1nptl(7) Miscellaneous Information Manual nptl(7)
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6 nptl - Native POSIX Threads Library
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9 NPTL (Native POSIX Threads Library) is the GNU C library POSIX threads
10 implementation that is used on modern Linux systems.
11 NPTL and signals
13 NPTL makes internal use of the first two real-time signals (signal num‐
14 bers 32 and 33). One of these signals is used to support thread cance‐
15 lation and POSIX timers (see timer_create(2)); the other is used as
16 part of a mechanism that ensures all threads in a process always have
17 the same UIDs and GIDs, as required by POSIX. These signals cannot be
18 used in applications.
19 To prevent accidental use of these signals in applications, which might
21 interfere with the operation of the NPTL implementation, various glibc
22 library functions and system call wrapper functions attempt to hide
23 these signals from applications, as follows:
24 • SIGRTMIN is defined with the value 34 (rather than 32).
26 • The sigwaitinfo(2), sigtimedwait(2), and sigwait(3) interfaces
28 silently ignore requests to wait for these two signals if they are
29 specified in the signal set argument of these calls.
30 • The sigprocmask(2) and pthread_sigmask(3) interfaces silently ignore
32 attempts to block these two signals.
33 • The sigaction(2), pthread_kill(3), and pthread_sigqueue(3) inter‐
35 faces fail with the error EINVAL (indicating an invalid signal num‐
36 ber) if these signals are specified.
37 • sigfillset(3) does not include these two signals when it creates a
39 full signal set.
40 NPTL and process credential changes
42 At the Linux kernel level, credentials (user and group IDs) are a per-
43 thread attribute. However, POSIX requires that all of the POSIX
44 threads in a process have the same credentials. To accommodate this
45 requirement, the NPTL implementation wraps all of the system calls that
46 change process credentials with functions that, in addition to invoking
47 the underlying system call, arrange for all other threads in the
48 process to also change their credentials.
49 The implementation of each of these system calls involves the use of a
51 real-time signal that is sent (using tgkill(2)) to each of the other
52 threads that must change its credentials. Before sending these sig‐
53 nals, the thread that is changing credentials saves the new creden‐
54 tial(s) and records the system call being employed in a global buffer.
55 A signal handler in the receiving thread(s) fetches this information
56 and then uses the same system call to change its credentials.
57 Wrapper functions employing this technique are provided for setgid(2),
59 setuid(2), setegid(2), seteuid(2), setregid(2), setreuid(2), setres‐
60 gid(2), setresuid(2), and setgroups(2).
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63 For details of the conformance of NPTL to the POSIX standard, see
64 pthreads(7).
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67 POSIX says that any thread in any process with access to the memory
68 containing a process-shared (PTHREAD_PROCESS_SHARED) mutex can operate
69 on that mutex. However, on 64-bit x86 systems, the mutex definition
70 for x86-64 is incompatible with the mutex definition for i386, meaning
71 that 32-bit and 64-bit binaries can't share mutexes on x86-64 systems.
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74 credentials(7), pthreads(7), signal(7), standards(7)
75Linux man-pages 6.05 2023-02-05 nptl(7)