linux man page for set_tid_address

SET_TID_ADDRESS(2)					     Linux Programmer's Manual						SET_TID_ADDRESS(2)

NAME
       set_tid_address - set pointer to thread ID

SYNOPSIS
       #include <linux/unistd.h>

       long set_tid_address(int *tidptr);

DESCRIPTION
       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.

CONFORMING TO
       This system call is Linux-specific.

SEE ALSO
       clone(2), futex(2)

COLOPHON
       This  page is part of release 3.27 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/.

Linux								    2004-09-10							SET_TID_ADDRESS(2)