The kernel keeps for each process two values called
set_child_tid
and
clear_child_tid
that are NULL by default.
set_child_tid
If a process is started using
clone(2)
with the
CLONE_CHILD_SETTID
flag,
set_child_tid
is set to
child_tidptr,
the fifth argument 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.
clear_child_tid
If a process is started using
clone(2)
with the
CLONE_CHILD_CLEARTID
flag,
clear_child_tid
is set to
child_tidptr,
the fifth argument 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 ignored.
RETURN VALUE
set_tid_address()
always returns the PID of the calling process.
ERRORS
set_tid_address()
always succeeds.
VERSIONS
This call is present since Linux 2.5.48.
Details as given here are valid since Linux 2.5.49.
This page is part of release 3.14 of the Linux
man-pages
project.
A description of the project,
and information about reporting bugs,
can be found at
http://www.kernel.org/doc/man-pages/.
