NAME

       set_tid_address - set pointer to thread ID

SYNOPSIS

       #include <sys/syscall.h>      /* Definition of SYS_* constants */
       #include <unistd.h>

       pid_t syscall(SYS_set_tid_address, int *tidptr);

       Note:   glibc  provides  no  wrapper  for  set_tid_address(),  necessitating  the  use  of
       syscall(2).

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 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 https://www.kernel.org/doc/man-pages/.