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NAME | DESCRIPTION | STANDARDS | NOTES | SEE ALSO | COLOPHON |
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nptl(7) Miscellaneous Information 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 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
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 the man-pages (Linux kernel and C library
user-space interface documentation) project. Information about
the project can be found at
⟨https://www.kernel.org/doc/man-pages/⟩. If you have a bug report
for this manual page, see
⟨https://git.kernel.org/pub/scm/docs/man-pages/man-pages.git/tree/CONTRIBUTING⟩.
This page was obtained from the tarball man-pages-6.9.1.tar.gz
fetched from
⟨https://mirrors.edge.kernel.org/pub/linux/docs/man-pages/⟩ on
2024-06-26. If you discover any rendering problems in this HTML
version of the page, or you believe there is a better or more up-
to-date source for the page, or you have corrections or
improvements to the information in this COLOPHON (which is not
part of the original manual page), send a mail to
man-pages@man7.org
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)
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HTML rendering created 2024-06-26 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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