nptl(7) Miscellaneous Information 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 num-
bers 32 and 33). One of these signals is used to support thread cance-
lation 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 require-
ment, 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 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), setres-
gid(2), setresuid(2), and setgroups(2).
STANDARDS
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 con-
taining 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)
Linux man-pages 6.9.1 2024-05-02 nptl(7)
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