macro. And C doesn't have anything like "include foo.h iff it exists"; it has to exist otherwise there is a compilation error.

Normally this would be sorted out by a script that you run before compilation. Your script checks locations like /usr/include, /usr/local/include, etc., to see if the OpenSSL headers are there; and then it outputs a Makefile which contains in the CFLAGS -DHAVE_OPENSSL. Then your code can check for that macro.

This is quite a bit of hullabaloo, to keep things simple you could require the user to manually edit a file , e.g. distribute your project with something called user_config.h that the user is supposed to edit before compiling, to specify where they put OpenSSL and so on.

There is a preset system called GNU Autoconf which contains a script that checks your system for everything under the sun. This has its advantages and disadvantages; it makes things easier for plebs downloading your source code, but it is bloaty and can be hard work for yourself.