man > pthreads(7)

pthreads(7)                       Miscellaneous Information Manual                       pthreads(7)

NAME
       pthreads - POSIX threads

DESCRIPTION
       POSIX.1 specifies a set of interfaces (functions, header files) for threaded programming com‐
       monly  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 num‐
       bers 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  speci‐
       fies that the function can never fail with the error EINTR.

   Thread IDs
       Each  of  the  threads  in  a  process  has  a  unique  thread identifier (stored in the type
       pthread_t).  This identifier is returned to the caller of pthread_create(3), and a thread can
       obtain its own thread identifier using pthread_self(3).

       Thread IDs are guaranteed to be unique only within a process.   (In  all  pthreads  functions
       that  accept a thread ID as an argument, that ID by definition refers to a thread in the same
       process as the caller.)

       The system may reuse a thread ID after a terminated thread has been  joined,  or  a  detached
       thread  has  terminated.   POSIX  says:  "If an 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  re‐
       gardless 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  asyn‐
       chronous 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()

   Cancelation points
       POSIX.1 specifies that certain functions must, and certain other functions may,  be  cancela‐
       tion  points.   If a thread is cancelable, its cancelability type is deferred, and a cancela‐
       tion request is pending for the thread, then the thread is canceled when it calls a  function
       that is a cancelation point.

       The  following  functions  are  required  to  be  cancelation  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  cancelation  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 cancelation
       points.  In particular, an implementation is likely to mark any nonstandard function that may
       block as a cancelation point.  (This includes most functions that can touch files.)

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

   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 bet‐
              ter 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 em‐
          ployed 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  getp‐
          pid(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 cre‐
          dentials 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-di‐
          rected 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 ini‐
          tially share an alternate signal stack.  (A new thread should start with no alternate sig‐
          nal  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 Linux 2.6.9).

       •  Threads do not share resource limits (fixed in Linux 2.6.10).

       •  Threads do not share interval timers (fixed in Linux 2.6.12).

       •  Only  the  main thread is permitted to start a new session using setsid(2) (fixed in Linux
          2.6.16).

       •  Only the main thread is permitted to make the process into a process  group  leader  using
          setpgid(2) (fixed in Linux 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 ini‐
          tially share an alternate signal stack (fixed in Linux 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  setr‐
          limit(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 im‐
       plementation, 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 nonconfor‐
       mant 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(3type), 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)

Linux man-pages 6.7                          2023-10-31                                  pthreads(7)