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man:nptl

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 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).

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)

COLOPHON

     This page is part of release 4.16 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
     https://www.kernel.org/doc/man-pages/.

Linux 2015-08-08 NPTL(7)

/data/webs/external/dokuwiki/data/pages/man/nptl.txt · Last modified: 2019/05/17 09:47 by 127.0.0.1

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