📅 最后修改于: 2020-12-17 05:17:23 🧑 作者: Mango
多线程是多任务的一种特殊形式，多任务是一种功能，它使您的计算机可以同时运行两个或多个程序。通常，多任务处理有两种类型：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一个程序的各个部分的并发执行。
多线程程序包含两个或多个可以同时运行的部分。这种程序的每个部分都称为一个线程，并且每个线程都定义了单独的执行路径。
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。
本教程假定您正在Linux OS上工作，并且我们将使用POSIX编写多线程C++程序。 POSIX线程或Pthread提供了许多类似Unix的POSIX系统上可用的API，例如FreeBSD，NetBSD，GNU / Linux，Mac OS X和Solaris。
以下例程用于创建POSIX线程-
#include
pthread_create (thread, attr, start_routine, arg)
在这里， 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多次调用此例程。这是参数的描述-
| Sr.No | Parameter & Description |
|---|---|
| 1 |
thread An opaque, unique identifier for the new thread returned by the subroutine. |
| 2 |
attr An opaque attribute object that may be used to set thread attributes. You can specify a thread attributes object, or NULL for the default values. |
| 3 |
start_routine The C++ routine that the thread will execute once it is created. |
| 4 |
arg A single argument that may be passed to start_routine. It must be passed by reference as a pointer cast of type void. NULL may be used if no argument is to be passed. |
进程可以创建的最大线程数取决于实现。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性。
我们使用以下例程终止POSIX线程-
#include
pthread_exit (status)
在这里， pthread_exit用于显式退出线程。通常，pthread_exit()例程在线程完成其工作之后被调用，并且不再需要存在。
如果main()在创建线程之前完成，并以pthread_exit()退出，则其他线程将继续执行。否则，它们将在main()完成时自动终止。
例
这个简单的示例代码使用pthread_create()例程创建5个线程。每个线程都打印一个“ Hello World！”。消息，然后以对pthread_exit()的调用终止。
#include
#include
#include
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，如下所示-
$gcc test.cpp -lpthread
现在，执行您的程序，它给出以下输出：
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
本示例说明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
#include
#include
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 < 编译并执行上述代码后，将产生以下结果-
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
我们可以使用以下两个例程来连接或分离线程：
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，其属性之一定义它是可连接的还是可分离的。只有创建为可连接的线程才能被连接。如果线程创建为分离线程，则永远无法加入。
此示例演示如何通过使用Pthread连接例程来等待线程完成。
#include
#include
#include
#include
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后，将产生以下结果-
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.