long set_tid_address(int *tidptr);
The kernel keeps for each process two values called set_child_tid and
clear_child_tid that are NULL by default.
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
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 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.
set_tid_address() always returns the PID of the calling process.
set_tid_address() always succeeds.
This call is present since Linux 2.5.48. Details as given here are
valid since Linux 2.5.49.
This system call is Linux-specific.
This page is part of release 3.35 of the Linux man-pages project. A
description of the project, and information about reporting bugs, can
be found at http://man7.org/linux/man-pages/.
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