nptl(7) — Linux manual page


nptl(7)             Miscellaneous Information 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 cancelation 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

       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).

STANDARDS         top

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

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

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Linux man-pages 6.9.1          2024-05-02                        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)libpsx(3)pthread_kill(3)pthread_sigmask(3)pthread_sigqueue(3)sigsetops(3)sigwait(3)credentials(7)landlock(7)pthreads(7)signal(7)