多线程是一种多任务处理的特殊形式,多任务处理是一种允许您的计算机同时运行两个或多个程序的功能.通常,有两种类型的多任务处理:基于进程和基于线程.
基于进程的多任务处理程序的并发执行.基于线程的多任务处理同时执行同一程序的各个部分.
多线程程序包含两个或多个可以并发运行的部分.这样一个程序的每个部分都称为一个线程,每个线程都定义了一个单独的执行路径.
C ++不包含对多线程应用程序的任何内置支持.相反,它完全依赖于操作系统来提供此功能.
本教程假设您正在使用Linux OS,我们将使用POSIX编写多线程C ++程序. POSIX Threads或Pthreads提供API,可在许多类似Unix的POSIX系统上使用,如FreeBSD,NetBSD,GNU/Linux,Mac OS X和Solaris.
以下例程用于创建POSIX线程 :
#include< pthread.h> pthread_create(thread,attr,start_routine,arg)
这里, pthread_create 创建一个新线程并使其可执行.可以在代码中的任何位置任意调用此例程.以下是参数的说明 :
Sr.No | 参数&说明 |
---|---|
1 | 线程 子程序返回的新线程的不透明唯一标识符. |
2 | attr 可用于设置线程属性的opaque属性对象.您可以指定线程属性对象,或者为默认值指定NULL. |
3 | start_routine 线程创建后将执行的C ++例程. |
4 | arg 可以传递给start_routine的单个参数.它必须通过引用传递为void类型的指针转换.如果不传递参数,则可以使用NULL. |
可能的最大线程数由进程创建是依赖于实现的.一旦创建,线程就是对等的,并且可以创建其他线程.线程之间没有隐含的层次结构或依赖关系.
我们使用以下例程来终止POSIX线程 :
#include< pthread.h> pthread_exit(status)
这里 pthread_exit 用于显式退出线程.通常,在线程完成其工作并且不再需要存在之后调用pthread_exit()例程.
如果main()在它创建的线程之前完成,并退出pthread_exit(),其他线程将继续执行.否则,当main()完成时,它们将自动终止.
示例
这个简单的示例代码创建了5个线程使用pthread_create()例程.每个主题都打印出一个"Hello World!"消息,然后通过调用pthread_exit()终止.
#include <iostream> #include <cstdlib> #include <pthread.h> using namespace std; #define NUM_THREADS 5 void *PrintHello(void *threadid) { long tid; tid = (long)threadid; cout << "Hello World! Thread ID, " << tid << endl; pthread_exit(NULL); } int main () { pthread_t threads[NUM_THREADS]; int rc; int i; for( i = 0; i < NUM_THREADS; i++ ) { cout << "main() : creating thread, " << i << endl; rc = pthread_create(&threads[i], NULL, PrintHello, (void *)i); if (rc) { cout << "Error:unable to create thread," << rc << endl; exit(-1); } } pthread_exit(NULL); }
使用-lpthread库编译以下程序如下 :
$gcc test.cpp -lpthread
现在,执行你的程序,它提供以下输出 :
main() : creating thread, 0 main() : creating thread, 1 main() : creating thread, 2 main() : creating thread, 3 main() : creating thread, 4 Hello World! Thread ID, 0 Hello World! Thread ID, 1 Hello World! Thread ID, 2 Hello World! Thread ID, 3 Hello World! Thread ID, 4
此示例显示如何通过结构传递多个参数.您可以在线程回调中传递任何数据类型,因为它指向void,如以下示例中所述 :
#include <iostream> #include <cstdlib> #include <pthread.h> using namespace std; #define NUM_THREADS 5 struct thread_data { int thread_id; char *message; }; void *PrintHello(void *threadarg) { struct thread_data *my_data; my_data = (struct thread_data *) threadarg; cout << "Thread ID : " << my_data->thread_id ; cout << " Message : " << my_data->message << endl; pthread_exit(NULL); } int main () { pthread_t threads[NUM_THREADS]; struct thread_data td[NUM_THREADS]; int rc; int i; for( i = 0; i < NUM_THREADS; i++ ) { cout <<"main() : creating thread, " << i << endl; td[i].thread_id = i; td[i].message = "This is message"; rc = pthread_create(&threads[i], NULL, PrintHello, (void *)&td[i]); if (rc) { cout << "Error:unable to create thread," << rc << endl; exit(-1); } } pthread_exit(NULL); }
编译并执行上述代码时,会产生以下结果 :
main() : creating thread, 0 main() : creating thread, 1 main() : creating thread, 2 main() : creating thread, 3 main() : creating thread, 4 Thread ID : 3 Message : This is message Thread ID : 2 Message : This is message Thread ID : 0 Message : This is message Thread ID : 1 Message : This is message Thread ID : 4 Message : This is message Joining and Detaching Threads
我们可以使用以下两个例程来加入或分离线程 :
pthread_join(threadid,status) pthread_detach(threadid)
pthread_join()子例程阻塞调用线程,直到指定的'threadid'线程终止.创建线程时,其中一个属性定义它是可连接还是已分离.只能创建作为可连接创建的线程.如果一个线程被创建为已分离,则它永远不会被连接.
此示例演示如何使用Pthread连接例程等待线程完成.
#include <iostream> #include <cstdlib> #include <pthread.h> #include <unistd.h> using namespace std; #define NUM_THREADS 5 void *wait(void *t) { int i; long tid; tid = (long)t; sleep(1); cout << "Sleeping in thread " << endl; cout << "Thread with id : " << tid << " ...exiting " << endl; pthread_exit(NULL); } int main () { int rc; int i; pthread_t threads[NUM_THREADS]; pthread_attr_t attr; void *status; // Initialize and set thread joinable pthread_attr_init(&attr); pthread_attr_setdetachstate(&attr, PTHREAD_CREATE_JOINABLE); for( i = 0; i < NUM_THREADS; i++ ) { cout << "main() : creating thread, " << i << endl; rc = pthread_create(&threads[i], &attr, wait, (void *)i ); if (rc) { cout << "Error:unable to create thread," << rc << endl; exit(-1); } } // free attribute and wait for the other threads pthread_attr_destroy(&attr); for( i = 0; i < NUM_THREADS; i++ ) { rc = pthread_join(threads[i], &status); if (rc) { cout << "Error:unable to join," << rc << endl; exit(-1); } cout << "Main: completed thread id :" << i ; cout << " exiting with status :" << status << endl; } cout << "Main: program exiting." << endl; pthread_exit(NULL); }
编译并执行上述代码时,会产生以下结果 :
main() : creating thread, 0 main() : creating thread, 1 main() : creating thread, 2 main() : creating thread, 3 main() : creating thread, 4 Sleeping in thread Thread with id : 0 .... exiting Sleeping in thread Thread with id : 1 .... exiting Sleeping in thread Thread with id : 2 .... exiting Sleeping in thread Thread with id : 3 .... exiting Sleeping in thread Thread with id : 4 .... exiting Main: completed thread id :0 exiting with status :0 Main: completed thread id :1 exiting with status :0 Main: completed thread id :2 exiting with status :0 Main: completed thread id :3 exiting with status :0 Main: completed thread id :4 exiting with status :0 Main: program exiting.