pthreads
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 vari-
ables).
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))
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 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.
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)]
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()
Cancellation Points
POSIX.1 specifies that certain functions must, and certain other func-
tions may, be cancellation points. If a thread is cancelable, its can-
celability 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:
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()
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()
gethostid()
gethostname()
getline() [Added in POSIX.1-2008]
getlogin()
getlogin_r()
getnameinfo()
getnetbyaddr()
getnetbyname()
getnetent()
getopt() (if opterr is non-zero)
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()
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()
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 non-standard function that may block as a cancella-
tion point. (This includes most functions that can touch files.)
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 require-
ments 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 implementa-
tions employ the Linux clone(2) system call. In NPTL, thread synchro-
nization primitives (mutexes, thread joining, etc.) 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 termi-
nation. (Problems can result if this thread is inadvertently
killed.)
- Signals are used internally by the implementation. On Linux 2.2 and
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 imple-
mentation 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 complica-
tions 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, arbitrar-
ily selected thread within the process. LinuxThreads does not sup-
port 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 alter-
nate signal stack defined. If two threads handle signals on their
shared alternate signal stack at the same time, unpredictable pro-
gram failures are likely to occur.)
NPTL
- 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 set-
sid(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 unlim-
ited, 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 $3}' ) | \
egrep -i 'threads|nptl'
Native POSIX Threads Library by Ulrich Drepper et al
Selecting the Threading Implementation: LD_ASSUME_KERNEL
On systems with a glibc that supports both LinuxThreads and NPTL (i.e.,
glibc 2.3.x), the LD_ASSUME_KERNEL environment variable can be used to
override the dynamic linker's default choice of threading implementa-
tion. This variable tells the dynamic linker to assume that it is run-
ning on top of a particular kernel version. By specifying a kernel
version that does not provide the support required by NPTL, we can
force the use of LinuxThreads. (The most likely reason for doing this
is to run a (broken) application that depends on some non-conformant
behavior in LinuxThreads.) For example:
bash$ $( LD_ASSUME_KERNEL=2.2.5 ldd /bin/ls | grep libc.so | \
awk '{print $3}' ) | egrep -i 'threads|ntpl'
COLOPHON
This page is part of release 3.23 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 2008-11-18 PTHREADS(7)
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