Wait for the completion of an asynchronous function that was
started with start_async().
+`run_hook`::
+
+ Run a hook.
+ The first argument is a pathname to an index file, or NULL
+ if the hook uses the default index file or no index is needed.
+ The second argument is the name of the hook.
+ The further arguments correspond to the hook arguments.
+ The last argument has to be NULL to terminate the arguments list.
+ If the hook does not exist or is not executable, the return
+ value will be zero.
+ If it is executable, the hook will be executed and the exit
+ status of the hook is returned.
+ On execution, .stdout_to_stderr and .no_stdin will be set.
+ (See below.)
+
Data structures
---------------