多线程使用

科技2022-07-13 129

1. POSIX 线程标准

POSIX线程标准(POXIS Threads，缩写为Pthreads)是POSIX的线程标准，定义了一系列操作线程的API。

1.1 POSIX标准

POSIX标准，POSIX表示可移植操作系统接口，即Portable Operating System Interface of UNIX，缩写为POSIX。POSIX 标准定义了操作系统为应用程序提供的接口标准，是IEEE为要在各种 UNIX操作系统上运行的软件而定义的一系列 API 标准的总称，其正式称呼为IEEE 1003，而国际标准名称为ISO/IEC 9945。

20世纪80年代，unix厂商视图增加一些新的、往往不兼容的特性使特们的程序体现差异化，这样导致通用性降低，为阻止这一趋势，IEEE开始努力使unix进行标准化开发，后来由 Richard Stallman 命名为“Posix”，从而就得到了一系列标准，称作Posix标准。这套标准涵盖了很多方面，比如Unix系统调用的C语言接口、shell程序和工具、线程及网络编程。

1.2 Linux上的线程库

Linux上没有真正意义的线程，使用进程来模拟，属于用户级线程，位于libpthread共享库，遵循POSIX标准。Linux最有名的两个线程库：LinuxThreads 和 NPTL。 Linux从内核2.6开始，多线程已经使用NPTL技术。查看系统使用的线程库，getconf GNU_LIBPTHREAD_VERSION。

1.3 线程API

当前我使用的操作系统CentOS Linux release 7.4.1708，线程库查看如下： [root@host122 ~]# getconf GNU_LIBPTHREAD_VERSION NPTL 2.17

查看该系统支持的线程API：

[root@host122 ~]# man -k pthread pthread_atfork (3p) - register fork handlers pthread_attr_destroy (3) - initialize and destroy thread attributes object pthread_attr_getaffinity_np (3) - set/get CPU affinity attribute in thread attributes object pthread_attr_getdetachstate (3) - set/get detach state attribute in thread attributes object pthread_attr_getguardsize (3) - set/get guard size attribute in thread attributes object pthread_attr_getinheritsched (3) - set/get inherit-scheduler attribute in thread attributes object pthread_attr_getschedparam (3) - set/get scheduling parameter attributes in thread attributes object pthread_attr_getschedpolicy (3) - set/get scheduling policy attribute in thread attributes object pthread_attr_getscope (3) - set/get contention scope attribute in thread attributes object pthread_attr_getstack (3) - set/get stack attributes in thread attributes object pthread_attr_getstackaddr (3) - set/get stack address attribute in thread attributes object pthread_attr_getstacksize (3) - set/get stack size attribute in thread attributes object pthread_attr_init (3) - initialize and destroy thread attributes object pthread_attr_setaffinity_np (3) - set/get CPU affinity attribute in thread attributes object pthread_attr_setdetachstate (3) - set/get detach state attribute in thread attributes object pthread_attr_setguardsize (3) - set/get guard size attribute in thread attributes object pthread_attr_setinheritsched (3) - set/get inherit-scheduler attribute in thread attributes object pthread_attr_setschedparam (3) - set/get scheduling parameter attributes in thread attributes object pthread_attr_setschedpolicy (3) - set/get scheduling policy attribute in thread attributes object pthread_attr_setscope (3) - set/get contention scope attribute in thread attributes object pthread_attr_setstack (3) - set/get stack attributes in thread attributes object pthread_attr_setstackaddr (3) - set/get stack address attribute in thread attributes object pthread_attr_setstacksize (3) - set/get stack size attribute in thread attributes object pthread_barrierattr_destroy (3p) - destroy and initialize the barrier attributes object (ADVANCED REALTIME THREADS) pthread_barrierattr_getpshared (3p) - get and set the process-shared attribute of the barrier attributes object (ADVANCED REALTIME THREADS) pthread_barrierattr_init (3p) - destroy and initialize the barrier attributes object (ADVANCED REALTIME THREADS) pthread_barrierattr_setpshared (3p) - get and set the process-shared attribute of the barrier attributes object (ADVANCED REALTIME THREADS) pthread_barrier_destroy (3p) - destroy and initialize a barrier object (ADVANCED REALTIME THREADS) pthread_barrier_init (3p) - destroy and initialize a barrier object (ADVANCED REALTIME THREADS) pthread_barrier_wait (3p) - synchronize at a barrier (ADVANCED REALTIME THREADS) pthread_cancel (3) - send a cancellation request to a thread pthread_cleanup_pop (3) - push and pop thread cancellation clean-up handlers pthread_cleanup_pop_restore_np (3) - push and pop thread cancellation clean-up handlers while saving cancelability type pthread_cleanup_push (3) - push and pop thread cancellation clean-up handlers pthread_cleanup_push_defer_np (3) - push and pop thread cancellation clean-up handlers while saving cancelability type pthread_condattr_destroy (3p) - destroy and initialize the condition variable attributes object pthread_condattr_getclock (3p) - get and set the clock selection condition variable attribute (ADVANCED REALTIME) pthread_condattr_getpshared (3p) - get and set the process-shared condition variable attributes pthread_condattr_init (3p) - destroy and initialize the condition variable attributes object pthread_condattr_setclock (3p) - get and set the clock selection condition variable attribute (ADVANCED REALTIME) pthread_condattr_setpshared (3p) - get and set the process-shared condition variable attributes pthread_cond_broadcast (3p) - broadcast or signal a condition pthread_cond_destroy (3p) - destroy and initialize condition variables pthread_cond_init (3p) - destroy and initialize condition variables pthread_cond_signal (3p) - broadcast or signal a condition pthread_cond_timedwait (3p) - wait on a condition pthread_cond_wait (3p) - wait on a condition pthread_create (3) - create a new thread pthread_detach (3) - detach a thread pthread_equal (3) - compare thread IDs pthread_exit (3) - terminate calling thread pthread_getaffinity_np (3) - set/get CPU affinity of a thread pthread_getattr_np (3) - get attributes of created thread pthread_getconcurrency (3) - set/get the concurrency level pthread_getcpuclockid (3) - retrieve ID of a thread's CPU time clock pthread_getname_np (3) - set/get the name of a thread pthread_getschedparam (3) - set/get scheduling policy and parameters of a thread pthread_getspecific (3p) - thread-specific data management pthread.h (0p) - threads pthread_join (3) - join with a terminated thread pthread_key_create (3p) - thread-specific data key creation pthread_key_delete (3p) - thread-specific data key deletion pthread_kill (3) - send a signal to a thread pthread_kill_other_threads_np (3) - terminate all other threads in process pthread_mutexattr_destroy (3p) - destroy and initialize the mutex attributes object pthread_mutexattr_getprioceiling (3p) - get and set the prioceiling attribute of the mutex attributes object (REALTIME THREADS) pthread_mutexattr_getprotocol (3p) - get and set the protocol attribute of the mutex attributes object (REALTIME THREADS) pthread_mutexattr_getpshared (3p) - get and set the process-shared attribute pthread_mutexattr_getrobust (3p) - get and set the mutex robust attribute pthread_mutexattr_gettype (3p) - get and set the mutex type attribute pthread_mutexattr_init (3p) - destroy and initialize the mutex attributes object pthread_mutexattr_setprioceiling (3p) - get and set the prioceiling attribute of the mutex attributes object (REALTIME THREADS) pthread_mutexattr_setprotocol (3p) - get and set the protocol attribute of the mutex attributes object (REALTIME THREADS) pthread_mutexattr_setpshared (3p) - get and set the process-shared attribute pthread_mutexattr_setrobust (3p) - get and set the mutex robust attribute pthread_mutexattr_settype (3p) - get and set the mutex type attribute pthread_mutex_consistent (3p) - mark state protected by robust mutex as consistent pthread_mutex_destroy (3p) - destroy and initialize a mutex pthread_mutex_getprioceiling (3p) - get and set the priority ceiling of a mutex (REALTIME THREADS) pthread_mutex_init (3p) - destroy and initialize a mutex pthread_mutex_lock (3p) - lock and unlock a mutex pthread_mutex_setprioceiling (3p) - get and set the priority ceiling of a mutex (REALTIME THREADS) pthread_mutex_timedlock (3p) - lock a mutex (ADVANCED REALTIME) pthread_mutex_trylock (3p) - lock and unlock a mutex pthread_mutex_unlock (3p) - lock and unlock a mutex pthread_once (3p) - dynamic package initialization pthread_rwlockattr_destroy (3p) - destroy and initialize the read-write lock attributes object pthread_rwlockattr_getpshared (3p) - get and set the process-shared attribute of the read-write lock attributes object pthread_rwlockattr_init (3p) - destroy and initialize the read-write lock attributes object pthread_rwlockattr_setpshared (3p) - get and set the process-shared attribute of the read-write lock attributes object pthread_rwlock_destroy (3p) - destroy and initialize a read-write lock object pthread_rwlock_init (3p) - destroy and initialize a read-write lock object pthread_rwlock_rdlock (3p) - lock a read-write lock object for reading pthread_rwlock_timedrdlock (3p) - lock a read-write lock for reading pthread_rwlock_timedwrlock (3p) - lock a read-write lock for writing pthread_rwlock_tryrdlock (3p) - lock a read-write lock object for reading pthread_rwlock_trywrlock (3p) - lock a read-write lock object for writing pthread_rwlock_unlock (3p) - unlock a read-write lock object pthread_rwlock_wrlock (3p) - lock a read-write lock object for writing pthreads (7) - POSIX threads pthread_self (3) - obtain ID of the calling thread pthread_setaffinity_np (3) - set/get CPU affinity of a thread pthread_setcancelstate (3) - set cancelability state and type pthread_setcanceltype (3) - set cancelability state and type pthread_setconcurrency (3) - set/get the concurrency level pthread_setname_np (3) - set/get the name of a thread pthread_setschedparam (3) - set/get scheduling policy and parameters of a thread pthread_setschedprio (3) - set scheduling priority of a thread pthread_setspecific (3p) - thread-specific data management pthread_sigmask (3) - examine and change mask of blocked signals pthread_sigqueue (3) - queue a signal and data to a thread pthread_spin_destroy (3p) - destroy or initialize a spin lock object (ADVANCED REALTIME THREADS) pthread_spin_init (3p) - destroy or initialize a spin lock object (ADVANCED REALTIME THREADS) pthread_spin_lock (3p) - lock a spin lock object (ADVANCED REALTIME THREADS) pthread_spin_trylock (3p) - lock a spin lock object (ADVANCED REALTIME THREADS) pthread_spin_unlock (3p) - unlock a spin lock object (ADVANCED REALTIME THREADS) pthread_testcancel (3) - request delivery of any pending cancellation request pthread_timedjoin_np (3) - try to join with a terminated thread pthread_tryjoin_np (3) - try to join with a terminated thread pthread_yield (3) - yield the processor vfs_aio_pthread (8) - implement async I/O in Samba vfs using a pthread pool

NPTL 源码 http://ftp.gnu.org/pub/gnu/glibc/glibc-2.9.tar.gz 或者 https://sourceforge.net/projects/nuttx/

2. 常用线程API介绍

2.1 创建线程 pthread_create

int pthread_create(pthread_t *thread, const pthread_attr_t *attr, void *(*start_routine) (void *), void *arg); 参数说明： thread ：线程号 attr ：线程属性 (*start_routine) (void *) ：线程运行函数的起始地址 arg ：运行函数的参数

2.2 等待线程结束 pthread_join

int pthread_join(pthread_t thread, void **retval); 阻塞直到等待id为thread的线程退出，可以通过传出参数value_ptr获取线程退出状态，线程退出时系统进行线程使用的资源的回收（如线程内部申请的动态内存、信号量等）。这个函数可以类比进程中使用的wait, waitpid函数。参数说明： thread : 线程号 retval ：线程运行函数的返回值

2.3 互斥锁加锁 pthread_mutex_lock

int pthread_mutex_lock(pthread_mutex_t *mutex); int pthread_mutex_trylock(pthread_mutex_t *mutex);

2.4 互斥锁解锁 pthread_mutex_unlock

int pthread_mutex_unlock(pthread_mutex_t *mutex);

2.5 线程分离 pthread_detach

int pthread_detach(pthread_t thread); 线程分两种：joinable 或者 detach。对于POSIX线程，除非线程是被分离了的，否则在线程退出时，它的资源是不会被释放的，需要调用pthread_join回收。 pthread_detach 函数使子线程主动与主线程断开关系，该子线程结束后其状态不能由其他线程获取，而子线程自身自动释放资源，多用于网络、多线程服务器。参数说明： thread : 线程号

2.6 线程退出 pthread_exit

void pthread_exit(void *retval); 线程可以隐式退出，也可以显式退出。参数说明： retval：只要pthread_join中的第二个参数不是NULL，这个值将被传递给pthread_join函数的第二个参数。

2.7 线程取消 pthread_cancel

int pthread_cancel(pthread_t thread); 功能是给线程发送取消信号，使线程从取消点退出。该函数执行成功也不一定能取消掉指定线程的执行，因为这个函数线程执行函数进到内核时才会被杀掉，如果用户一直运行在用户态不进到内核态也就无法取消线程的执行，此时可以使用 pthread_testcancel函数进到内核从而能够取消掉该线程的执行。线程处于PTHREAD_CANCEL_ENABLE 状态，它就接受取消请求，如果线程处处于 PTHREAD_CANCEL_DISABLE 状态，取消请求就会被保持在挂起状态。默认情况下，线程处于 PTHREAD_CANCEL_ENABLE状态。根据POSIX标准，pthread_join()、pthread_testcancel()、pthread_cond_wait()、 pthread_cond_timedwait()、sem_wait()、sigwait()等函数以及read()、write()等会引起阻塞的系统调用都是Cancelation-point，而其他pthread函数都不会引起Cancelation动作。使用man -7 pthreads 查看取消点相关内容。参数说明： thread ：要取消的线程id

2.7.1 线程取消涉及的内部属性

线程取消涉及的函数如下：

设置取消状态： int pthread_setcancelstate(int state, int *oldstate); 设置取消类型 int pthread_setcanceltype(int type, int *oldtype); 当线程将退出作为对取消请求的响应时，取消类型允许线程控制它在什么地方退出，包括： PTHREAD_CANCEL_DEFERRED 时，线程只能在特定的几个取消点上响应取消请求，默认值 PTHREAD_CANCEL_ASYNCHRONOUS 时，线程在任何时候都可以响应取消请求

2.8 线程条件处理发送信号 pthread_cond_signal

pthread_cond_signal函数的作用是发送一个信号给另外一个正在处于阻塞等待状态的线程,使其脱离阻塞状态,继续执行.如果没有线程处在阻塞等待状态, pthread_cond_signal也会成功返回。

使用pthread_cond_signal不会有“惊群现象”产生，他最多只给一个线程发信号。假如有多个线程正在阻塞等待着这个条件变量的话，那么是根据各等待线程优先级的高低确定哪个线程接收到信号开始继续执行。如果各线程优先级相同，则根据等待时间的长短来确定哪个线程获得信号。

在LinuxThreads或者NPTL里面有两个队列，分别是cond_wait队列和mutex_lock队列， cond_signal只是让线程从cond_wait队列移到mutex_lock队列，而不用返回到用户空间，不会有性能的损耗。

该函数对应pthread_cond_wait, 同类型函数 pthread_cond_broadcast。

int pthread_cond_signal(pthread_cond_t *cond); 参数说明： cond ：线程条件变量

2.9 线程条件处理广播信号 pthread_cond_broadcast

int pthread_cond_broadcast(pthread_cond_t *cond); 参数说明： cond ：线程条件变量

2.10 线程等待条件变量 pthread_cond_wait

int pthread_cond_wait(pthread_cond_t *restrict cond, pthread_mutex_t *restrict mutex); 线程等待处于挂起状态。为什么需要mutex见参考中列出的内容。参数说明： cond ：线程条件变量 mutex ：互斥锁超时等待条件变量函数： int pthread_cond_timedwait(pthread_cond_t *restrict cond, pthread_mutex_t *restrict mutex, const struct timespec *restrict abstime); abstime 参数需要注意： POSIX提供了多种时钟类型,其中包括以下两种: CLOCK_REALTIME： Systemwide realtime clock. 系统范围内的实时时钟,是个软件时钟, 可以通过命令等方式修改该系统时间. CLOCK_MONOTONIC：Represents monotonic time. Cannot be set. 表示单调时间，为系统起机时到现在的时间,不能被设置跟修改. pthread_cond_timedwait()在没有设置条件变量属性的时候,默认用的是CLOCK_REALTIME软件时间,因此在极端情况下会出现实际等待的时间与设置的超时时间不同。

2.11 pthread_key_create

分配用于标识进程中线程特定数据的键。进程中并不能创建无限个的pthread_key_t变量。Linux中可以通过PTHREAD_KEY_MAX（定义于limits.h文件中）或者系统调用sysconf(_SC_THREAD_KEYS_MAX) 来确定当前系统最多支持多少个键。Linux中默认是1024个键。

查看pthread_key_create.c 源码实现文件，可以更好的理解 pthread_key_create 函数，里面用了个for循环，实际内部创建的是一个数组。

int pthread_key_create(pthread_key_t *key, void (*destructor)(void*)); 参数说明： key ：线程特定数据的标识键 (*destructor)(void*) ：线程结束时，系统将调用这个函数来释放绑定在这个键上的内存对应的销毁函数 int pthread_key_delete (pthread_key_t key);

2.12 设置线程数据共享 pthread_setspecific

设置变量value 与 pthread_key_t 类型的变量key进行关联。 pthread_getpecific和pthread_setspecific提供了在同一个线程中不同函数间共享数据即线程存储的一种方法。 c++11 提供了一个新的数据类型 thread_local，该数据类型的变量在每个线程中都是一个副本，进行修改等处理不影响其他线程内保存的该变量的值。

int pthread_setspecific(pthread_key_t key, const void *value);

2.13 读取线程数据共享 pthread_getspecific

void *pthread_getspecific(pthread_key_t key);

2.14 pthread_once

只执行1次的设置函数

int pthread_once(pthread_once_t *once_control, void (*init_routine)(void)); pthread_once_t once_control = PTHREAD_ONCE_INIT;

2.15 线程读写锁

读锁： int pthread_rwlock_rdlock(pthread_rwlock_t *rwlock); int pthread_rwlock_tryrdlock(pthread_rwlock_t *rwlock); 写锁： int pthread_rwlock_trywrlock(pthread_rwlock_t *rwlock); int pthread_rwlock_wrlock(pthread_rwlock_t *rwlock); 解锁： int pthread_rwlock_unlock(pthread_rwlock_t *rwlock);

3. 参考

POSIX 线程详解（经典必看） https://www.cnblogs.com/sunminmin/p/4479952.html

GLIBC中NPTL线程实现代码阅读 https://blog.csdn.net/whuzm08/article/details/53537631

pthread_cond_wait 为什么需要传递 mutex 参数？ https://www.zhihu.com/question/24116967

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