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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(7))

       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.  (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.)

       The  system  may  reuse  a  thread ID after a terminated thread has been joined, or a detached thread has
       terminated.  POSIX says: "If an application attempts to use a thread ID whose  lifetime  has  ended,  the
       behavior is undefined."

   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, and so on) 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 allows only 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 be sent only 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; these signals cannot be used in applications.
       See nptl(7) for further details.

       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 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|nptl'
                   linuxthreads-0.10 by Xavier Leroy

SEE ALSO

       clone(2), fork(2), futex(2), gettid(2), proc(5), attributes(7), futex(7), nptl(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

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       information about reporting bugs, and the latest version of this page, can be found at
       http://www.kernel.org/doc/man-pages/.