man2/set_tid_address.2

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  25 .TH SET_TID_ADDRESS 2 2017-09-15 "Linux" "Linux Programmer's Manual"
  26 .SH NAME
  27 set_tid_address \- set pointer to thread ID
  28 .SH SYNOPSIS
  29 .nf
  30 .B #include <linux/unistd.h>
  31 .PP
  32 .BI "long set_tid_address(int *" tidptr );
  33 .fi
  34 .PP
  35 .IR Note :
  36 There is no glibc wrapper for this system call; see NOTES.
  37 .SH DESCRIPTION
  38 For each thread, the kernel maintains two attributes (addresses) called
  39 .I set_child_tid
  40 and
  41 .IR clear_child_tid .
  42 These two attributes contain the value NULL by default.
  43 .TP
  44 .I set_child_tid
  45 If a thread is started using
  46 .BR clone (2)
  47 with the
  48 .B CLONE_CHILD_SETTID
  49 flag,
  50 .I set_child_tid
  51 is set to the value passed in the
  52 .I ctid
  53 argument of that system call.
  54 .IP
  55 When
  56 .I set_child_tid
  57 is set, the very first thing the new thread does
  58 is to write its thread ID at this address.
  59 .TP
  60 .I clear_child_tid
  61 If a thread is started using
  62 .BR clone (2)
  63 with the
  64 .B CLONE_CHILD_CLEARTID
  65 flag,
  66 .I clear_child_tid
  67 is set to the value passed in the
  68 .I ctid
  69 argument of that system call.
  70 .PP
  71 The system call
  72 .BR set_tid_address ()
  73 sets the
  74 .I clear_child_tid
  75 value for the calling thread to
  76 .IR tidptr .
  77 .PP
  78 When a thread whose
  79 .I clear_child_tid
  80 is not NULL terminates, then,
  81 if the thread is sharing memory with other threads,
  82 then 0 is written at the address specified in
  83 .I clear_child_tid
  84 and the kernel performs the following operation:
  85 .PP
  86     futex(clear_child_tid, FUTEX_WAKE, 1, NULL, NULL, 0);
  87 .PP
  88 The effect of this operation is to wake a single thread that
  89 is performing a futex wait on the memory location.
  90 Errors from the futex wake operation are ignored.
  91 .SH RETURN VALUE
  92 .BR set_tid_address ()
  93 always returns the caller's thread ID.
  94 .SH ERRORS
  95 .BR set_tid_address ()
  96 always succeeds.
  97 .SH VERSIONS
  98 This call is present since Linux 2.5.48.
  99 Details as given here are valid since Linux 2.5.49.
 100 .SH CONFORMING TO
 101 This system call is Linux-specific.
 102 .SH NOTES
 103 Glibc does not provide a wrapper for this system call; call it using
 104 .BR syscall (2).
 105 .SH SEE ALSO
 106 .BR clone (2),
 107 .BR futex (2),
 108 .BR gettid (2)