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NPTL(7) Linux Programmer's Manual NPTL(7)
NAME
nptl - Native POSIX Threads Library
DESCRIPTION
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 er-
ror 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 creden-
tials. 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 un-
derlying 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 creden-
tials. 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), sete-
gid(2), seteuid(2), setregid(2), setreuid(2), setresgid(2), setresuid(2), and set-
groups(2).
CONFORMING TO
For details of the conformance of NPTL to the POSIX standard, see pthreads(7).
NOTES
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
credentials(7), pthreads(7), signal(7), standards(7)
COLOPHON
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 https://www.kernel.org/doc/man-pages/.
Linux 2015-08-08 NPTL(7)
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