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.
Basically there are the two types of multitasking:
Process-based multitasking handles the programs execution at the same time.
Thread-based multitasking deals with the concurrent execution of peices of the same program.
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.