Thread Join In C

Why thread Join??

As we have seen in the first multithreaded program that we have the while(1) loop in the main function ,that is to ensure that the application/program keeps runnning because if main ends the program ends...

Now if we dun want to waste the time in the main program and only want to run the threads we can use the thread join to achieve this task.

what is a join??

Join means we are telling the thread to join with the other thread that is we are telling that wait for the other thread to finish..

So two threads are included in the join

first thread -to whom we are saying to join

second thread-the thread to join with

suppose there is Thread1 and Thread2,and we require the Thread2 must complete before Thread1 so we have to tell the Thread1 to join with the Thread2..

So in our Example,We have to say main to join with the Thread1 and Thread2 ,that is telling main to wait for both the Threads to complete,

So, By using join we need not to worry about the main,the main will end only after the completion of the both threads..

Funtion prototype:

int pthread_join(pthread_t tid, void **status);

The first argument is the thread ID.

The second argument is a pointer to the data your thread function returned,keep it as NULL for now.

Example:

#include<stdio.h>
#include<stdlib.h>
#include<pthread.h>
void* myfunc1(void *ptr)
{
    char *msg;
    int i=0;
    msg= (char*)ptr;
    while(1)
    {
        printf("In the %s\n",msg);
        sleep(1);
    if(i++> 7)
    break;
    }
printf("Thread1 completed\n");
}
void *myfunc2(void *ptr)
{
    char *msg;
    int i=0;
    msg= (char*)ptr;
    while(1)
    {
        printf("In the %s\n",msg);
        sleep(1);
    if(i++>10 )
    break;
    }
printf("Thread2 completed\n");
}
int main()
{
    pthread_t thread1,thread2;
    int val1,val2;
    int status;
    char *msg1="Thread1";
    char *msg2="Thread2";
    void **ret_val;
    printf("In the main funciton\n");
    val1=pthread_create(&thread1,NULL,myfunc1,(void*)msg1);
    val2=pthread_create(&thread2,NULL,myfunc2,(void*)msg2);
    printf("the value returned by thread1=%d\n",val1);
    printf("the value returned by thread2=%d\n",val2);
    pthread_join(thread1,NULL);//telling main to join with thread1
    pthread_join(thread2,NULL);//telling main to join with thread2
        printf("Now main ends:\n");
    return 0;
}

First Multi Threaded Program in C

Today i have written my first multithreaded program in C on linux,

It is very easy to write multithreded program using the pthread library..

Let us take a look which function we need to create a multithreded program in c...

The function to create the thread:

int pthread_create(pthread_t *tid,const pthread_attr_t *attr,void *(*func)(void *), void *arg)

Note-Each thread will have its own unique thread ID.

The first argument is the variable where its thread ID will be stored. 

The second argument contains attributes describing the thread. Keep this as NULL now,will explain later.

The third argument is a pointer to the function you want to run as a thread.

The final argument is a pointer to data you want to pass to the function.

Having this information in mind you can write a simple multithreaded program in C...

Let us take a look at a example:

#include<stdio.h>
#include<stdlib.h>
#include<pthread.h>
void* myfunc1(void *ptr)
{
char *msg;
msg= (char*)ptr;
printf("%s started\n",msg);
while(1)
{
printf("In the %s\n",msg);
sleep(1);
}
}
void *myfunc2(void *ptr)
{
char *msg;
msg= (char*)ptr;
printf("%s started\n",msg);
while(1)
{
printf("In the %s\n",msg);
sleep(1);
}
}
int main()
{
pthread_t thread1,thread2;
int val1,val2;
char *msg1="Thread1";
char *msg2="Thread2";
printf("In the main funciton\n");
printf("My first thread program\n");
val1= pthread_create(&thread1,NULL,myfunc1,(void*)msg1);
val2= pthread_create(&thread2,NULL,myfunc2,(void*)msg2);
printf("The value returned by thread1=%d\n",val1);
printf("The value returned by thread2=%d\n",val2);
while(1)
{
printf("in the main function\n");
sleep(1);
}
return 0;
}