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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

       The  kernel  keeps for each process two values called set_child_tid and
       clear_child_tid that are 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 child_tidptr, the fifth parameter 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 child_tidptr, the  fifth  parameter  of
       that system call.

       The  system  call  set_tid_address() sets the clear_child_tid value for
       the calling process to tidptr.

       When clear_child_tid is set, and the process exits, and the process was
       sharing  memory  with  other processes or threads, then 0 is written at
       this address, and a futex(child_tidptr, FUTEX_WAKE, 1, NULL, NULL,  0);
       call  is done.  (That is, wake a single process waiting on this futex.)
       Errors 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 3.01 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/.