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 location. 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 3.44 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/.