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.

   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 (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
   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
   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

   Wrapper  functions employing this technique are provided for setgid(2),
   setuid(2),   setegid(2),    seteuid(2),    setregid(2),    setreuid(2),
   setresgid(2), setresuid(2), and setgroups(2).


   For  details  of  the  conformance  of  NPTL to the POSIX standard, see


   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.


   credentials(7), pthreads(7), signal(7), standards(7)


   This page is part of release 4.09 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


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