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 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.  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() [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.)

       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
       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
       This page is part of release 5.08 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                              2020-08-13                        PTHREADS(7)