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PTHREADS(7) Linux Programmer's Manual PTHREADS(7)
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_setscheduler(2) and
sched_setparam(2))
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 of failure.
Note that the pthreads functions do not set errno. For each of the pthreads
functions that can return an error, POSIX.1-2001 specifies that the function
can never fail with the error EINTR.
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 only guaranteed to be unique within a
process. A thread ID may be reused after a terminated thread has been joined,
or a detached thread has terminated. In all pthreads functions that accept a
thread ID as an argument, that ID by definition refers to a thread in the same
process as the caller.
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.)
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, etc.) 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 only
allows the thread that created the child to wait(2) on it.
- When a thread calls execve(2), all other threads are terminated (as
required by POSIX.1). However, the resulting process has the same PID as
the thread that called execve(2): it should have the same PID as the main
thread.
- Threads do not share user and group IDs. This can cause complications with
set-user-ID programs and can cause failures in Pthreads functions if an
application changes its credentials using seteuid(2) or similar.
- Threads do not share a common session ID and process group ID.
- Threads do not share record locks created using fcntl(2).
- The information returned by times(2) and getrusage(2) is per-thread rather
than process-wide.
- Threads do not share semaphore undo values (see semop(2)).
- Threads do not share interval timers.
- Threads do not share a common nice value.
- POSIX.1 distinguishes the notions of signals that are directed to the
process as a whole and signals that are directed to individual threads.
According to POSIX.1, a process-directed signal (sent using kill(2), for
example) should be handled by a single, arbitrarily selected thread within
the process. LinuxThreads does not support the notion of process-directed
signals: signals may only be sent 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
(see also signal(7)); these signals cannot be used in applications.
NPTL still has at least one nonconformance with POSIX.1:
- Threads do not share a common nice value.
Some NPTL nonconformances only occur 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|ntpl'
linuxthreads-0.10 by Xavier Leroy
clone(2), futex(2), gettid(2), proc(5), futex(7), sigevent(7), signal(7),
and various Pthreads manual pages, for example: pthread_attr_init(3),
pthread_atfork(3), pthread_cancel(3), pthread_cleanup_push(3),
pthread_cond_signal(3), pthread_cond_wait(3), pthread_create(3),
pthread_detach(3), pthread_equal(3), pthread_exit(3), pthread_key_create(3),
pthread_kill(3), pthread_mutex_lock(3), pthread_mutex_unlock(3),
pthread_once(3), pthread_setcancelstate(3), pthread_setcanceltype(3),
pthread_setspecific(3), pthread_sigmask(3), pthread_sigqueue(3), and
pthread_testcancel(3)
This page is part of release 3.32 of the Linux man-pages project. A
description of the project, and information about reporting bugs, can be found
at http://www.kernel.org/doc/man-pages/.
Linux 2010-11-14 PTHREADS(7)
HTML rendering created 2010-12-03 by Michael Kerrisk, author of The Linux Programming Interface