- Workers of the World
- Lock the Door Behind You
- On One Condition
- Reading, Writing, and Arithmetic
- Loose Threads
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Reading, Writing, and Arithmetic
Condition variables are useful for synchronization, and mutexes are good for protecting critical sections. Often, however, you’ll have a resource that several threads want to be able to read at once. This isn’t a problem; as many threads as need to can read a memory location at once, without issue. The problem comes when one thread wants to write to that memory location.
If one thread modifies a shared resource, any other threads that are reading that resource will be in an inconsistent state. The locking model required for this scenario has the following properties:
- Anyone can gain read access to a resource, as long as no one has write access.
- Write access can be granted only when no one has read access.
Again, pthreads come with a built-in mechanism for this situation, in the form of read/write locks. These locks have one extra feature that make it easier to acquire write locks: No one is allowed to gain a read lock when someone is waiting for a write lock. This means that a thread trying to gain write access will block any thread trying to gain read access until it has finished.
The example in Listing 4 again has four threads. The first one fills an array with random numbers. While it’s doing this, it has a write lock. Once the thread has finished, it broadcasts on a condition variable, waking up three other threads. These threads then acquire a read lock and generate the total, average, and maximum value for the array.
Listing 4 calculate.c.
#include <stdio.h>
#include <stdlib.h>
#include <pthread.h>
#include <math.h>
#include <unistd.h>
#include <limits.h>
pthread_mutex_t mutex = PTHREAD_MUTEX_INITIALIZER;
pthread_cond_t condition = PTHREAD_COND_INITIALIZER;
pthread_rwlock_t lock;
#define NUMBERS 100
int numbers[NUMBERS];
int averageRun = 0;
int sumRun = 0;
int maxRun = 0;
void * makerandom(void* spare)
{
while(1)
{
pthread_mutex_lock(&mutex);
pthread_rwlock_wrlock(&lock);
printf("Generating numbers...\n");
for(unsigned int i=0 ; i<NUMBERS ; i++)
{
numbers[i] = random();
}
//Unlock the array when we have finished with it
pthread_rwlock_unlock(&lock);
//Set a flag so that the other functions run if they miss the condition variable
maxRun = averageRun = sumRun = 1;
//Wake everyone else up...
pthread_cond_broadcast(&condition);
//...and let them run
pthread_mutex_unlock(&mutex);
sleep(1);
}
}
void * sum(void * spare)
{
while(1)
{
pthread_mutex_lock(&mutex);
//Only wait on the condition variable if there is nothing else to do
if(sumRun == 0)
{
pthread_cond_wait(&condition, &mutex);
}
//Protect the array while we look at it
pthread_rwlock_rdlock(&lock);
if(sumRun == 1)
{
//Unset the flag before releasing the mutex so we don’t miss it being set again
sumRun = 0;
pthread_mutex_unlock(&mutex);
long long total = 0;
for(unsigned int i=0 ; i<NUMBERS ; i++)
{
total += numbers[i];
}
printf("Total = %lld\n",total);
}
else
{
//Release the mutex if we didn’t do anything with the array
pthread_mutex_unlock(&mutex);
}
sleep(1);
//Unlock the array
pthread_rwlock_unlock(&lock);
}
}
void * average(void * spare)
{
while(1)
{
pthread_mutex_lock(&mutex);
if(averageRun == 0)
{
pthread_cond_wait(&condition, &mutex);
}
pthread_rwlock_rdlock(&lock);
if(averageRun == 1)
{
averageRun = 0;
pthread_mutex_unlock(&mutex);
long long total = 0;
for(unsigned int i=0 ; i<NUMBERS ; i++)
{
total += numbers[i];
}
printf("Average = %lld\n",total / NUMBERS);
}
else
{
pthread_mutex_unlock(&mutex);
}
pthread_rwlock_unlock(&lock);
}
}
void * max(void * spare)
{
while(1)
{
pthread_mutex_lock(&mutex);
if(maxRun == 0)
{
pthread_cond_wait(&condition, &mutex);
}
pthread_rwlock_rdlock(&lock);
if(maxRun == 1)
{
maxRun = 0;
pthread_mutex_unlock(&mutex);
int max = -INT_MAX;
for(unsigned int i=0 ; i<NUMBERS ; i++)
{
if(numbers[i] > max)
{
max = numbers[i];
}
}
printf("Max = %d\n",max);
}
else
{
pthread_mutex_unlock(&mutex);
}
pthread_rwlock_unlock(&lock);
}
}
int main(void)
{
pthread_t thread1, thread2;
//Dynamically initialise the rwlock with default attributes
pthread_rwlock_init(&lock, NULL);
pthread_create(&thread1, NULL, sum, (void*)1);
pthread_create(&thread1, NULL, average, (void*)2);
pthread_create(&thread1, NULL, max, (void*)3);
pthread_create(&thread2, NULL, makerandom, NULL);
pthread_join(thread1, NULL);
pthread_join(thread2, NULL);
return 0;
}
This example shows all three forms of synchronization control available to POSIX threads:
- A mutex is used to ensure that only one thread is modifying the run flag at a time.
- A condition variable is used to notify flags that new values are in an array.
- A read/write lock is used to protect the array itself.