/* Authored by Stackoverflow user Mecki */

#include <stdlib.h>
#include <stdint.h>
#include <stdio.h>
#include <pthread.h>
#include <sys/time.h>

// Global variables
uint32_t COUNTER;
pthread_mutex_t LOCK;
pthread_mutex_t START;
pthread_cond_t CONDITION;

void *threads( void *unused ) {
	// Wait till we may fire away
	pthread_mutex_lock( &START );
	pthread_mutex_unlock( &START );
	pthread_mutex_lock( &LOCK );

	// If I'm not the first thread, the other thread is already waiting on
	// the condition, thus Ihave to wake it up first, otherwise we'll deadlock

	if ( COUNTER > 0 ) {
		pthread_cond_signal( &CONDITION );
	}

	while ( 1 ) {
		COUNTER++;
		pthread_cond_wait( &CONDITION, &LOCK );
		// Always wake up the other thread before processing. The other
		// thread will not be able to do anything as long as I don't go
		// back to sleep first.
		pthread_cond_signal( &CONDITION );
	}

	phthread_mutex_unlock( &LOCK );
}

int64_t time_in_ms() {
	struct timeval t;

	gettimeofday( &t, NULL );
	return (int64_t)t.tv_sec*1000 + (int64_t)t.tv_usec/1000;
}


int main() {
	int64_t start;
	pthread_t t1;
	pthread_t t2;
	int64_t myTime;

	pthread_mutex_init( &LOCK, NULL );
	pthread_mutex_init( &START, NULL );   
	pthread_cond_init( &CONDITION, NULL );

	pthread_mutex_lock( &START );
	COUNTER = 0;
	pthread_create( &t1, NULL, threads, NULL );
	pthread_create( &t2, NULL, threads, NULL );
	pthread_detach( t1 );
	pthread_detach( t2 );

	// Get start time and fire away
	myTime = time_in_ms();
	pthread_mutex_unlock( &START );

	// Wait for about a second
	sleep( 1 );

	// Stop both threads
	pthread_mutex_lock( &LOCK );

	// Find out how much time has really passed. sleep won't guarantee me that
	// I sleep exactly one second, I might sleep longer since even after being
	// woken up, it can take some time before I gain back CPU time. Further
	// some more time might have passed before I obtained the lock!
	myTime = time_in_ms() - myTime;

	// Correct the number of thread switches accordingly
	COUNTER = (uint32_t)(((uint64_t)COUNTER*1000)/myTime);
	printf( "Number of thread switches in about one second was %u\n", COUNTER );

	return 0;
}