>> pthreads (7) ( Linux man: Макропакеты и соглашения )
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):
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.
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)]
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()
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:
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 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()
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 non-standard function that may block as a cancellation 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 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.
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 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.)
NPTL
With NPTL, all of the threads in a process are placed
in the same thread group;
all members of a thread groups 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 non-conformance with POSIX.1:
-
Threads do not share a common nice value.
Some NPTL non-conformances 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).
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 aq{print $3}aq ) | \
egrep -i aqthreads|nptlaq
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 non-conformant behavior
in LinuxThreads.)
For example:
This page is part of release 3.14 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/.
