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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))

   Pthreads function return values
       Most pthreads functions return 0 on success, and  an  error  number  of
       failure.   Note that the pthreads functions do not set errno.  For each
       of the pthreads  functions  that  can  return  an  error,  POSIX.1-2001
       specifies that the function can never fail with the error EINTR.

   Thread IDs
       Each of the threads in a process has a unique thread identifier (stored
       in the type pthread_t).  This identifier is returned to the  caller  of
       pthread_create(3),  and  a  thread can obtain its own thread identifier
       using pthread_self(3).  Thread IDs are only  guaranteed  to  be  unique
       within  a process.  A thread ID may be reused after a terminated thread
       has been joined, or a detached thread has terminated.  In all  pthreads
       functions that accept a thread ID as an argument, that ID by definition
       refers to a thread in the same process as the caller.

   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 and POSIX.1-2008 require 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() [POSIX.1-2001 only (function removed in POSIX.1-2008)]
           encrypt()
           endgrent()
           endpwent()
           endutxent()
           fcvt() [POSIX.1-2001 only (function removed in POSIX.1-2008)]
           ftw()
           gcvt() [POSIX.1-2001 only (function removed in POSIX.1-2008)]
           getc_unlocked()
           getchar_unlocked()
           getdate()
           getenv()
           getgrent()
           getgrgid()
           getgrnam()
           gethostbyaddr() [POSIX.1-2001 only (function removed in POSIX.1-2008)]
           gethostbyname() [POSIX.1-2001 only (function removed in POSIX.1-2008)]
           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()
           strsignal() [Added in POSIX.1-2008]
           strtok()
           system() [Added in POSIX.1-2008]
           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()

   Cancellation Points
       POSIX.1  specifies  that  certain  functions  must,  and  certain other
       functions may, be cancellation points.  If a thread is cancelable,  its
       cancelability  type  is deferred, and a cancellation request is pending
       for the thread, then the thread is canceled when it  calls  a  function
       that is a cancellation point.

       The  following  functions  are  required  to  be cancellation points by
       POSIX.1-2001 and/or POSIX.1-2008:

           accept()
           aio_suspend()
           clock_nanosleep()
           close()
           connect()
           creat()
           fcntl() F_SETLKW
           fdatasync()
           fsync()
           getmsg()
           getpmsg()
           lockf() F_LOCK
           mq_receive()
           mq_send()
           mq_timedreceive()
           mq_timedsend()
           msgrcv()
           msgsnd()
           msync()
           nanosleep()
           open()
           openat() [Added in POSIX.1-2008]
           pause()
           poll()
           pread()
           pselect()
           pthread_cond_timedwait()
           pthread_cond_wait()
           pthread_join()
           pthread_testcancel()
           putmsg()
           putpmsg()
           pwrite()
           read()
           readv()
           recv()
           recvfrom()
           recvmsg()
           select()
           sem_timedwait()
           sem_wait()
           send()
           sendmsg()
           sendto()
           sigpause() [POSIX.1-2001 only (moves to "may" list in POSIX.1-2008)]
           sigsuspend()
           sigtimedwait()
           sigwait()
           sigwaitinfo()
           sleep()
           system()
           tcdrain()
           usleep() [POSIX.1-2001 only (function removed in POSIX.1-2008)]
           wait()
           waitid()
           waitpid()
           write()
           writev()

       The  following  functions  may  be  cancellation  points  according  to
       POSIX.1-2001 and/or POSIX.1-2008:

           access()
           asctime()
           asctime_r()
           catclose()
           catgets()
           catopen()
           chmod() [Added in POSIX.1-2008]
           chown() [Added in POSIX.1-2008]
           closedir()
           closelog()
           ctermid()
           ctime()
           ctime_r()
           dbm_close()
           dbm_delete()
           dbm_fetch()
           dbm_nextkey()
           dbm_open()
           dbm_store()
           dlclose()
           dlopen()
           dprintf() [Added in POSIX.1-2008]
           endgrent()
           endhostent()
           endnetent()
           endprotoent()
           endpwent()
           endservent()
           endutxent()
           faccessat() [Added in POSIX.1-2008]
           fchmod() [Added in POSIX.1-2008]
           fchmodat() [Added in POSIX.1-2008]
           fchown() [Added in POSIX.1-2008]
           fchownat() [Added in POSIX.1-2008]
           fclose()
           fcntl() (for any value of cmd argument)
           fflush()
           fgetc()
           fgetpos()
           fgets()
           fgetwc()
           fgetws()
           fmtmsg()
           fopen()
           fpathconf()
           fprintf()
           fputc()
           fputs()
           fputwc()
           fputws()
           fread()
           freopen()
           fscanf()
           fseek()
           fseeko()
           fsetpos()
           fstat()
           fstatat() [Added in POSIX.1-2008]
           ftell()
           ftello()
           ftw()
           futimens() [Added in POSIX.1-2008]
           fwprintf()
           fwrite()
           fwscanf()
           getaddrinfo()
           getc()
           getc_unlocked()
           getchar()
           getchar_unlocked()
           getcwd()
           getdate()
           getdelim() [Added in POSIX.1-2008]
           getgrent()
           getgrgid()
           getgrgid_r()
           getgrnam()
           getgrnam_r()
           gethostbyaddr() [SUSv3 only (function removed in POSIX.1-2008)]
           gethostbyname() [SUSv3 only (function removed in POSIX.1-2008)]
           gethostent()
           gethostid()
           gethostname()
           getline() [Added in POSIX.1-2008]
           getlogin()
           getlogin_r()
           getnameinfo()
           getnetbyaddr()
           getnetbyname()
           getnetent()
           getopt() (if opterr is non-zero)
           getprotobyname()
           getprotobynumber()
           getprotoent()
           getpwent()
           getpwnam()
           getpwnam_r()
           getpwuid()
           getpwuid_r()
           gets()
           getservbyname()
           getservbyport()
           getservent()
           getutxent()
           getutxid()
           getutxline()
           getwc()
           getwchar()
           getwd() [SUSv3 only (function removed in POSIX.1-2008)]
           glob()
           iconv_close()
           iconv_open()
           ioctl()
           link()
           linkat() [Added in POSIX.1-2008]
           lio_listio() [Added in POSIX.1-2008]
           localtime()
           localtime_r()
           lockf() [Added in POSIX.1-2008]
           lseek()
           lstat()
           mkdir() [Added in POSIX.1-2008]
           mkdirat() [Added in POSIX.1-2008]
           mkdtemp() [Added in POSIX.1-2008]
           mkfifo() [Added in POSIX.1-2008]
           mkfifoat() [Added in POSIX.1-2008]
           mknod() [Added in POSIX.1-2008]
           mknodat() [Added in POSIX.1-2008]
           mkstemp()
           mktime()
           nftw()
           opendir()
           openlog()
           pathconf()
           pclose()
           perror()
           popen()
           posix_fadvise()
           posix_fallocate()
           posix_madvise()
           posix_openpt()
           posix_spawn()
           posix_spawnp()
           posix_trace_clear()
           posix_trace_close()
           posix_trace_create()
           posix_trace_create_withlog()
           posix_trace_eventtypelist_getnext_id()
           posix_trace_eventtypelist_rewind()
           posix_trace_flush()
           posix_trace_get_attr()
           posix_trace_get_filter()
           posix_trace_get_status()
           posix_trace_getnext_event()
           posix_trace_open()
           posix_trace_rewind()
           posix_trace_set_filter()
           posix_trace_shutdown()
           posix_trace_timedgetnext_event()
           posix_typed_mem_open()
           printf()
           psiginfo() [Added in POSIX.1-2008]
           psignal() [Added in POSIX.1-2008]
           pthread_rwlock_rdlock()
           pthread_rwlock_timedrdlock()
           pthread_rwlock_timedwrlock()
           pthread_rwlock_wrlock()
           putc()
           putc_unlocked()
           putchar()
           putchar_unlocked()
           puts()
           pututxline()
           putwc()
           putwchar()
           readdir()
           readdir_r()
           readlink() [Added in POSIX.1-2008]
           readlinkat() [Added in POSIX.1-2008]
           remove()
           rename()
           renameat() [Added in POSIX.1-2008]
           rewind()
           rewinddir()
           scandir() [Added in POSIX.1-2008]
           scanf()
           seekdir()
           semop()
           setgrent()
           sethostent()
           setnetent()
           setprotoent()
           setpwent()
           setservent()
           setutxent()
           sigpause() [Added in POSIX.1-2008]
           stat()
           strerror()
           strerror_r()
           strftime()
           symlink()
           symlinkat() [Added in POSIX.1-2008]
           sync()
           syslog()
           tmpfile()
           tmpnam()
           ttyname()
           ttyname_r()
           tzset()
           ungetc()
           ungetwc()
           unlink()
           unlinkat() [Added in POSIX.1-2008]
           utime() [Added in POSIX.1-2008]
           utimensat() [Added in POSIX.1-2008]
           utimes() [Added in POSIX.1-2008]
           vdprintf() [Added in POSIX.1-2008]
           vfprintf()
           vfwprintf()
           vprintf()
           vwprintf()
           wcsftime()
           wordexp()
           wprintf()
           wscanf()

       An  implementation  may  also mark other functions not specified in the
       standard as cancellation points.  In particular, an  implementation  is
       likely   to  mark  any  non-standard  function  that  may  block  as  a
       cancellation point.  (This  includes  most  functions  that  can  touch
       files.)

   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  (see  also
          signal(7)).   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 (see also signal(7));  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), proc(5), futex(7), signal(7),
       and various Pthreads manual pages, for  example:  pthread_attr_init(3),
       pthread_atfork(3),      pthread_cancel(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

       This  page  is  part of release 3.15 of the Linux man-pages project.  A
       description of the project, and information about reporting  bugs,  can
       be found at http://www.kernel.org/doc/man-pages/.