pthreads - POSIX threads
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))
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.
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."
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()
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()
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.
On Linux, programs that use the Pthreads API should be compiled using
cc
-pthread.
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.
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.)
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).
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 $3}' ) | \
egrep -i 'threads|nptl'
Native POSIX Threads Library by Ulrich Drepper et al
On systems with a glibc that supports both LinuxThreads and NPTL (i.e., glibc
2.3.
x), the
LD_ASSUME_KERNEL environment variable can be used
to override the dynamic linker's default choice of threading implementation.
This variable tells the dynamic linker to assume that it is running on top of
a particular kernel version. By specifying a kernel version that does not
provide the support required by NPTL, we can force the use of LinuxThreads.
(The most likely reason for doing this is to run a (broken) application that
depends on some nonconformant behavior in LinuxThreads.) For example:
bash$ $( 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
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)