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Name

set_tid_address - set pointer to thread ID

Synopsis


#include <linux/unistd.h>
long set_tid_address(int *tidptr);

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 argument 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.

See Also

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

Colophon

This page is part of release 3.78 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 http://www.kernel.org/doc/man-pages/.

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