int tkill(int tid, int sig);

       int tgkill(int tgid, int tid, int sig);

       Note: There are no glibc wrappers for these system calls; see NOTES.

       tgkill()  sends  the signal sig to the thread with the thread ID tid in
       the thread group tgid.  (By contrast, kill(2) can be  used  to  send  a
       signal  only to a process (i.e., thread group) as a whole, and the sig-
       nal will be delivered to an arbitrary thread within that process.)

       tkill() is an obsolete predecessor to tgkill().   It  allows  only  the
       target  thread ID to be specified, which may result in the wrong thread
       being signaled if a thread terminates and its thread  ID  is  recycled.
       Avoid using this system call.

       These  are  the  raw  system call interfaces, meant for internal thread
       library use.

       On success, zero is returned.  On error, -1 is returned, and  errno  is
       set appropriately.

       EINVAL An invalid thread ID, thread group ID, or signal was specified.

       EPERM  Permission denied.  For the required permissions, see kill(2).

       ESRCH  No  process  with  the specified thread ID (and thread group ID)

       tkill() is supported since Linux 2.4.19 / 2.5.4.  tgkill() was added in
       Linux 2.5.75.

       tkill()  and tgkill() are Linux-specific and should not be used in pro-
       grams that are intended to be portable.

       See the description of CLONE_THREAD in clone(2) for an  explanation  of
       thread groups.

       Glibc does not provide wrappers for these system calls; call them using

       clone(2), gettid(2), kill(2), rt_sigqueueinfo(2)

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