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CentOS 7.0 - man page for pthreads (centos section 7)

PTHREADS(7)			    Linux Programmer's Manual			      PTHREADS(7)

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 guaranteed to
       be unique only 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 cancel-
       lation points.  If a thread is cancelable, its cancelability type is deferred, and a  can-
       cellation  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 cancella-
       tion  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, and so on) are imple-
       mented 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  inad-
	  vertently 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  sepa-
	  rate 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 sin-
	  gle, arbitrarily selected thread within the process.	LinuxThreads does not support the
	  notion of process-directed signals: signals may be sent only to specific threads.

       -  Threads  have distinct alternate signal stack settings.  However, a new thread's alter-
	  nate 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  alter-
	  nate	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  mem-
       bers  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 occur only 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 ker-
	  nel 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  alter-
	  nate	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  setr-
	  limit(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 deter-
       mine 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),

       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
       This page is part of release 3.53 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/.

Linux					    2010-11-14				      PTHREADS(7)


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