nptl

       NPTL  (Native POSIX Threads Library) is the GNU C library POSIX threads
       implementation that is used on modern Linux systems.

   NPTL and signals
       NPTL makes internal use of the first two real-time signals (signal num-
       bers  32  and 33).  One of these signals is used to support thread can-
       cellation and POSIX timers (see timer_create(2)); the other is used  as
       part  of  a mechanism that ensures all threads in a process always have
       the same UIDs and GIDs, as required by POSIX.  These signals cannot  be
       used in applications.

       To prevent accidental use of these signals in applications, which might
       interfere with the operation of the NPTL implementation, various  glibc
       library  functions  and  system  call wrapper functions attempt to hide
       these signals from applications, as follows:

       *  SIGRTMIN is defined with the value 34 (rather than 32).

       *  The  sigwaitinfo(2),  sigtimedwait(2),  and  sigwait(3)   interfaces
          silently  ignore  requests to wait for these two signals if they are
          specified in the signal set argument of these calls.

       *  The sigprocmask(2) and pthread_sigmask(3) interfaces silently ignore
          attempts to block these two signals.

       *  The  sigaction(2),  pthread_kill(3),  and pthread_sigqueue(3) inter-
          faces fail with the error EINVAL (indicating an invalid signal  num-
          ber) if these signals are specified.

       *  sigfillset(3)  does  not include these two signals when it creates a
          full signal set.

   NPTL and process credential changes
       At the Linux kernel level, credentials (user and group IDs) are a  per-
       thread  attribute.   However,  POSIX  requires  that  all  of the POSIX
       threads in a process have the same credentials.   To  accommodate  this
       requirement, the NPTL implementation wraps all of the system calls that
       change process credentials with functions that, in addition to invoking
       the  underlying  system  call,  arrange  for  all  other threads in the
       process to also change their credentials.

       The implementation of each of these system calls involves the use of  a
       real-time  signal  that  is sent (using tgkill(2)) to each of the other
       threads that must change its credentials.  Before  sending  these  sig-
       nals,  the  thread  that  is changing credentials saves the new creden-
       tial(s) and records the system call being employed in a global  buffer.
       A  signal  handler  in the receiving thread(s) fetches this information
       and then uses the same system call to change its credentials.

       Wrapper functions employing this technique are provided for  setgid(2),
       setuid(2),  setegid(2),  seteuid(2),  setregid(2), setreuid(2), setres-
       gid(2), setresuid(2), and setgroups(2).

CONFORMING TO

COLOPHON
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       latest     version     of     this    page,    can    be    found    at
       http://www.kernel.org/doc/man-pages/.

Linux                             2015-08-08                           NPTL(7)
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