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);

       Note: There is no glibc wrapper for this system call; see NOTES.

DESCRIPTION

       For  each  thread,  the kernel maintains two attributes (addresses) called set_child_tid and clear_child_tid.  These two attributes contain
       the value NULL by default.

       set_child_tid
	      If a thread is started using clone(2) with the CLONE_CHILD_SETTID flag, set_child_tid is set to the value passed in the  ctid  argu-
	      ment of that system call.

	      When set_child_tid is set, the very first thing the new thread does is to write its thread ID at this address.

       clear_child_tid
	      If  a  thread  is  started using clone(2) with the CLONE_CHILD_CLEARTID flag, clear_child_tid is set to the value passed in the ctid
	      argument of that system call.

       The system call set_tid_address() sets the clear_child_tid value for the calling thread to tidptr.

       When a thread whose clear_child_tid is not NULL terminates, then, if the thread is sharing memory with other threads, then 0 is written	at
       the address specified in clear_child_tid and the kernel performs the following operation:

	   futex(clear_child_tid, FUTEX_WAKE, 1, NULL, NULL, 0);

       The effect of this operation is to wake a single thread that is performing a futex wait on the memory location.	Errors from the futex wake
       operation are ignored.

RETURN VALUE

       set_tid_address() always returns the caller's thread ID.

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.

NOTES

       Glibc does not provide a wrapper for this system call; call it using syscall(2).

SEE ALSO

       clone(2), futex(2), gettid(2)

COLOPHON

       This page is part of release 4.15 of the Linux man-pages project.  A description of the project, information about reporting bugs, and  the
       latest version of this page, can be found at https://www.kernel.org/doc/man-pages/.

Linux								    2017-09-15							SET_TID_ADDRESS(2)