From Linux Man Pages
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 pro-
gram. 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())
- 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())
- 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))
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 (now obsolete) Pthreads implementation.
- NPTL (Native POSIX Threads Library) This is the modern Pthreads implementation. By comparison with Linux-
Threads, NPTL provides closer conformance to the requirements of the POSIX.1 specification and better perfor-
mance when creating large numbers of threads. NPTL requires features that are present in the Linux 2.6 ker-
nel.
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 synchronisation primitives
(mutexes, thread joining, etc.) are implemented using the Linux futex(2) system call.
Modern GNU C libraries provide both LinuxThreads and NPTL, with the latter being the default (if supported by the
underlying kernel).
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(), 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. 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 fol-
lowing:
- 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 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 set-
tings 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; these signals cannot be used in applications.
NPTL still has a few non-conformances 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 set-
tings 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 default 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 thread-
ing implementation:
bash$ $( ldd /bin/ls | grep libc.so | awk '{print $3}' ) | \
egrep -i 'threads|ntpl'
Native POSIX Threads Library by Ulrich Drepper et al
Selecting the Threading Implementation: LD_ASSUME_KERNEL
On systems with a glibc that supports both LinuxThreads and NPTL, the LD_ASSUME_KERNEL environment variable can
be used to override the dynamic linker's default choice of threading implementation. This variable tells the
dynamic linker to assume that it is running on top of a particular kernel version. By specifying a kernel ver-
sion that does not provide the support required by NPTL, we can force the use of LinuxThreads. (The most likely
reason for doing this is to run a (broken) application that depends on some non-conformant behavior in Linux-
Threads.) For example:
bash$ $( 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
RELATED
clone(2), futex(2), gettid(2), futex(7), and various Pthreads manual pages, for example: pthread_atfork(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), and pthread_testcancel(3).
CATEGORY