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In the Linux threads series, we discussed on the ways in which a thread can terminate and how the return status is passed on from the terminating thread to its parent thread. In this article we will throw some light on an important aspect known as thread synchronization. Thread Synchronization ProblemsLets take an example code to study synchronization problems : #include<stdio.h>
#include<string.h>
#include<pthread.h>
#include<stdlib.h>
#include<unistd.h>
pthread_t tid[2];
int counter;
void* doSomeThing(void *arg)
{
unsigned long i = 0;
counter += 1;
printf("\n Job %d started\n", counter);
for(i=0; i<(0xFFFFFFFF);i++);
printf("\n Job %d finished\n", counter);
return NULL;
}
int main(void)
{
int i = 0;
int err;
while(i < 2)
{
err = pthread_create(&(tid[i]), NULL, &doSomeThing, NULL);
if (err != 0)
printf("\ncan't create thread :[%s]", strerror(err));
i++;
}
pthread_join(tid[0], NULL);
pthread_join(tid[1], NULL);
return 0;
}
The above code is a simple one in which two threads(jobs) are created and in the start function of these threads, a counter is maintained through which user gets the logs about job number which is started and when it is completed. The code and the flow looks fine but when we see the output : $ ./tgsthreads Job 1 started Job 2 started Job 2 finished Job 2 finished If you focus on the last two logs, you will see that the log ‘Job 2 finished’ is repeated twice while no log for ‘Job 1 finished’ is seen. Now, if you go back at the code and try to find any logical flaw, you’ll probably not find any flaw easily. But if you’ll have a closer look and visualize the execution of the code, you’ll find that :
The actual problem was the usage of the variable ‘counter’ by second thread when the first thread was using or about to use it. In other words we can say that lack of synchronization between the threads while using the shared resource ‘counter’ caused the problems or in one word we can say that this problem happened due to ‘Synchronization problem’ between two threads. MutexesNow since we have understood the base problem, lets discuss the solution to it. The most popular way of achieving thread synchronization is by using Mutexes. A Mutex is a lock that we set before using a shared resource and release after using it. When the lock is set, no other thread can access the locked region of code. So we see that even if thread 2 is scheduled while thread 1 was not done accessing the shared resource and the code is locked by thread 1 using mutexes then thread 2 cannot even access that region of code. So this ensures a synchronized access of shared resources in the code. Internally it works as follows :
A mutex is initialized and then a lock is achieved by calling the following two functions : int pthread_mutex_init(pthread_mutex_t *restrict mutex, const pthread_mutexattr_t *restrict attr); int pthread_mutex_lock(pthread_mutex_t *mutex); The first function initializes a mutex and through second function any critical region in the code can be locked. The mutex can be unlocked and destroyed by calling following functions : int pthread_mutex_unlock(pthread_mutex_t *mutex); int pthread_mutex_destroy(pthread_mutex_t *mutex); The first function above releases the lock and the second function destroys the lock so that it cannot be used anywhere in future. A Practical ExampleLets see a piece of code where mutexes are used for thread synchronization #include<stdio.h>
#include<string.h>
#include<pthread.h>
#include<stdlib.h>
#include<unistd.h>
pthread_t tid[2];
int counter;
pthread_mutex_t lock;
void* doSomeThing(void *arg)
{
pthread_mutex_lock(&lock);
unsigned long i = 0;
counter += 1;
printf("\n Job %d started\n", counter);
for(i=0; i<(0xFFFFFFFF);i++);
printf("\n Job %d finished\n", counter);
pthread_mutex_unlock(&lock);
return NULL;
}
int main(void)
{
int i = 0;
int err;
if (pthread_mutex_init(&lock, NULL) != 0)
{
printf("\n mutex init failed\n");
return 1;
}
while(i < 2)
{
err = pthread_create(&(tid[i]), NULL, &doSomeThing, NULL);
if (err != 0)
printf("\ncan't create thread :[%s]", strerror(err));
i++;
}
pthread_join(tid[0], NULL);
pthread_join(tid[1], NULL);
pthread_mutex_destroy(&lock);
return 0;
}
In the code above :
Now if we look at the output, we find : $ ./threads Job 1 started Job 1 finished Job 2 started Job 2 finished So we see that this time the start and finish logs of both the jobs were present. So thread synchronization took place by the use of Mutex. If you enjoyed this article, you might also like..
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来自： astrotycoon > 《thread》
