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 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 speci-
	  fied 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 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 signals, 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 informa-
       tion 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).

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)

Linux								    2015-08-08								   NPTL(7)