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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. 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()
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() [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
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
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