It is not. In general it is not safe.
Reentrant libraries are very uncommon in the unix world, sadly enough.
It collects garbage and detects leaks, and is thread and signal safe. The answer is very implementation dependent. If the docs don't explicitly say that the calls are reentrant, then assume they are not. The man page states that the option causes all calls to the malloc routines to be protected by signal masks which block all signals during the execution of the function. I've used this option with success.