man2/set_tid_address.2

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  23 .TH SET_TID_ADDRESS 2 2004-09-10 "Linux 2.6" "Linux Programmer's Manual"
  24 .SH NAME
  25 set_tid_address \- set pointer to thread ID
  26 .SH SYNOPSIS
  27 .nf
  28 .B #include <linux/unistd.h>
  29 .sp
  30 .BI "long set_tid_address(int *" tidptr );
  31 .SH DESCRIPTION
  32 The kernel keeps for each process two values called
  33 .I set_child_tid
  34 and
  35 .I clear_child_tid
  36 that are NULL by default.
  37
  38 .SS set_child_tid
  39 If a process is started using
  40 .BR clone (2)
  41 with the
  42 .B CLONE_CHILD_SETTID
  43 flag,
  44 .I set_child_tid
  45 is set to
  46 .IR child_tidptr ,
  47 the fifth parameter of that system call.
  48 .LP
  49 When
  50 .I set_child_tid
  51 is set, the very first thing the new process does
  52 is writing its PID at this address.
  53
  54 .SS clear_child_tid
  55 If a process is started using
  56 .BR clone (2)
  57 with the
  58 .B CLONE_CHILD_CLEARTID
  59 flag,
  60 .I clear_child_tid
  61 is set to
  62 .IR child_tidptr ,
  63 the fifth parameter of that system call.
  64 .LP
  65 The system call
  66 .BR set_tid_address ()
  67 sets the
  68 .I clear_child_tid
  69 value for the calling process to
  70 .IR tidptr .
  71 .LP
  72 When
  73 .I clear_child_tid
  74 is set, and the process exits, and the process was sharing memory
  75 with other processes or threads, then 0 is written at this address,
  76 and a
  77 .I futex(child_tidptr, FUTEX_WAKE, 1, NULL, NULL, 0);
  78 call is done. (That is, wake a single process waiting on this futex.)
  79 Errors are ignored.
  80 .SH "RETURN VALUE"
  81 .BR set_tid_address ()
  82 returns the PID of the current process.
  83 .SH HISTORY
  84 This call is present since Linux 2.5.48.
  85 Details as given here are valid since Linux 2.5.49.
  86 .SH "CONFORMING TO"
  87 This system call is Linux specific.
  88 .SH "SEE ALSO"
  89 .BR clone (2),
  90 .BR futex (2)