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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 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 em‐
       ployed 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).

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.  How‐
       ever, 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)