# Dining Philosophers Problem III

This post ends the mini-series about the dining philosophers problem by Andre Adrian. Today, he applies powerful locks and semaphores.

By Benjamin D. Esham / Wikimedia Commons, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=56559

Here is a quick reminder about where Andre’s analysis ended last time.

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## `std::lock_guard` and Synchronized Output with Resource Hierarchy

```// dp_10.cpp
#include <iostream>
#include <chrono>
#include <mutex>

int myrand(int min, int max) {
return rand()%(max-min)+min;
}

std::mutex mo;

void phil(int ph, std::mutex& ma, std::mutex& mb) {
while(true) {
int duration=myrand(1000, 2000);
{
std::lock_guard<std::mutex> g(mo);
std::cout<<ph<<" thinks "<<duration<<"ms\n";
}

std::lock_guard<std::mutex> ga(ma);
{
std::lock_guard<std::mutex> g(mo);
std::cout<<"\t\t"<<ph<<" got ma\n";
}

std::lock_guard<std::mutex> gb(mb);
{
std::lock_guard<std::mutex> g(mo);
std::cout<<"\t\t"<<ph<<" got mb\n";
}

duration=myrand(1000, 2000);
{
std::lock_guard<std::mutex> g(mo);
std::cout<<"\t\t\t\t"<<ph<<" eats "<<duration<<"ms\n";
}
}
}

int main() {
std::cout<<"dp_10\n";
srand(time(nullptr));

std::mutex m1, m2, m3, m4;

std::thread t1([&] {phil(1, m1, m2);});
std::thread t2([&] {phil(2, m2, m3);});
std::thread t3([&] {phil(3, m3, m4);});
std::thread t4([&] {phil(4, m1, m4);});

t1.join();
t2.join();
t3.join();
t4.join();
}
```

The global mutex mo controls the console output resource. Every cout statement is in its block and the` lock_guard(`) template ensures that console output is not garbled.

## A `std::unique_lock` using deferred locking

C++ offers alternative solutions next to resource hierarchy. We currently have two separate operations to acquire the two resources. If there is only one operation to acquire the two resources, there is no longer the danger of deadlock. The first “all or nothing” solution used` unique_lock()` with `defer_lock`. The actual resource acquire happens in the `lock()` statement. See `dp_12.cpp:`

```int myrand(int min, int max) {
static std::mt19937 rnd(std::time(nullptr));
return std::uniform_int_distribution<>(min,max)(rnd);
}

std::mutex mo;

void phil(int ph, std::mutex& ma, std::mutex& mb) {
while(true) {
int duration=myrand(1000, 2000);
{
std::lock_guard<std::mutex> g(mo);
std::cout<<ph<<" thinks "<<duration<<"ms\n";
}

std::unique_lock<std::mutex> ga(ma, std::defer_lock);
{
std::lock_guard<std::mutex> g(mo);
std::cout<<"\t\t"<<ph<<" got ma\n";
}

std::unique_lock<std::mutex> gb(mb,std::defer_lock);
std::lock(ga, gb);
{
std::lock_guard<std::mutex> g(mo);
std::cout<<"\t\t"<<ph<<" got mb\n";
}

duration=myrand(1000, 2000);
{
std::lock_guard<std::mutex> g(mo);
std::cout<<"\t\t\t\t"<<ph<<" eats "<<duration<<"ms\n";
}
}
}
```

So far, we have generated random numbers using the` rand()` function. This function is not reentrant. Not reentrant means not threadable. This error is fixed with a modified` myrand()` function. The static function object `rnd` is a Mersenne Twister random number generator. With static, we avoid a global function object. Scaling to a value between `min` and `max` is now done with `uniform_int_distribution<>`. Using the library is better than writing your code. Who would have thought simple things like cout output and the random number are so tricky in programs with threads?

## A `std::scoped_lock `with Resource Hierarchy

The second “all or nothing” solution is even more straightforward. The C++17 function` scoped_lock()` allows acquiring multiple resources. This powerful function gives us the shortest dining philosophers solution. See `dp_13.cpp`:

```void phil(int ph, std::mutex& ma, std::mutex& mb) {
while(true) {
int duration=myrand(1000, 2000);
{
std::lock_guard<std::mutex> g(mo);
std::cout<<ph<<" thinks "<<duration<<"ms\n";
}

std::scoped_lock sco(ma, mb);

{
std::lock_guard<std::mutex> g(mo);
std::cout<<"\t\t"<<ph<<" got ma, mb\n";
}

duration=myrand(1000, 2000);
{
std::lock_guard<std::mutex> g(mo);
std::cout<<"\t\t\t\t"<<ph<<" eats "<<duration<<"ms\n";
}
}
}
```

There are more solutions. The original Dijkstra solution used one mutex, one semaphore per philosopher, and one state variable per philosopher. [ref 1971; Dijkstra; EWD310 Hierarchical Ordering of Sequential Processes; https://www.cs.utexas.edu/users/EWD/transcriptions/EWD03xx/EWD310.html]. Andrew S. Tanenbaum provided a C language solution. [ref 2006; Tanenbaum; Operating Systems. Design and Implementation, 3rd edition; chapter 2.3.1]

## The Original Dining Philosopher’s Problem using Semaphores

File `dp_14.cpp` is the Tanenbaum solution rewritten in C++20:

```// dp_14.cpp
#include <iostream>
#include <chrono>
#include <mutex>
#include <semaphore>
#include <random>

int myrand(int min, int max) {
static std::mt19937 rnd(std::time(nullptr));
return std::uniform_int_distribution<>(min,max)(rnd);
}

enum {
N=5,                  // number of philosophers
THINKING=0,           // philosopher is thinking
HUNGRY=1,             // philosopher is trying to get forks
EATING=2,             // philosopher is eating
};

#define LEFT (i+N-1)%N  // number of i's left neighbor
#define RIGHT (i+1)%N   // number of i's right neighbor

int state[N];           // array to keep track of everyone's state
std::mutex mutex_;      // mutual exclusion for critical regions
std::binary_semaphore s[N]{0, 0, 0, 0, 0};
// one semaphore per philosopher

void test(int i)        // i: philosopher number, from 0 to N-1
{
if (state[i] == HUNGRY
&& state[LEFT] != EATING && state[RIGHT] != EATING) {
state[i] = EATING;
s[i].release();
}
}

void take_forks(int i)  // i: philosopher number, from 0 to N-1
{
mutex_.lock();        // enter critical region
state[i] = HUNGRY;    // record fact that philosopher i is hungry
test(i);              // try to acquire 2 forks
mutex_.unlock();      // exit critical region
s[i].acquire();       // block if forks were not acquired
}

void put_forks(int i)   // i: philosopher number, from 0 to N-1
{
mutex_.lock();        // enter critical region
state[i] = THINKING;  // philosopher has finished eating
test(LEFT);           // see if left neighbor can now eat
test(RIGHT);          // see if right neighbor can now eat
mutex_.unlock();      // exit critical region
}

std::mutex mo;

void think(int i) {
int duration = myrand(1000, 2000);
{
std::lock_guard<std::mutex> g(mo);
std::cout<<i<<" thinks "<<duration<<"ms\n";
}
}

void eat(int i) {
int duration = myrand(1000, 2000);
{
std::lock_guard<std::mutex> g(mo);
std::cout<<"\t\t\t\t"<<i<<" eats "<<duration<<"ms\n";
}
}

void philosopher(int i) // i: philosopher number, from 0 to N-1
{
while (true) {        // repeat forever
think(i);           // philosopher is thinking
take_forks(i);      // acquire two forks or block
eat(i);             // yum-yum, spaghetti
put_forks(i);       // put both forks back on table
}
}

int main() {
std::cout<<"dp_14\n";

t0.join();
t1.join();
t2.join();
t3.join();
t4.join();
}
```

By the way, the semaphore is the oldest thread synchronization primitive. Dijkstra defined the P() and V() operation in 1965: “It is the P-operation, which represents the potential delay, viz. when a process initiates a P-operation on a semaphore, that at that moment is = 0, in that case, this P-operation cannot be completed until another process has performed a V-operation on the same semaphore and has given it the value ‘1’.” Today P() is called` release()` and V() is called` acquire()`. [ref 1965; Dijkstra; EWD123 Cooperating sequential processes; https://www.cs.utexas.edu/users/EWD/transcriptions/EWD01xx/EWD123.html]

## A C++20 Compatible Semaphore

You need  a C++20 compiler like LLVM (clang++) version 13.0.0 or newer to compile `dp_14.cpp`. Or you change the` #include <semaphore`> into `#include "semaphore.h"` and provide the following header file. Then a C++11 compiler is sufficient.

```// semaphore.h
#include <mutex>
#include <condition_variable>
#include <limits>

namespace std {
template <std::ptrdiff_t least_max_value
= std::numeric_limits<std::ptrdiff_t>::max()>
class counting_semaphore {
public:
counting_semaphore(std::ptrdiff_t desired) : counter(desired) {}

counting_semaphore(const counting_semaphore&) = delete;
counting_semaphore& operator=(const counting_semaphore&) = delete;

inline void release(ptrdiff_t update = 1) {
std::unique_lock<std::mutex> lock(mutex_);
counter += update;
cv.notify_one();
}

inline void acquire() {
std::unique_lock<std::mutex> lock(mutex_);
while (0 == counter) cv.wait(lock);
--counter;
}

private:
ptrdiff_t counter;
std::mutex mutex_;
std::condition_variable cv;
};

using binary_semaphore = counting_semaphore<1>;
}
```

The C++ semaphore consists of a counter, a `mutex`, and a `condition_variable.` After 14 program versions, we leave this topic. The programs versions 1 to 6 have problems. I presented them to show bad multi-thread programming. Don’t copy that!

## What’s next?

` constexpr` functions have much in common with templates and become more powerful with C++20. I will write about them in my next post.

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