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 3.54 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/.