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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(2))

       -  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()

   Async-cancel-safe functions
       An  async-cancel-safe  function  is one that can be safely called in an
       application  where   asynchronous   cancelability   is   enabled   (see
       pthread_setcancelstate(3)).

       Only  the  following  functions are required to be async-cancel-safe by
       POSIX.1-2001 and POSIX.1-2008:

           pthread_cancel()
           pthread_setcancelstate()
           pthread_setcanceltype()

   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 nonzero)
           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  nonstandard  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 group 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 nonconformance with POSIX.1:

       -  Threads do not share a common nice value.

       Some NPTL nonconformances 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
       nonconformant 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), sigevent(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),
       pthread_sigqueue(3), and pthread_testcancel(3)

COLOPHON

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       description of the project, and information about reporting bugs, can
       be found at http://man7.org/linux/man-pages/.