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       pthreads - POSIX threads


       POSIX.1  specifies  a  set  of interfaces (functions, header files) for
       threaded programming commonly known as POSIX threads, or  Pthreads.   A
       single process can contain multiple threads, all of which are executing
       the same program.  These threads share the same global memory (data and
       heap   segments),   but  each  thread  has  its  own  stack  (automatic

       POSIX.1 also requires that threads share a range  of  other  attributes
       (i.e., these attributes are process-wide rather than per-thread):

       -  process ID

       -  parent process ID

       -  process group ID and session ID

       -  controlling terminal

       -  user and group IDs

       -  open file descriptors

       -  record locks (see fcntl(2))

       -  signal dispositions

       -  file mode creation mask (umask(2))

       -  current directory (chdir(2)) and root directory (chroot(2))

       -  interval timers (setitimer(2)) and POSIX timers (timer_create())

       -  nice value (setpriority(2))

       -  resource limits (setrlimit(2))

       -  measurements of the consumption of CPU time (times(2)) and resources

       As well as the stack, POSIX.1 specifies that various  other  attributes
       are distinct for each thread, including:

       -  thread ID (the pthread_t data type)

       -  signal mask (pthread_sigmask())

       -  the errno variable

       -  alternate signal stack (sigaltstack(2))

       -  real-time  scheduling policy and priority (sched_setscheduler(2) and

       The following Linux-specific features are also per-thread:

       -  capabilities (see capabilities(7))

       -  CPU affinity (sched_setaffinity(2))

   Compiling on Linux
       On Linux, programs that use the Pthreads API should be  compiled  using
       cc -pthread.

   Linux Implementations of POSIX Threads
       Over  time, two threading implementations have been provided by the GNU
       C library on Linux:

       -  LinuxThreads  This  is  the   original   (now   obsolete)   Pthreads

       -  NPTL  (Native  POSIX  Threads  Library)  This is the modern Pthreads
          implementation.  By  comparison  with  LinuxThreads,  NPTL  provides
          closer  conformance to the requirements of the POSIX.1 specification
          and better performance when creating large numbers of threads.  NPTL
          requires features that are present in the Linux 2.6 kernel.

       Both  of  these  are  so-called  1:1 implementations, meaning that each
       thread maps to a kernel scheduling entity.

       Both threading implementations employ the Linux clone(2)  system  call.
       In  NPTL,  thread  synchronisation primitives (mutexes, thread joining,
       etc.) are implemented using the Linux futex(2) system call.

       Modern GNU C libraries provide both LinuxThreads  and  NPTL,  with  the
       latter being the default (if supported by the underlying kernel).

       The notable features of this implementation are the following:

       -  In  addition  to the main (initial) thread, and the threads that the
          program creates using pthread_create(), the implementation creates a
          "manager"   thread.    This   thread  handles  thread  creation  and
          termination.  (Problems can result if this thread  is  inadvertently

       -  Signals are used internally by the implementation.  On Linux 2.2 and
          later, the first three real-time signals are used.  On  older  Linux
          kernels,  SIGUSR1 and SIGUSR2 are used.  Applications must avoid the
          use of whichever set of signals is employed by the implementation.

       -  Threads do not share process IDs.  (In effect, LinuxThreads  threads
          are  implemented  as  processes  which  share  more information than
          usual, but which do not share a common  process  ID.)   LinuxThreads
          threads  (including  the  manager  thread)  are  visible as separate
          processes using ps(1).

       The LinuxThreads implementation deviates from the POSIX.1 specification
       in a number of ways, including the following:

       -  Calls to getpid(2) return a different value in each thread.

       -  Calls to getppid(2) in threads other than the main thread return the
          process ID of  the  manager  thread;  instead  getppid(2)  in  these
          threads  should  return  the  same  value  as getppid(2) in the main

       -  When one thread creates a  new  child  process  using  fork(2),  any
          thread  should  be  able  to  wait(2)  on  the  child.  However, the
          implementation only allows the thread  that  created  the  child  to
          wait(2) on it.

       -  When  a thread calls execve(2), all other threads are terminated (as
          required by POSIX.1).  However, the resulting process has  the  same
          PID as the thread that called execve(2): it should have the same PID
          as the main thread.

       -  Threads  do  not  share  user  and  group  IDs.   This   can   cause
          complications  with  set-user-ID  programs and can cause failures in
          Pthreads functions if an application changes its  credentials  using
          seteuid(2) or similar.

       -  Threads do not share a common session ID and process group ID.

       -  Threads do not share record locks created using fcntl(2).

       -  The  information returned by times(2) and getrusage(2) is per-thread
          rather than process-wide.

       -  Threads do not share semaphore undo values (see semop(2)).

       -  Threads do not share interval timers.

       -  Threads do not share a common nice value.

       -  POSIX.1 distinguishes the notions of signals that  are  directed  to
          the  process  as  a  whole  and  signals  are directed to individual
          threads.  According to  POSIX.1,  a  process-directed  signal  (sent
          using   kill(2),  for  example)  should  be  handled  by  a  single,
          arbitrarily selected thread within the process.   LinuxThreads  does
          not support the notion of process-directed signals: signals may only
          be sent to specific threads.

       -  Threads have distinct alternate signal stack settings.   However,  a
          new  thread’s  alternate  signal  stack settings are copied from the
          thread that created it, so  that  the  threads  initially  share  an
          alternate  signal  stack.   (A  new  thread  should  start  with  no
          alternate signal stack defined.  If two threads  handle  signals  on
          their  shared alternate signal stack at the same time, unpredictable
          program failures are likely to occur.)

       With NPTL, all of the threads in a  process  are  placed  in  the  same
       thread  group; all members of a thread groups share the same PID.  NPTL
       does not employ a manager thread.  NPTL makes internal use of the first
       two real-time signals; these signals cannot be used in applications.

       NPTL still has a few non-conformances with POSIX.1:

       -  Threads  have  distinct alternate signal stack settings.  However, a
          new thread’s alternate signal stack settings are copied from the the
          thread  that  created  it,  so  that  the threads initially share an
          alternate signal stack.

       -  Threads do not share a common nice value.

       -  Only the main thread is permitted  to  start  a  new  session  using

       -  Only the main thread is permitted to make the process into a process
          group leader using setpgid(2).

       Some NPTL non-conformances only occur with older kernels:

       -  The information returned by times(2) and getrusage(2) is  per-thread
          rather than process-wide (fixed in kernel 2.6.9).

       -  Threads do not share resource limits (fixed in kernel 2.6.10).

       -  Threads do not share interval timers (fixed in kernel 2.6.12).

   Determining the Threading Implementation
       Since  glibc 2.3.2, the getconf(1) command can be used to determine the
       system’s default threading implementation, for example:

           bash$ getconf GNU_LIBPTHREAD_VERSION
           NPTL 2.3.4

       With older glibc versions, a command such as the  following  should  be
       sufficient to determine the default threading implementation:

           bash$ $( ldd /bin/ls | grep | awk ’{print $3}’ ) | \
                           egrep -i ’threads|ntpl’
                   Native POSIX Threads Library by Ulrich Drepper et al

   Selecting the Threading Implementation: LD_ASSUME_KERNEL
       On  systems  with a glibc that supports both LinuxThreads and NPTL, the
       LD_ASSUME_KERNEL environment variable  can  be  used  to  override  the
       dynamic  linker’s  default  choice  of  threading implementation.  This
       variable tells the dynamic linker to assume that it is running  on  top
       of  a  particular  kernel version.  By specifying a kernel version that
       does not provide the support required by NPTL, we can force the use  of
       LinuxThreads.   (The  most  likely  reason  for  doing this is to run a
       (broken) application that depends on some  non-conformant  behavior  in
       LinuxThreads.)  For example:

           bash$ $( LD_ASSUME_KERNEL=2.2.5 ldd /bin/ls | grep | \
                           awk ’{print $3}’ ) | egrep -i ’threads|ntpl’
                   linuxthreads-0.10 by Xavier Leroy


       clone(2),  futex(2),  gettid(2),  futex(4), and various Pthreads manual
       pages,   for   example:   pthread_atfork(3),   pthread_cleanup_push(3),
       pthread_cond_signal(3),     pthread_cond_wait(3),    pthread_create(3),
       pthread_detach(3),          pthread_equal(3),          pthread_exit(3),
       pthread_key_create(3),      pthread_kill(3),     pthread_mutex_lock(3),
       pthread_mutex_unlock(3),  pthread_once(3),   pthread_setcancelstate(3),
       pthread_setcanceltype(3),  pthread_setspecific(3),  pthread_sigmask(3),
       and pthread_testcancel(3).