pthreads

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(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 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)]
           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  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, 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 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  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 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  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  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; these sig-
       nals 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 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|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
       This page is part of release 4.04 of the Linux man-pages project.  A
       description of the project, information about reporting bugs, and the
       latest version of this page, can be found at
       http://www.kernel.org/doc/man-pages/.

Linux                             2015-05-07                       PTHREADS(7)
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