set_tid_address(2)

1SET_TID_ADDRESS(2)         Linux Programmer's Manual        SET_TID_ADDRESS(2)
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NAME

6       set_tid_address - set pointer to thread ID
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SYNOPSIS

9       #include <linux/unistd.h>
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11       long set_tid_address(int *tidptr);
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DESCRIPTION

14       For  each  process,  the  kernel  maintains  two attributes (addresses)
15       called set_child_tid and clear_child_tid.  These two attributes contain
16       the value NULL by default.
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18       set_child_tid
19              If a process is started using clone(2) with the CLONE_CHILD_SET‐
20              TID flag, set_child_tid is set to the value passed in  the  ctid
21              argument of that system call.
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23              When  set_child_tid is set, the very first thing the new process
24              does is writing its PID at this address.
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26       clear_child_tid
27              If   a   process   is   started   using   clone(2)   with    the
28              CLONE_CHILD_CLEARTID  flag,  clear_child_tid is set to the value
29              passed in the ctid argument of that system call.
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31       The system call set_tid_address() sets the  clear_child_tid  value  for
32       the calling process to tidptr.
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34       When  a  process whose clear_child_tid is not NULL terminates, then, if
35       the process is sharing memory with other processes or threads,  then  0
36       is  written  at the address specified in clear_child_tid and the kernel
37       performs the following operation:
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39           futex(clear_child_tid, FUTEX_WAKE, 1, NULL, NULL, 0);
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41       The effect of this operation is to wake a single process that  is  per‐
42       forming  a  futex  wait  on the memory location.  Errors from the futex
43       wake operation are ignored.
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RETURN VALUE

46       set_tid_address() always returns the PID of the calling process.
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ERRORS

49       set_tid_address() always succeeds.
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VERSIONS

52       This call is present since Linux 2.5.48.  Details  as  given  here  are
53       valid since Linux 2.5.49.
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CONFORMING TO

56       This system call is Linux-specific.
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COLOPHON

62       This  page  is  part of release 3.53 of the Linux man-pages project.  A
63       description of the project, and information about reporting  bugs,  can
64       be found at http://www.kernel.org/doc/man-pages/.
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68Linux                             2012-07-19                SET_TID_ADDRESS(2)