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 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)
          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 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
       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

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

       For details of the conformance of NPTL to the POSIX standard, see

       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 5.05 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

Linux                             2015-08-08                           NPTL(7)