+ occurs.
+
+ Restrictions for the cleanup function:
+ - The cleanup function can do all kinds of system calls.
+ - It can also access application dependent memory locations and data
+ structures provided they are in a consistent state. One way to ensure
+ this is through block_fatal_signals()/unblock_fatal_signals(), see
+ below. Another - more tricky - way to ensure this is the careful use
+ of 'volatile'.
+ However,
+ - malloc() and similarly complex facilities are not safe to be called
+ because they are not guaranteed to be in a consistent state.
+ - Also, the cleanup function must not block the catchable fatal signals
+ and leave them blocked upon return.
+
+ The cleanup function is executed asynchronously. It is unspecified
+ whether during its execution the catchable fatal signals are blocked
+ or not. */