Signal handlers and async safety

I am still a little confused as to why exactly it is unsafe to receive a signal and call a non async safe function from within that signal handler. Could someone explain the reasoning behind this and possibly try and give me some references that I can follow to read up more on this myself?

In other words I am asking why it is unsafe to say call printf from within a signal handler. Is it because of intra-process issues and possible race conditions resulting from two possible calls to printf without protection or is it because of inter process races to the same resource (in this example stdout). Say a thread within process A is calling printf and another thread receives the signal and then calls printf. Is it possibly because the kernel here will not know what to do because it will not be able to distinguish between the two calls.