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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 vari‐
       ables).

       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 speci‐
       fies 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 func‐
       tions may, be cancellation points.  If a thread is cancelable, its can‐
       celability  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 cancella‐
       tion 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  require‐
	      ments  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 implementa‐
       tions employ the Linux clone(2) system call.  In NPTL, thread  synchro‐
       nization	 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	termi‐
	  nation.   (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 sig‐
	  nal(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 pro‐
	  cesses 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 imple‐
	  mentation 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  complica‐
	  tions	 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, arbitrar‐
	  ily selected thread within the process.  LinuxThreads does not  sup‐
	  port	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 alter‐
	  nate signal stack defined.  If two threads handle signals  on	 their
	  shared  alternate  signal stack at the same time, unpredictable pro‐
	  gram 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 set‐
	  sid(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 unlim‐
	  ited, 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 implementa‐
       tion.  This variable tells the dynamic linker to assume that it is run‐
       ning  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), sig‐
       nal(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_cre‐
       ate(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 3.27 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				  2008-11-18			   PTHREADS(7)
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