pthreads


DESCRIPTION
       POSIX.1  specifies  a  set  of interfaces (functions, header files) for
       threaded programming commonly known as POSIX threads, or  Pthreads.   A
       single process can contain multiple threads, all of which are executing
       the same program.  These threads share the same global memory (data and
       heap  segments),  but  each  thread  has its own stack (automatic vari-
       ables).

       POSIX.1 also requires that threads share a range  of  other  attributes
       (i.e., these attributes are process-wide rather than per-thread):

       -  process ID

       -  parent process ID

       -  process group ID and session ID

       -  controlling terminal

       -  user and group IDs

       -  open file descriptors

       -  record locks (see fcntl(2))

       -  signal dispositions

       -  file mode creation mask (umask(2))

       -  current directory (chdir(2)) and root directory (chroot(2))

       -  interval timers (setitimer(2)) and POSIX timers (timer_create(2))

       -  nice value (setpriority(2))

       -  resource limits (setrlimit(2))

       -  measurements of the consumption of CPU time (times(2)) and resources
          (getrusage(2))

       As well as the stack, POSIX.1 specifies that various  other  attributes
       are distinct for each thread, including:

       -  thread ID (the pthread_t data type)

       -  signal mask (pthread_sigmask(3))

       -  the errno variable

       -  alternate signal stack (sigaltstack(2))

       -  real-time  scheduling policy and priority (sched_setscheduler(2) and
          sched_setparam(2))

   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)]
           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()

           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)]
           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()
           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]
           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()
           symlinkat() [Added in POSIX.1-2008]
           sync()
           syslog()
           tmpfile()
           tmpnam()
           ttyname()
           ttyname_r()
           tzset()
           ungetc()
           ungetwc()
           unlink()
           unlinkat() [Added in POSIX.1-2008]
           utime() [Added in POSIX.1-2008]
           utimensat() [Added in POSIX.1-2008]
           utimes() [Added in POSIX.1-2008]
           vdprintf() [Added in POSIX.1-2008]
           vfprintf()
           vfwprintf()
           vprintf()
           vwprintf()
           wcsftime()
           wordexp()
           wprintf()
           wscanf()

       An implementation may also mark other functions not  specified  in  the
       standard  as  cancellation points.  In particular, an implementation is
       likely to mark any nonstandard function that may block as  a  cancella-
       tion point.  (This includes most functions that can touch files.)

   Compiling on Linux
       On  Linux,  programs that use the Pthreads API should be compiled using
       cc -pthread.

   Linux Implementations of POSIX Threads
       Over time, two threading implementations have been provided by the  GNU
       C library on Linux:

       LinuxThreads
              This  is the original Pthreads implementation.  Since glibc 2.4,
              this implementation is no longer supported.

       NPTL (Native POSIX Threads Library)
              This is the modern Pthreads implementation.  By comparison  with
              LinuxThreads,  NPTL  provides closer conformance to the require-
              ments of the POSIX.1 specification and better  performance  when
              creating  large  numbers  of  threads.   NPTL is available since
              glibc 2.3.2, and requires features that are present in the Linux
              2.6 kernel.

       Both  of  these  are  so-called  1:1 implementations, meaning that each
       thread maps to a kernel scheduling entity.  Both threading  implementa-
       tions  employ the Linux clone(2) system call.  In NPTL, thread synchro-
       nization primitives (mutexes, thread  joining,  etc.)  are  implemented
          later, the first three real-time signals are  used  (see  also  sig-
          nal(7)).   On  older  Linux  kernels,  SIGUSR1 and SIGUSR2 are used.
          Applications must avoid the use  of  whichever  set  of  signals  is
          employed by the implementation.

       -  Threads  do not share process IDs.  (In effect, LinuxThreads threads
          are implemented as  processes  which  share  more  information  than
          usual,  but  which  do not share a common process ID.)  LinuxThreads
          threads (including the manager thread) are visible as separate  pro-
          cesses using ps(1).

       The LinuxThreads implementation deviates from the POSIX.1 specification
       in a number of ways, including the following:

       -  Calls to getpid(2) return a different value in each thread.

       -  Calls to getppid(2) in threads other than the main thread return the
          process  ID  of  the  manager  thread;  instead  getppid(2) in these
          threads should return the same  value  as  getppid(2)  in  the  main
          thread.

       -  When  one  thread  creates  a  new  child process using fork(2), any
          thread should be able to wait(2) on the child.  However, the  imple-
          mentation  only  allows the thread that created the child to wait(2)
          on it.

       -  When a thread calls execve(2), all other threads are terminated  (as
          required  by  POSIX.1).  However, the resulting process has the same
          PID as the thread that called execve(2): it should have the same PID
          as the main thread.

       -  Threads  do  not share user and group IDs.  This can cause complica-
          tions with set-user-ID programs and can cause failures  in  Pthreads
          functions if an application changes its credentials using seteuid(2)
          or similar.

       -  Threads do not share a common session ID and process group ID.

       -  Threads do not share record locks created using fcntl(2).

       -  The information returned by times(2) and getrusage(2) is  per-thread
          rather than process-wide.

       -  Threads do not share semaphore undo values (see semop(2)).

       -  Threads do not share interval timers.

       -  Threads do not share a common nice value.

       -  POSIX.1  distinguishes  the  notions of signals that are directed to
          the process as a whole and signals that are directed  to  individual
          threads.   According  to  POSIX.1,  a  process-directed signal (sent
          using kill(2), for example) should be handled by a single, arbitrar-
          ily  selected thread within the process.  LinuxThreads does not sup-

       With  NPTL,  all  of  the  threads  in a process are placed in the same
       thread group; all members of a thread group share the same  PID.   NPTL
       does not employ a manager thread.  NPTL makes internal use of the first
       two real-time signals (see also signal(7));  these  signals  cannot  be
       used in applications.

       NPTL still has at least one nonconformance with POSIX.1:

       -  Threads do not share a common nice value.

       Some NPTL nonconformances only occur with older kernels:

       -  The  information returned by times(2) and getrusage(2) is per-thread
          rather than process-wide (fixed in kernel 2.6.9).

       -  Threads do not share resource limits (fixed in kernel 2.6.10).

       -  Threads do not share interval timers (fixed in kernel 2.6.12).

       -  Only the main thread is permitted to start a new session using  set-
          sid(2) (fixed in kernel 2.6.16).

       -  Only the main thread is permitted to make the process into a process
          group leader using setpgid(2) (fixed in kernel 2.6.16).

       -  Threads have distinct alternate signal stack settings.   However,  a
          new  thread's  alternate  signal  stack settings are copied from the
          thread that created it, so  that  the  threads  initially  share  an
          alternate signal stack (fixed in kernel 2.6.16).

       Note the following further points about the NPTL implementation:

       -  If  the  stack  size  soft  resource  limit  (see the description of
          RLIMIT_STACK in setrlimit(2)) is set to a value  other  than  unlim-
          ited,  then  this  value  defines  the  default  stack  size for new
          threads.  To be effective, this limit must be set before the program
          is  executed,  perhaps  using  the  ulimit -s shell built-in command
          (limit stacksize in the C shell).

   Determining the Threading Implementation
       Since glibc 2.3.2, the getconf(1) command can be used to determine  the
       system's threading implementation, for example:

           bash$ getconf GNU_LIBPTHREAD_VERSION
           NPTL 2.3.4

       With  older  glibc  versions, a command such as the following should be
       sufficient to determine the default threading implementation:

           bash$ $( ldd /bin/ls | grep libc.so | awk '{print $3}' ) | \
                           egrep -i 'threads|nptl'
                   Native POSIX Threads Library by Ulrich Drepper et al

   Selecting the Threading Implementation: LD_ASSUME_KERNEL
                   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/.



Linux                             2010-11-14                       PTHREADS(7)
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