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For review: nptl(7) man page

Hello all,

I wrote a short man page documented some of the details of 
NPTL that might affect application developers. Review comments
would be most welcome.



.\" Copyright (c) 2015 by Michael Kerrisk <>
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.TH NPTL 7 2015-03-21 "Linux" "Linux Programmer's Manual"
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.
.SS 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;
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:
.IP * 3
is defined with the value 34 (rather than 32).
.IP *
.BR sigwaitinfo (2),
.BR sigtimedwait (2),
.BR sigwait (3)
interfaces silently ignore requests to wait for these two signals
if they are specified in the signal set argument of these calls.
.IP *
.BR sigprocmask (2)
.BR pthread_sigmask (3)
interfaces silently ignore attempts to block these two signals.
.IP *
.BR sigaction (2),
.BR pthread_kill (3),
.BR pthread_sigqueue (3)
interfaces fail with the error
(indicating an invalid signal number) if these signals are specified.
.IP *
.BR sigfillset (3)
does not include these two signals when it creates a full signal set.
.SS 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
.BR tgkill (2))
to each of the other threads that must change 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
.BR setgid (2),
.BR setuid (2),
.BR setegid (2),
.BR seteuid (2),
.BR setregid (2),
.BR setreuid (2),
.BR setresgid (2),
.BR setresuid (2),
.BR setgroups (2).
For details of the conformance of NPTL to the POSIX standard, see
.BR pthreads (7).
.BR credentials (7),
.BR pthreads (7),
.BR signal (7),
.BR standards (7)

Michael Kerrisk
Linux man-pages maintainer;
Linux/UNIX System Programming Training:

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