void cleanUp_delete(void * p)
{
	delete p;
	p = NULL;
	cout << "cleanUp_delete" << endl;
}

void connectionReader(void *thread,void*arg)
{	
	connection* c = (connection*) arg;
	pthread_cleanup_push(cleanUp_delete,(void*)c);
	int bytes;
	char *buffer = new (char [512]);
	pthread_cleanup_push(cleanUp_delete,(void*)buffer);
	cout << "listening for " << c->socket << " " << inet_ntoa(c->addr) << endl;
	while(true) {
		if ((bytes = read(c->socket,(void*)buffer,sizeof(char[512]))) == -1){
			cout << "receive() error" << endl; 
			break;
		} 
		if(bytes == 0) break;
		c->buffer.append(buffer);
		cout << "received " << bytes << " bytes from " << inet_ntoa(c->addr) << endl;
		cout << buffer << endl;
		bzero(buffer,sizeof(char[512]));
	}
	cout << "closing connection to " << c->socket << " " << inet_ntoa(c->addr) << endl;
	pthread_cleanup_pop(1);
	pthread_cleanup_pop(1);
	//c->~connection();
	cout << "closed" << endl;
	pthread_exit(NULL);
	return;
}

i forgot to mention: the handlers should be called when i pthread_cancel the thread running the above while it is waiting for a read().

You need either two different handlers to deal with the different types, or one handler that deals with different types..

struct mydata
{
  connection *c;
  char *buffer;
};

static void mycleanup(void *arg)
{
struct mydata *data=(struct mydata *)arg;
  delete data->c;
  delete data->buffer;
}

....
{
struct mydata data;

     data.c=(connection *)arg;
     data.buffer=new char[...]

     pthread_cleanup_push(mycleanup,&mydata);

     .....

     pthread_cleanup_pop(1);

     ...
}