C++ Multithreading

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Multithreading is simply a form of multitasking and a multitasking is simply the feature that allows your computer to run two or more programs at the same time.

Types of Multitasking

Basically there are the two types of multitasking:

process-based
thread-based

Process-based Multitasking

Process-based multitasking handles the programs execution at the same time.

Thread-based Multitasking

Thread-based multitasking deals with the concurrent execution of peices of the same program.

C++ Multithreading Example

Here is an example program, illustrating the concept of multithreading in C++ practically.

/* C++ Multithreading - Example Program */

#include<iostream.h>
#include<stdlib.h>
#include<conio.h>
#include<pthread.h>
void *display_hello(void *thread_id)
{
	long t_id;
	t_id=(long)thread_id;
	cout<<"Thread ID, "<<t_id<<"\n";
	pthread_exit(NULL);
}
void main()
{
	clrscr();
	pthread_t threads[5];
	int rc;
	for(int i=0; i<5; i++)
	{
		cout<<"main(): creating the thread, "<<i<<"\n";
		rc=pthread_create(&threads[i], NULL, display_hello, (void *)i);
		if(rc)
		{
			cout<<"Error: unable to create thread, "<<rc<<"\n";
			exit(-1);
		}
	}
	pthread_exit(NULL);
	getch();
}

Here is the sample output of the above C++ program:

main(): creating the thread, 0
main(): creating the thread, 1
main(): creating the thread, 2
main(): creating the thread, 3
main(): creating the thread, 4
Thread ID, 0
Thread ID, 1
Thread ID, 2
Thread ID, 3
Thread ID, 4

As you can see from the above output, this C++ program creates 5 threads with the routine pthread_create() and each thread simply prints a "Hello World." message, and then terminates with call to the pthread_exit() routine.

Let's take another C++ program, also demonstrating multithreading in C++. This example program simply shows how to pass multiple arguments via a structure.

/* C++ Multithreading - Example Program */

#include<iostream.h>
#include<conio.h>
#include<stdlib.h>
#include<pthread.h>
struct THREAD_DATA
{
	int  thrd_id;
	char *msg;
};
void *display_hello(void *thread_arg)
{
	struct THREAD_DATA *mydata;
	mydata=(struct THREAD_DATA *) threadarg;
	cout<<"Thread ID: "<<mydata->thrd_id;
	cout<<"Message: "<<mydata->msg<<"\n";
	pthread_exit(NULL);
}
void main()
{
	clrscr();
	pthread_t threads[5];
	struct THREAD_DATA td[5];
	int rc, i;
	for(i=0; i<5; i++)
	{
		cout<<"main(): creating the thread, "<<i<<"\n";
		td[i].thread_id=i;
		td[i].msg="This is the message";
		rc=pthread_create(&threads[i], NULL, display_hello, (void *)&td[i]);
		if(rc)
		{
			cout<<"Error: unable to create thread, "<<rc<<"\n";
			exit(-1);
		}
	}
	pthread_exit(NULL);
	getch();
}

Below is the sample output of this C++ program:

main(): creating the thread, 0
main(): creating the thread, 1
main(): creating the thread, 2
main(): creating the thread, 3
main(): creating the thread, 4
Thread ID: 3 Message: This is the message
Thread ID: 2 Message: This is the message
Thread ID: 0 Message: This is the message
Thread ID: 1 Message: This is the message
Thread ID: 4 Message: This is the message

Following is one more example program, illustrates how to wait for the thread completions:

/* C++ Multithreading - Example Program */

#include<iostream.h>
#include<conio.h>
#include<stdlib.h>
#include<pthread.h>
#include<dos.h>
#include<unistd.h>
void *wait(void *tm)
{
	int i;
	long tm_id;
	tm_id=(long)tm;
	sleep(1);
	cout<<"Sleeping in the thread\n";
	cout<<"Thread having id: "<<tm_id<<"  ...exiting from this\n";
	pthread_exit(NULL);
}
void main()
{
	clrscr();
	int rc, i;
	pthread_t threads[5];
	pthread_attr_t attr;
	void *status;
	pthread_attr_init(&attr);
	pthread_attr_setdetachstate(&attr, PTHREAD_CREATE_JOINABLE);
	for(i=0;i<5; i++)
	{
		cout<<"main(): creating the thread, "<<i<<"\n";
		rc=pthread_create(&threads[i], NULL, wait, (void *)i);
		if(rc)
		{
			cout<<"Error: unable to create thread, "<<rc<<"\n";
			exit(-1);
		}
	}
	pthread_attr_destroy(&attr);
	for(i=0; i<5; i++)
	{
		rc=pthread_join(threads[i], &status);
		if(rc)
		{
			cout<<"Error: unable to join, "<<rc<<"\n";
			exit(-1);
		}
		cout<<"Main: completed thread id: "<<i;
		cout<<"  exiting with status: "<<status<<"\n";
	}
	cout<<"Main: program exiting.\n";
	pthread_exit(NULL);
	getch();
}

When the above program is compile and executed, it will produce the following output:

main(): creating thread, 0
main(): creating the thread, 1
main(): creating the thread, 2
main(): creating the thread, 3
main(): creating the thread, 4
Sleeping in the thread
Thread having id: 0 .... exiting from this
Sleeping in the thread
Thread having id: 1 .... exiting from this
Sleeping in the thread
Thread having id: 2 .... exiting from this
Sleeping in the thread
Thread having id: 3 .... exiting from this
Sleeping in the thread
Thread having id: 4 .... exiting from this
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.