|
HTML rendering created 2025-09-06 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
|
|
NAME | DESCRIPTION | SEE ALSO | COLOPHON |
|
|
|
pthreads(7) Miscellaneous Information 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(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()
Cancelation points
POSIX.1 specifies that certain functions must, and certain other
functions may, be cancelation points. If a thread is cancelable,
its cancelability type is deferred, and a cancelation 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 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 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
initially 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 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 $3}' ) | \
egrep -i 'threads|nptl'
Native POSIX Threads Library by Ulrich Drepper et al
Selecting the threading implementation: LD_ASSUME_KERNEL
On systems with a glibc that supports both LinuxThreads and NPTL
(i.e., glibc 2.3.*), the LD_ASSUME_KERNEL environment variable can
be used to override the dynamic linker's default choice of
threading implementation. This variable tells the dynamic linker
to assume that it is running on top of a particular kernel
version. By specifying a kernel version that does not provide the
support required by NPTL, we can force the use of LinuxThreads.
(The most likely reason for doing this is to run a (broken)
application that depends on some nonconformant behavior in
LinuxThreads.) For example:
bash$ $( 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(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)
This page is part of the man-pages (Linux kernel and C library
user-space interface documentation) project. Information about
the project can be found at
⟨https://www.kernel.org/doc/man-pages/⟩. If you have a bug report
for this manual page, see
⟨https://git.kernel.org/pub/scm/docs/man-pages/man-pages.git/tree/CONTRIBUTING⟩.
This page was obtained from the tarball man-pages-6.15.tar.gz
fetched from
⟨https://mirrors.edge.kernel.org/pub/linux/docs/man-pages/⟩ on
2025-08-11. If you discover any rendering problems in this HTML
version of the page, or you believe there is a better or more up-
to-date source for the page, or you have corrections or
improvements to the information in this COLOPHON (which is not
part of the original manual page), send a mail to
man-pages@man7.org
Linux man-pages 6.15 2025-05-17 pthreads(7)
Pages that refer to this page: clone(2), timer_create(2), vfork(2), intro(3), libpsx(3), pthread_atfork(3), pthread_attr_init(3), pthread_attr_setaffinity_np(3), pthread_attr_setdetachstate(3), pthread_attr_setguardsize(3), pthread_attr_setinheritsched(3), pthread_attr_setschedparam(3), pthread_attr_setschedpolicy(3), pthread_attr_setscope(3), pthread_attr_setsigmask_np(3), pthread_attr_setstack(3), pthread_attr_setstackaddr(3), pthread_attr_setstacksize(3), pthread_cancel(3), pthread_cleanup_push(3), pthread_cleanup_push_defer_np(3), pthread_create(3), pthread_detach(3), pthread_equal(3), pthread_exit(3), pthread_getattr_default_np(3), pthread_getattr_np(3), pthread_getcpuclockid(3), pthread_join(3), pthread_kill(3), pthread_kill_other_threads_np(3), pthread_mutexattr_getpshared(3), pthread_mutexattr_init(3), pthread_mutexattr_setrobust(3), pthread_mutex_consistent(3), pthread_rwlockattr_setkind_np(3), pthread_self(3), pthread_setaffinity_np(3), pthread_setcancelstate(3), pthread_setconcurrency(3), pthread_setname_np(3), pthread_setschedparam(3), pthread_setschedprio(3), pthread_sigmask(3), pthread_sigqueue(3), pthread_spin_init(3), pthread_spin_lock(3), pthread_testcancel(3), pthread_tryjoin_np(3), pthread_yield(3), core(5), proc_pid_status(5), attributes(7), capabilities(7), credentials(7), futex(7), nptl(7), sem_overview(7), signal(7), ld.so(8), xfs_copy(8)
|
HTML rendering created 2025-09-06 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
|