Calling a standard library function in signal handler

Question

Why is calling a standard library function inside a signal handler discouraged?

Answer 1

This is explained in the GNU LibC documentation.

If you call a function in the handler, make sure it is reentrant with respect to signals, or else make sure that the signal cannot interrupt a call to a related function.

And just in case, here's the Wikipedia page on reentrant functions.

A computer program or routine is described as reentrant if it can be safely called again before its previous invocation has been completed (i.e it can be safely executed concurrently).