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NAME

       pthreads - POSIX threads

DESCRIPTION

       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
       variables).

       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(3))

       -  nice value (setpriority(2))

       -  resource limits (setrlimit(2))

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

       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(3))

       -  the errno variable

       -  alternate signal stack (sigaltstack(2))

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

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

       -  capabilities (see capabilities(7))

       -  CPU affinity (sched_setaffinity(2))

   Thread-safe functions
       A thread-safe function is one that can be safely (i.e., it will deliver
       the  same  results  regardless  of  whether it is) called from multiple
       threads at the same time.

       POSIX.1-2001 requires that all  functions  specified  in  the  standard
       shall be thread-safe, except for the following functions:

           asctime()
           basename()
           catgets()
           crypt()
           ctermid() if passed a non-NULL argument
           ctime()
           dbm_clearerr()
           dbm_close()
           dbm_delete()
           dbm_error()
           dbm_fetch()
           dbm_firstkey()
           dbm_nextkey()
           dbm_open()
           dbm_store()
           dirname()
           dlerror()
           drand48()
           ecvt()
           encrypt()
           endgrent()
           endpwent()
           endutxent()
           fcvt()
           ftw()
           gcvt()
           getc_unlocked()
           getchar_unlocked()
           getdate()
           getenv()
           getgrent()
           getgrgid()
           getgrnam()
           gethostbyaddr()
           gethostbyname()
           gethostent()
           getlogin()
           getnetbyaddr()
           getnetbyname()
           getnetent()
           getopt()
           getprotobyname()
           getprotobynumber()
           getprotoent()
           getpwent()
           getpwnam()
           getpwuid()
           getservbyname()
           getservbyport()
           getservent()
           getutxent()
           getutxid()
           getutxline()
           gmtime()
           hcreate()
           hdestroy()
           hsearch()
           inet_ntoa()
           l64a()
           lgamma()
           lgammaf()
           lgammal()
           localeconv()
           localtime()
           lrand48()
           mrand48()
           nftw()
           nl_langinfo()
           ptsname()
           putc_unlocked()
           putchar_unlocked()
           putenv()
           pututxline()
           rand()
           readdir()
           setenv()
           setgrent()
           setkey()
           setpwent()
           setutxent()
           strerror()
           strtok()
           tmpnam() if passed a non-NULL argument
           ttyname()
           unsetenv()
           wcrtomb() if its final argument is NULL
           wcsrtombs() if its final argument is NULL
           wcstombs()
           wctomb()

   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 Pthreads implementation.  Since glibc 2.4,
              this implementation is no longer supported.

       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 is available since
              glibc 2.3.2, and 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
       synchronization  primitives  (mutexes,  thread   joining,   etc.)   are
       implemented using the Linux futex(2) system call.

   LinuxThreads
       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(3), the implementation  creates
          a  "manager"  thread.   This  thread  handles  thread  creation  and
          termination.  (Problems can result if this thread  is  inadvertently
          killed.)

       -  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
          thread.

       -  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 that 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.)

   NPTL
       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 at least one non-conformance with POSIX.1:

       -  Threads do not share a common nice value.

       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).

       -  Only the main thread is permitted  to  start  a  new  session  using
          setsid(2) (fixed in kernel 2.6.16).

       -  Only the main thread is permitted to make the process into a process
          group leader using setpgid(2) (fixed in kernel 2.6.16).

       -  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 (fixed in kernel 2.6.16).

       Note the following further points about the NPTL implementation:

       -  If  the  stack  size  soft  resource  limit  (see the description of
          RLIMIT_STACK  in  setrlimit(2))  is  set  to  a  value  other   than
          unlimited,  then  this  value defines the default stack size for new
          threads.  To be effective, this limit must be set before the program
          is  executed,  perhaps  using  the  ulimit -s shell built-in command
          (limit stacksize in the C shell).

   Determining the Threading Implementation
       Since glibc 2.3.2, the getconf(1) command can be used to determine  the
       system’s 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 libc.so | awk '{print $3}' ) | \
                           egrep -i 'threads|nptl'
                   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 (i.e.,
       glibc  2.3.x), 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 libc.so | \
                           awk '{print $3}' ) | egrep -i 'threads|ntpl'
                   linuxthreads-0.10 by Xavier Leroy

SEE ALSO

       clone(2), futex(2), gettid(2), futex(7), 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).

COLOPHON

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       description of the project, and information about reporting  bugs,  can
       be found at http://www.kernel.org/doc/man-pages/.