NAME

       set_tid_address - set pointer to thread ID

SYNOPSIS

       #include <linux/unistd.h>

       long set_tid_address(int *tidptr);

       Note: There is no glibc wrapper for this system call; see NOTES.

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.

NOTES

       Glibc does not provide a wrapper for this system call; call it using syscall(2).

SEE ALSO

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

COLOPHON

       This  page  is  part of release 4.15 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/.