SET_TID_ADDRESS(2)                                  Linux Programmer's Manual                                 SET_TID_ADDRESS(2)



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
       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  child_tidptr,  the
       fifth argument 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 3.25 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/.



Linux                                                      2004-09-10                                         SET_TID_ADDRESS(2)