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


     pthreads - POSIX threads


     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-
     POSIX.1 also requires that threads share a range  of  other  attributes
     (i.e., these attributes are process-wide rather than per-thread):
  1. process ID
  1. parent process ID
  1. process group ID and session ID
  1. controlling terminal
  1. user and group IDs
  1. open file descriptors
  1. record locks (see fcntl(2))
  1. signal dispositions
  1. file mode creation mask (umask(2))
  1. current directory (chdir(2)) and root directory (chroot(2))
  1. interval timers (setitimer(2)) and POSIX timers (timer_create(2))
  1. nice value (setpriority(2))
  1. resource limits (setrlimit(2))
  1. measurements of the consumption of CPU time (times(2)) and resources


     As well as the stack, POSIX.1 specifies that various  other  attributes
     are distinct for each thread, including:
  1. thread ID (the pthread_t data type)
  1. signal mask (pthread_sigmask(3))
  1. the errno variable
  1. alternate signal stack (sigaltstack(2))
  1. real-time scheduling policy and priority (sched(7))
     The following Linux-specific features are also per-thread:
  1. capabilities (see capabilities(7))
  1. CPU affinity (sched_setaffinity(2))
 Pthreads function return values
     Most  pthreads  functions  return  0 on success, and an error number on
     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 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 appli-
     cation attempts to use a thread ID whose lifetime has ended, the behav-
     ior 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)]  gethostby-
         name() [POSIX.1-2001 only (function removed in POSIX.1-2008)] geth-
         ostent()  getlogin()  getnetbyaddr()   getnetbyname()   getnetent()
         getopt()  getprotobyname()  getprotobynumber()  getprotoent() getp-
         went() getpwnam() getpwuid() getservbyname()  getservbyport()  get-
         servent()  getutxent()  getutxid()  getutxline() gmtime() hcreate()
         hdestroy() hsearch() inet_ntoa() l64a()  lgamma()  lgammaf()  lgam-
         mal()  localeconv() localtime() lrand48() mrand48() nftw() nl_lang-
         info()  ptsname()   putc_unlocked()   putchar_unlocked()   putenv()
         pututxline()  rand()  readdir()  setenv() setgrent() setkey() setp-
         went() 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()
 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
     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 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_timed-
         wait()   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()
     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() get-
         grnam_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() get-
         netbyaddr()  getnetbyname()  getnetent()  getopt()  (if  opterr  is
         nonzero)  getprotobyname()  getprotobynumber()  getprotoent() getp-
         went() getpwnam() getpwnam_r() getpwuid() getpwuid_r() gets()  get-
         servbyname()  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] mkdi-
         rat()  [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_with-
         log() posix_trace_eventtypelist_getnext_id() posix_trace_eventtype-
         list_rewind()       posix_trace_flush()      posix_trace_get_attr()
         posix_trace_get_filter() posix_trace_get_status()  posix_trace_get-
         next_event()         posix_trace_open()        posix_trace_rewind()
         posix_trace_set_filter() posix_trace_shutdown()  posix_trace_timed-
         getnext_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() set-
         grent() sethostent() setnetent() setprotoent()  setpwent()  setser-
         vent()  setutxent()  sigpause() [Added in POSIX.1-2008] stat() str-
         error() strerror_r() strftime()  symlink()  symlinkat()  [Added  in
         POSIX.1-2008]  sync()  syslog()  tmpfile()  tmpnam() ttyname() tty-
         name_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() wcsf-
         time() 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:
            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, and  so  on)  are  imple-
     mented using the Linux futex(2) system call.
     The notable features of this implementation are the following:
  1. 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
  1. 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.
  1. 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:
  1. Calls to getpid(2) return a different value in each thread.
  1. 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
  1. 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  allows  only the thread that created the child to wait(2)
        on it.
  1. 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.
  1. 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.
  1. Threads do not share a common session ID and process group ID.
  1. Threads do not share record locks created using fcntl(2).
  1. The information returned by times(2) and getrusage(2) is per-thread

rather than process-wide.

  1. Threads do not share semaphore undo values (see semop(2)).
  1. Threads do not share interval timers.
  1. Threads do not share a common nice value.
  1. 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  be  sent
        only to specific threads.
  1. 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.)
     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 sig-
     nals cannot be used in applications.  See nptl(7) for further  details.
     NPTL still has at least one nonconformance with POSIX.1:
  1. Threads do not share a common nice value.
     Some NPTL nonconformances occur only with older kernels:
  1. The information returned by times(2) and getrusage(2) is per-thread

rather than process-wide (fixed in kernel 2.6.9).

  1. Threads do not share resource limits (fixed in kernel 2.6.10).
  1. Threads do not share interval timers (fixed in kernel 2.6.12).
  1. Only the main thread is permitted to start a new session using set-

sid(2) (fixed in kernel 2.6.16).

  1. Only the main thread is permitted to make the process into a process

group leader using setpgid(2) (fixed in kernel 2.6.16).

  1. 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:
  1. 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 | 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 | \
                         awk '{print $3}' ) | egrep -i 'threads|nptl'
                 linuxthreads-0.10 by Xavier Leroy


     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_atfork(3),
     pthread_attr_init(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_mutexattr_destroy(3),
     pthread_mutexattr_init(3), pthread_once(3), pthread_spin_init(3),
     pthread_spin_lock(3), pthread_rwlockattr_setkind_np(3),
     pthread_setcancelstate(3), pthread_setcanceltype(3),
     pthread_setspecific(3), pthread_sigmask(3), pthread_sigqueue(3), and


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Linux 2017-09-15 PTHREADS(7)

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