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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

       This page is part of release 3.35 of the Linux man-pages project.  A description of the
       project, and information about reporting bugs, can be found at http://man7.org/linux/man-
       pages/.