# nptl

## 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.
    


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