NAME | DESCRIPTION | CONFORMING TO | NOTES | SEE ALSO | COLOPHON

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

NAME         top

       nptl - Native POSIX Threads Library

DESCRIPTION         top

       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
       credentials.

       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).

CONFORMING TO         top

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

NOTES         top

       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.

SEE ALSO         top

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

COLOPHON         top

       This page is part of release 4.10 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
       https://www.kernel.org/doc/man-pages/.

Linux                            2015-08-08                          NPTL(7)

Pages that refer to this page: getgroups(2)setgid(2)setresuid(2)setreuid(2)setuid(2)sigaction(2)sigprocmask(2)sigwaitinfo(2)timer_create(2)pthread_kill(3)pthread_sigmask(3)pthread_sigqueue(3)sigsetops(3)sigwait(3)credentials(7)pthreads(7)signal(7)