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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
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).
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 4.12 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)