set_tid_address — set pointer to thread ID


#include <sys/syscall.h>       /* Definition of  SYS_*  constants */
#include <unistd.h>
pid_t syscall( SYS_set_tid_address,
  int *tidptr);
[Note] Note
glibc provides no wrapper for
.BR set_tid_address (),
necessitating the use of


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.


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.


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.


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


set_tid_address() always succeeds.


This call is present since Linux 2.5.48. Details as given here are valid since Linux 2.5.49.


This system call is Linux-specific.


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


This page is part of release 5.13 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−pages/.

  Copyright (C) 2004 Andries Brouwer (

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