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       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)


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