set_tid_address


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_SET-
              TID 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  per-
       forming  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)
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