Wait notify pthreads unix C++ - c++

Question

I have n threads , each modifying an object O(k) where k can be 0 to n-1.
Now there is a listener thread l, that needs to get an alert when any of the thread,k, has modified its object O(k)
What is the fastest way to implement this situation ?

Answer 1

Use a Posix (or even better, std C++) condition variable, as one commentor already suggested. You can use the related mutex to protect a std::array of flags, one flag per worker thread. When a worker thread modifies its object, it acquires mutex and raises its flag. When the listener thread is notified, it will service the k:th object (corresponding to k:th flag in array) and lower the flag, then release mutex.
Be sure to read examples for condvars so you understand when mutex is automatically acquired/released.
In general, std C++ threading primitives are easier to use, since they use e.g. RAII for automatic unlocking of mutexes etc. Also portable to non-Posix environments. But here is a pthreads example from
http://www.yolinux.com/TUTORIALS/LinuxTutorialPosixThreads.html
#include <stdio.h>
#include <stdlib.h>
#include <pthread.h>
pthread_mutex_t count_mutex = PTHREAD_MUTEX_INITIALIZER;
pthread_cond_t condition_var = PTHREAD_COND_INITIALIZER;
void *functionCount1();
void *functionCount2();
int count = 0;
#define COUNT_DONE 10
#define COUNT_HALT1 3
#define COUNT_HALT2 6
main()
{
pthread_t thread1, thread2;
pthread_create( &thread1, NULL, &functionCount1, NULL);
pthread_create( &thread2, NULL, &functionCount2, NULL);
pthread_join( thread1, NULL);
pthread_join( thread2, NULL);
printf("Final count: %d\n",count);
exit(EXIT_SUCCESS);
}
// Write numbers 1-3 and 8-10 as permitted by functionCount2()
void *functionCount1()
{
for(;;)
{
// Lock mutex and then wait for signal to relase mutex
pthread_mutex_lock( &count_mutex );
// Wait while functionCount2() operates on count
// mutex unlocked if condition varialbe in functionCount2() signaled.
pthread_cond_wait( &condition_var, &count_mutex );
count++;
printf("Counter value functionCount1: %d\n",count);
pthread_mutex_unlock( &count_mutex );
if(count >= COUNT_DONE) return(NULL);
}
}
// Write numbers 4-7
void *functionCount2()
{
for(;;)
{
pthread_mutex_lock( &count_mutex );
if( count < COUNT_HALT1 || count > COUNT_HALT2 )
{
// Condition of if statement has been met.
// Signal to free waiting thread by freeing the mutex.
// Note: functionCount1() is now permitted to modify "count".
pthread_cond_signal( &condition_var );
}
else
{
count++;
printf("Counter value functionCount2: %d\n",count);
}
pthread_mutex_unlock( &count_mutex );
if(count >= COUNT_DONE) return(NULL);
}
}