NAME | DESCRIPTION | SEE ALSO | COLOPHON

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

NAME         top

       pthreads - POSIX threads

DESCRIPTION         top

       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.  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, 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 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 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 (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 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|ntpl'
                   linuxthreads-0.10 by Xavier Leroy

SEE ALSO         top

       clone(2), futex(2), gettid(2), proc(5), futex(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         top

       This page is part of release 3.75 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
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Linux                            2014-05-21                      PTHREADS(7)