NPTL(7)                             Linux Programmer's Manual                             NPTL(7)

       nptl - Native POSIX Threads Library

       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  numbers  32  and  33).
       One  of  these  signals  is  used  to  support  thread  cancellation 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

       *  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) interfaces fail with the
          error EINVAL (indicating an invalid signal number) 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 creden‐
       tials.  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

       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  creden‐
       tials.   Before  sending  these signals, the thread that is changing credentials saves the
       new credential(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),  sete‐
       gid(2),   seteuid(2),  setregid(2),  setreuid(2),  setresgid(2),  setresuid(2),  and  set‐

       For details of the conformance of NPTL to the POSIX standard, see pthreads(7).

       POSIX says that any thread in any process with access to the memory containing a  process-
       shared  (PTHREAD_PROCESS_SHARED)  mutex can operate on that mutex.  However, on 64-bit x86
       systems, the mutex definition for x86-64 is incompatible with  the  mutex  definition  for
       i386, meaning that 32-bit and 64-bit binaries can't share mutexes on x86-64 systems.

       credentials(7), pthreads(7), signal(7), standards(7)

       This  page  is  part of release 4.04 of the Linux man-pages project.  A description of the
       project, information about reporting bugs, and the 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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