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  cancelation  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 credentials.

       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).

STANDARDS

       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)