man2/tkill.2

   1 .\" Copyright (C) 2008 Michael Kerrisk <tmk.manpages@gmail.com>
   2 .\" and Copyright 2003 Abhijit Menon-Sen <ams@wiw.org>
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  26 .\" 2004-05-31, added tgkill, ahu, aeb
  27 .\" 2008-01-15 mtk -- rewrote DESCRIPTION
  28 .\"
  29 .TH TKILL 2 2019-08-02 "Linux" "Linux Programmer's Manual"
  30 .SH NAME
  31 tkill, tgkill \- send a signal to a thread
  32 .SH SYNOPSIS
  33 .nf
  34 .BI "int tkill(int " tid ", int " sig );
  35 .PP
  36 .BI "int tgkill(int " tgid ", int " tid ", int " sig );
  37 .fi
  38 .PP
  39 .IR Note :
  40 There is no glibc wrapper for
  41 .BR tkill ();
  42 see NOTES.
  43 .SH DESCRIPTION
  44 .BR tgkill ()
  45 sends the signal
  46 .I sig
  47 to the thread with the thread ID
  48 .I tid
  49 in the thread group
  50 .IR tgid .
  51 (By contrast,
  52 .BR kill (2)
  53 can be used to send a signal only to a process (i.e., thread group)
  54 as a whole, and the signal will be delivered to an arbitrary
  55 thread within that process.)
  56 .PP
  57 .BR tkill ()
  58 is an obsolete predecessor to
  59 .BR tgkill ().
  60 It allows only the target thread ID to be specified,
  61 which may result in the wrong thread being signaled if a thread
  62 terminates and its thread ID is recycled.
  63 Avoid using this system call.
  64 .\" FIXME Maybe say something about the following:
  65 .\" http://sourceware.org/bugzilla/show_bug.cgi?id=12889
  66 .\"
  67 .\" Quoting Rich Felker <bugdal@aerifal.cx>:
  68 .\"
  69 .\" There is a race condition in pthread_kill: it is possible that,
  70 .\" between the time pthread_kill reads the pid/tid from the target
  71 .\" thread descriptor and the time it makes the tgkill syscall,
  72 .\" the target thread terminates and the same tid gets assigned
  73 .\" to a new thread in the same process.
  74 .\"
  75 .\" (The tgkill syscall was designed to eliminate a similar race
  76 .\" condition in tkill, but it only succeeded in eliminating races
  77 .\" where the tid gets reused in a different process, and does not
  78 .\" help if the same tid gets assigned to a new thread in the
  79 .\" same process.)
  80 .\"
  81 .\" The only solution I can see is to introduce a mutex that ensures
  82 .\" that a thread cannot exit while pthread_kill is being called on it.
  83 .\"
  84 .\" Note that in most real-world situations, like almost all race
  85 .\" conditions, this one will be extremely rare. To make it
  86 .\" measurable, one could exhaust all but 1-2 available pid values,
  87 .\" possibly by lowering the max pid parameter in /proc, forcing
  88 .\" the same tid to be reused rapidly.
  89 .PP
  90 These are the raw system call interfaces, meant for internal
  91 thread library use.
  92 .SH RETURN VALUE
  93 On success, zero is returned.
  94 On error, \-1 is returned, and \fIerrno\fP
  95 is set appropriately.
  96 .SH ERRORS
  97 .TP
  98 .B EAGAIN
  99 The
 100 .B RLIMIT_SIGPENDING
 101 resource limit was reached and
 102 .I sig
 103 is a real-time signal.
 104 .TP
 105 .B EAGAIN
 106 Insufficient kernel memory was available and
 107 .I sig
 108 is a real-time signal.
 109 .TP
 110 .B EINVAL
 111 An invalid thread ID, thread group ID, or signal was specified.
 112 .TP
 113 .B EPERM
 114 Permission denied.
 115 For the required permissions, see
 116 .BR kill (2).
 117 .TP
 118 .B ESRCH
 119 No process with the specified thread ID (and thread group ID) exists.
 120 .SH VERSIONS
 121 .BR tkill ()
 122 is supported since Linux 2.4.19 / 2.5.4.
 123 .BR tgkill ()
 124 was added in Linux 2.5.75.
 125 .PP
 126 Library support for
 127 .BR tgkill ()
 128 was added to glibc in version 2.30.
 129 .SH CONFORMING TO
 130 .BR tkill ()
 131 and
 132 .BR tgkill ()
 133 are Linux-specific and should not be used
 134 in programs that are intended to be portable.
 135 .SH NOTES
 136 See the description of
 137 .B CLONE_THREAD
 138 in
 139 .BR clone (2)
 140 for an explanation of thread groups.
 141 .PP
 142 Glibc does not provide a wrapper for
 143 .BR tkill ();
 144 call it using
 145 .BR syscall (2).
 146 Before glibc 2.30, there was also no wrapper function for
 147 .BR tgkill ().
 148 .SH SEE ALSO
 149 .BR clone (2),
 150 .BR gettid (2),
 151 .BR kill (2),
 152 .BR rt_sigqueueinfo (2)