pthreads(7) — Linux manual page

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
       on failure.  The error numbers that can be returned have the same
       meaning as the error numbers returned in errno by conventional
       system calls and C library functions.  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() [POSIX.1-2001 only (function removed in
                            POSIX.1-2008)]
           gethostbyname() [POSIX.1-2001 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() [POSIX.1-2001 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.)

       It should be noted that even if an application is not using
       asynchronous cancellation, that calling a function from the above
       list from an asynchronous signal handler may cause the equivalent
       of asynchronous cancellation.  The underlying user code may not
       expect asynchronous cancellation and the state of the user data
       may become inconsistent.  Therefore signals should be used with
       caution when entering a region of deferred cancellation.

   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         top

       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 pthread_testcancel(3)

COLOPHON         top

       This page is part of release 5.13 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
       https://www.kernel.org/doc/man-pages/.

Linux                          2021-03-22                    PTHREADS(7)

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