kernel_Notes/Zim/Programme/APUE/pthreads_-_POSIX_threads.txt

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====== pthreads - POSIX threads ======
Created Tuesday 23 October 2012
http://linux.die.net/man/7/pthreads

===== 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_setscheduler(2) and sched_setparam(2))

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 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()
........

===== 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()
.........

The following functions __may be cancellation points__ according to POSIX.1-2001 and/or POSIX.1-2008:
access()
asctime()
..........
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.)

===== 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（**futex和实时信号**）.

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.注意，管理线程是函数库**内部自动生成**的，而不是指调用进程。)

- 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:
下面是LinuxThreads实现的与POSIX.1规范__不符__的地方：

- 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.   这里的main thread指的是最开始的(initial)线程。

- 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 group 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 nonconformance__ with POSIX.1:

- Threads do not share a common nice value.

Some NPTL nonconformances 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 '{print $3}' ) | \
                egrep -i 'threads|nptl'
        Native POSIX Threads Library by Ulrich Drepper et al

[geekard@kb310 ~]$ __/lib/libpthread.so.0__ 
Native POSIX Threads Library by Ulrich Drepper et al
Copyright (C) 2006 Free Software Foundation, Inc.
This is free software; see the source for copying conditions.
There is NO warranty; not even for MERCHANTABILITY or FITNESS FOR A
PARTICULAR PURPOSE.
Forced unwind support included.
[geekard@kb310 ~]$ 

===== 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|ntpl'
        linuxthreads-0.10 by Xavier Leroy

===== See Also =====
clone(2), futex(2), gettid(2), proc(5), futex(7), sigevent(7), signal(7),
and various Pthreads manual pages, for example: pthread_attr_init(3), pthread_atfork(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_once(3), pthread_setcancelstate(3), pthread_setcanceltype(3), pthread_setspecific(3), pthread_sigmask(3), pthread_sigqueue(3), and pthread_testcancel(3)

===== Referenced By =====
core(5), intro(3), pthread_attr_getdetachstate(3), pthread_attr_getscope(3), pthread_attr_getstackaddr(3), pthread_attr_setaffinity_np(3), pthread_attr_setguardsize(3), pthread_attr_setinheritsched(3), pthread_attr_setschedparam(3), pthread_attr_setschedpolicy(3), pthread_attr_setstack(3), pthread_attr_setstacksize(3), pthread_cleanup_push_defer_np(3), pthread_getattr_np(3), pthread_getcpuclockid(3), pthread_join(3), pthread_kill_other_threads_np(3), pthread_setaffinity_np(3), pthread_setconcurrency(3), pthread_setschedparam(3), pthread_setschedprio(3), pthread_tryjoin_np(3), pthread_yield(3), sem_overview(7), vfork(2), xfs_copy(8)