If a process is started using clone(2) with the CLONE_CHILD_SETTID
flag, set_child_tid is set to child_tidptr, the fifth parameter
of that system call.
When set_child_tid is set, the very first thing the new process does is
writing its PID at this address.
If a process is started using clone(2) with the
CLONE_CHILD_CLEARTID flag, clear_child_tid is set to
child_tidptr, the fifth parameter of that system call.
The system call set_tid_address sets the clear_child_tid value for
the calling process to tidptr.
When clear_child_tid is set, and the process exits, and the process was
sharing memory with other processes or threads, then 0 is written at this
address, and a futex(child_tidptr, FUTEX_WAKE, 1, NULL, NULL, 0); call
is done. (That is, wake a single process waiting on this futex.) Errors are