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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13       Note: There is no glibc wrapper for this system call; see NOTES.
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DESCRIPTION

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

48       set_tid_address() always returns the caller's thread ID.
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ERRORS

51       set_tid_address() always succeeds.
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VERSIONS

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

58       This system call is Linux-specific.
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NOTES

61       Glibc  does  not  provide a wrapper for this system call; call it using
62       syscall(2).
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COLOPHON

68       This page is part of release 4.15 of the Linux  man-pages  project.   A
69       description  of  the project, information about reporting bugs, and the
70       latest    version    of    this    page,    can     be     found     at
71       https://www.kernel.org/doc/man-pages/.
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75Linux                             2017-09-15                SET_TID_ADDRESS(2)