[SOLVED] pthreads in C. critical section

franmf · 10-24-2012, 06:17 PM

hi people, I need the following

Code:

#include <stdio.h>
#include <stdlib.h>
#include <pthread.h>

int size;
static pthread_mutex_t cs_mutex = PTHREAD_MUTEX_INITIALIZER;

void *f1(void *vargp)
{
	while(1){
		pthread_mutex_lock( &cs_mutex );
		printf("processing size\n");
		size = 0;
		sleep(10);
		size = 10;
		printf("finishing processing size\n");
		pthread_mutex_unlock( &cs_mutex );
		sleep(40);
	}
}

void *f2(void *vargp){

      while(1){

           --ANY INSTRUCCION USING SIZE

          sleep(60);
      }



}
int main(int argc, char *argv[])
{ 
	pthread_create(&tid, NULL, f1, NULL);
	pthread_create(&tid2, NULL, f2, NULL);
	pthread_join(tid,NULL);
	pthread_join(tid2,NULL);

	return 0;
}

What I need is that f2 CAN'T use size when f1 is in the "critical section" or just the same, when f1 is using or modifing size.

How can I manage to reach that?

thank you

dwhitney67 · 10-24-2012, 06:19 PM

Quote:

Originally Posted by franmf

hi people, I need the following

Please include a question with your post!

Also, why would you place a sleep() in-between a Mutex lock() and unlock() calls????

And your second thread loops infinitely without delay. Again, why???

Edit:

Forget my second comment; you edited your post.

franmf · 10-24-2012, 06:20 PM

Quote:

Originally Posted by dwhitney67

Please include a question with your post!

Also, why would you place a sleep() in-between a Mutex lock() and unlock() calls????

And your second thread loops infinitely without delay. Again, why???

sorry, I press enter and posted it unintentionally without finishing the post and you didn't give me time to edit it

dwhitney67 · 10-24-2012, 06:21 PM

Quote:

Originally Posted by franmf

What I need is that f2 CAN'T use size when f1 is in the "critical section" or just the same, when f1 is using or modifing size.

How can I manage to reach that?

thank you

Of course 'size' can be used within the second thread. Why would you think it cannot?

dwhitney67 · 10-24-2012, 06:23 PM

If you want to prevent f2 from using 'size' while f1 is using/modifying it, then use the mutex (in both threads) to prevent concurrent access to it.

franmf · 10-24-2012, 06:23 PM

Quote:

Originally Posted by dwhitney67

Of course 'size' can be used within the second thread. Why would you think it cannot?

what I mean is that when f1() is in the critical section, and modifing size, f2() can't use it. f2() only can use it when f1() has finished the critical section.

I don't know if I'm explaining myself. Sorry for my english

---------- Post added 10-24-12 at 06:24 PM ----------

Quote:

Originally Posted by dwhitney67

If you want to prevent f2 from using 'size' while f1 is using/modifying it, then use the mutex (in both threads) to prevent concurrent access to it.

I need just that, could you tell me how to do it? thank you dwhitney67

dwhitney67 · 10-24-2012, 06:24 PM

Quote:

Originally Posted by franmf

what I mean is that when f1() is in the critical section, and modifing size, f2() can't use it. f2() only can use it when f1() has finished the critical section.

I don't know if I'm explaining myself. Sorry for my english

It took me a moment to understand you; see my follow-up post about protecting access to 'size' using the mutex.

franmf · 10-24-2012, 06:29 PM

could you show me how to do that dwhitney67? I've seen several examples where both pthreads use the same code, but in my case both pthreads use different codes, so I don't know how to do it exactly

dwhitney67 · 10-24-2012, 06:29 PM

For example:

Code:

#include <stdio.h>     /* for printf */
#include <unistd.h>    /* for sleep */
#include <pthread.h>
#include <stdbool.h>   /* for boolean type */

volatile int size = 0;

static pthread_mutex_t cs_mutex = PTHREAD_MUTEX_INITIALIZER;

void* f1(void* arg)
{
    while (true)
    {
        pthread_mutex_lock(&cs_mutex);
        ++size;
        pthread_mutex_unlock(&cs_mutex);

        sleep(1);
    }

    return NULL;
}

void* f2(void* arg)
{
    while (true)
    {
        pthread_mutex_lock(&cs_mutex);
        printf("size is: %d\n", size);
        pthread_mutex_unlock(&cs_mutex);

        sleep(1);
    }

    return NULL;
}

int main()
{
    pthread_t t1, t2;

    pthread_create(&t1, NULL, f1, NULL);
    pthread_create(&t2, NULL, f2, NULL);

    pthread_join(t1, NULL);
    pthread_join(t2, NULL);

    return 0;
}

franmf · 10-24-2012, 06:41 PM

fucking genious dwhiteney67. Tested and running!

thank you very very much, for being so fast answering and doing it properly.

I own you one. hope to see you around here.