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dining-philosophers
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24
README.md
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@ -1,3 +1,25 @@
# dining-philosophers
A commented example of solving the dining philosophers multithreading problem.
A commented example of solving the dining philosophers multithreading problem.
## Description of the dining philosophers problem.
The dinner philosophers problem consists of a round table where
N number of philosophers are seated, every philosopher has a fork
on each side that they are supposed to share with their neighbours
to eat a spaghetti that can only eated by the usage of two
forks.
The problem consists on achieving the table to be able to continuously
eat without getting stuck in a deadlock without the chance of philosophers to
talk between themselves.
## Description of the solution.
For the solution I inspired on the Dijkstra's solution but using the OOP nature
of C++ to make it more readable and near to the natural language.
The code is commented in its tricky parts in order to make the solution easier to understand.
[https://en.wikipedia.org/wiki/Dining_philosophers_problem](https://en.wikipedia.org/wiki/Dining_philosophers_problem)

48
include/philosopher.hpp
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@ -17,47 +17,81 @@ typedef std::shared_ptr<ListOfPhilosophers> ListOfPhilosophersPtr;
extern std::mutex changingForkState;
extern std::mutex printing;
/**
* \brief A class representing a single philosopher having his dinner.
*
* Philosophers are seated in a round table.
* In order of being able to eat a philosopher should be able to take
* the two forks on his sides so no other of its neighbour philosophers
* should be eating at this time.
*/
class Philosopher {
private:
//! The position of the philosopher in the table.
int numberOfPhilosopher;
//! What the philosopher is doing.
PhilosopherState state{PhilosopherState::THINKING};
//! The vector of philosophers in the table.
ListOfPhilosophersPtr philosophers;
//! The function to set the state of the philosopher.
void
setState(PhilosopherState state);
//! Method to guess the numberOfPhilosopher of the left neighbour of this philosopher.
size_t
leftPhilosopherNumber(void);
//! Method to guess the numberOfPhilosopher of the right neighbour of this philosopher.
size_t
rightPhilosopherNumber(void);
//! Retrieve the left neighbour.
PhilosopherPtr
leftPhilosopher(void);
//! Retrieve the right neighbour.
PhilosopherPtr
rightPhilosopher(void);
//! Makes the philosopher think for a while.
void
think();
think(void);
//! The philosopher waits until the forks of his neighbours are free and then takes the two forks.
void
takeForks();
takeForks(void);
//! The philosopher eats the spaghetti with the two forks.
void
eat();
eat(void);
//! The philosopher puts the forks in the table to be used by his neighbours.
void
putForks();
putForks(void);
public:
//! Retrieves the philosopher state.
PhilosopherState
getState();
getState(void);
/**
* \brief Philosopher constructor
* @param philosophers The vector of in the table philosophers.
* @param numberOfPhilosopher The position of this philosopher in the table.
*/
Philosopher(ListOfPhilosophersPtr philosophers, int numberOfPhilosopher);
//! Whenever this philosopher was the two forks currently.
std::binary_semaphore hasBothForks{0};
void startThread();
void test();
//! Starts the philosopher dinner.
void startThread(void);
//! Test if the philosopher should be able to take both forks currently.
void test(void);
};

43
src/main.cpp
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@ -14,33 +14,56 @@ findNumberOfPhilosophersInParams(const int argc, char **argv, int *const numberO
int
main(int argc, char **argv) {
ListOfPhilosophersPtr philosophers;
philosophers = ListOfPhilosophersPtr(new ListOfPhilosophers());
// std::shared_ptr<std::vector<std::shared_ptr<Philosopher>>
ListOfPhilosophersPtr philosophers(new ListOfPhilosophers());
// Default value for the number of dining philosophers.
int numberOfPhilosophers = 5;
// Attempts to find a tailored value provided by the user for numberOfPhilosophers.
findNumberOfPhilosophersInParams(argc, argv, &numberOfPhilosophers);
printf("%d philosophers to create.\n", numberOfPhilosophers);
// Pushes a number of philosophers equal to numberOfPhilosophers to the table.
for (int i = 0; i < numberOfPhilosophers; i++) {
printf("Creating philosopher: %d.\n", i);
philosophers->push_back(PhilosopherPtr(new Philosopher(philosophers, i)));
}
// If the jthreads go out of scope is equal than joining, which is undersirable on the
// insides of a for loop if we want they to execute in parallel.
std::vector<std::jthread> threads;
for (unsigned long int i = 0; i < philosophers->size(); i++) {
std::shared_ptr<unsigned long> numberOfPhilosopher(new unsigned long(i));
// A pitfall would be to use here i directly since it would be incremented before
// it is used by jthread, so a pointer is needed.
std::shared_ptr<unsigned long> numberOfPhilosopher(new unsigned long());
*numberOfPhilosopher = i;
threads.push_back(std::jthread([philosophers, numberOfPhilosopher] {
// This starts the dinner for the philosopher *numberOfPhilosopher.
// While philosophers are yet not dinning the state for them is thinking
// anyway, so they have to problem with the neighbours dining already.
(*philosophers)[*numberOfPhilosopher]->startThread();
} ));
}
}
/*
* Checks if the parameter list is greater or equal than 2 and takes the second parameter.-
* The first if ignoring the current executable name.- And with this parameter attempts to
* get a number to be used as the number of philosophers in the table.
*
* In case of failure, numberOfPhilosophers is not changed, else is set to the integer value
* of the parameter.
*/
void
findNumberOfPhilosophersInParams(const int argc, char **argv, int *const numberOfPhilosophers) {
if (argc >= 2) {
const char *numberOfPhilosophersAsString = argv[1];
try {
*numberOfPhilosophers = std::stoi(numberOfPhilosophersAsString, NULL, 10);
} catch (std::exception &ex) {
printf("Unable to read the number of philosophers, continuing\n");
}
// The user did not gave us enough arguments, early return.
if (argc < 2) {
return;
}
// numberOfPhilosophersAsString is now equal to the supposed number the user gave us.
const char *numberOfPhilosophersAsString = argv[1];
try {
// Converts param to number, can throw an exception.
*numberOfPhilosophers = std::stoi(numberOfPhilosophersAsString, NULL, 10);
} catch (std::exception &ex) {
printf("The number of philosophers is not a valid number, continuing\n");
}
}

30
src/philosopher.cpp
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@ -8,56 +8,79 @@ Philosopher::setState(PhilosopherState state) {
}
size_t
Philosopher::leftPhilosopherNumber(void) {
// Circular implementation of the table to the left.
// (0 - 1 + 9) % 9 = 8
// (1 - 1 + 9) % 9 = 0
return (numberOfPhilosopher - 1 + philosophers->size()) % philosophers->size();
}
size_t
Philosopher::rightPhilosopherNumber(void) {
// Circular implementation of the table to the right.
// (8 + 1) % 9 = 0
// (7 + 1) % 9 = 8
return (numberOfPhilosopher + 1) % philosophers->size();
}
PhilosopherPtr
Philosopher::leftPhilosopher(void) {
return (*philosophers)[leftPhilosopherNumber()];
}
PhilosopherPtr
Philosopher::rightPhilosopher(void) {
return (*philosophers)[rightPhilosopherNumber()];
}
void
Philosopher::think() {
// The number of milliseconds thinking.
size_t duration = random(100, 1000);
{
// Holds the lock for printing.
std::lock_guard<std::mutex> lk{printing};
std::cout << "Philosopher " << numberOfPhilosopher << " is thinking for " << duration << "." << std::endl;
}
// Sleeps.
std::this_thread::sleep_for(std::chrono::milliseconds(duration));
}
void
Philosopher::takeForks() {
{
// Holds the lock for state manipulation and read.
std::lock_guard<std::mutex> lk{changingForkState};
// Sets the philosopher to be hungry.
setState(PhilosopherState::HUNGRY);
{
// Holds the lock for printing.
std::lock_guard<std::mutex> lk{printing};
std::cout << "Philosopher " << numberOfPhilosopher << " is hungry." << std::endl;
}
// Attempts to release the binary semaphore for forks locking if still up.
test();
}
// Attempts to adquire the forks lock.
hasBothForks.acquire();
}
void Philosopher::eat() {
// Sets the time eating.
size_t duration = random(100, 1000);
{
// Holds the lock for printing.
std::lock_guard<std::mutex> lk{printing};
std::cout << "Philosopher " << numberOfPhilosopher << " is eating." << std::endl;
}
// Sleeps the time the philosopher last to eat.
std::this_thread::sleep_for(std::chrono::milliseconds(duration));
}
void Philosopher::putForks() {
// Holds the lock for state manipulation and read.
std::lock_guard<std::mutex> lk{changingForkState};
state = PhilosopherState::THINKING;
// Set the philosopher in thinking state.
setState(PhilosopherState::THINKING);
// Attempts to free the lock of neighbours if they are locked because of this philosopher.
leftPhilosopher()->test();
rightPhilosopher()->test();
}
@ -80,15 +103,20 @@ Philosopher::startThread() {
std::cout << "Philosopher " << numberOfPhilosopher << " has sit in the table." << std::endl;
}
while (true) {
// The philosopher starts thinking.
think();
// Then gets hungry and attempts to eat.
takeForks();
// When the two forks are available the philosopher eats.
eat();
// Then the philosopher puts the two forks on the table to be used by the philosopher's neighbours.
putForks();
}
}
void
Philosopher::test() {
// This is self-documenting.
if (state == PhilosopherState::HUNGRY &&
leftPhilosopher()->getState() != PhilosopherState::EATING &&
rightPhilosopher()->getState() != PhilosopherState::EATING) {