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CentOS 7.0 - man page for set_tid_address (centos section 2)

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
For each process, 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 process is started using clone(2) with the CLONE_CHILD_SETTID flag, set_child_tid is set to the value passed in the ctid 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 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 process to tidptr. When a process whose clear_child_tid is not NULL terminates, then, if the process is sharing memory with other processes or 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 process that is performing a futex wait on the memory loca- tion. Errors from the futex wake operation 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 3.53 of the Linux man-pages project. A description of the project, and informa- tion about reporting bugs, can be found at http://www.kernel.org/doc/man-pages/. Linux 2012-07-19 SET_TID_ADDRESS(2)


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