man > nptl(7)

nptl(7)                           Miscellaneous Information 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  cancelation  and  POSIX  timers  (see  timer_cre‐
       ate(2));  the  other is used as part of a mechanism that ensures all threads in a process al‐
       ways have the same UIDs and GIDs, as required by POSIX.  These signals cannot be used in  ap‐
       plications.

       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.  How‐
       ever,  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.  Be‐
       fore sending these signals, 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), 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 sys‐
       tems,  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 man-pages 6.7                          2023-10-31                                      nptl(7)