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 num-
bers 32 and 33). One of these signals is used to support thread can-
cellation 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) inter-
faces fail with the error EINVAL (indicating an invalid signal num-
ber) 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 sig-
nals, the thread that is changing credentials saves the new creden-
tial(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), setres-
gid(2), setresuid(2), and setgroups(2).
This page is part of release 4.04 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
Linux 2015-08-08 NPTL(7)
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