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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 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

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       project,  and information about reporting bugs, can be found at http://man7.org/linux/man-
       pages/.