Can cause some performance decrease due to the getenv call. Not much, but testing on huge loads is necessary.

No-no, that isn't great.

Please make a pointer to a debug print function, and a macros which checks whether the pointer is NULL and calls the function.
The arguments parser then sets the function pointer to a debug print function if this option is enabled (or if environment variable is set, etc).

Do we really need environment variable? This seems less convenient, given that none other options are set with envs.

Do we really need environment variable? This seems less convenient, given that none other options are set with envs.

I know, this is one of the options that I was able to test quickly. It's more complicated with the cmd argument, since it needs to be passed on, and I don't really want to create a global variable for this (even as func pointer). I'll look at other ways. It would be better to cancel the pull request then.