Lua 5.2.2 Broken Threading System - multithreading

Ey,
I'm making threading system in lua and I have a crash when I resume a lot threads...
I'm noob in C Lua and I don't really know what is the good or bad what I'm doing in the next codes...
I flagged crash places with "//maybe here"
#include<cstdio>
#include<ala_lua.h>
extern"C"{
void __stdcall Sleep(unsigned long);
};
int main(){
lua_State*L=luaL_newstate();
luaL_openlibs(L);
luaA_libs(L);
lua_State*T=lua_newthread(L);
luaL_loadfile(T,"c:/clua.lua");
co_resume(T);
do{
Sleep(1);
}while(co_update());
lua_close(L);
gets(new char[1]);
return 0;
};
#ifndef ALA_LUA_H
#define ALA_LUA_H
#include<ala_lua_co.h>
void luaA_libs(lua_State*);
static int luaA_sleep(lua_State*handle){
co_sleep(handle,lua_tointeger(handle,1));
return 0;
};
static int luaA_rthread(lua_State*handle){
if(lua_isthread(handle,1)){
co_resume(lua_tothread(handle,1));
};
return 0;
};
static int luaA_mthread(lua_State*handle){
if(lua_isfunction(handle,1)){
lua_State*thread=lua_newthread(handle);
lua_pushvalue(handle,1);
lua_xmove(handle,thread,1);
return 1;
};
return 0;
};
void luaA_libs(lua_State*handle){
lua_register(handle,"mthread",luaA_mthread);
lua_register(handle,"rthread",luaA_rthread);
lua_register(handle,"sleep",luaA_sleep);
};
#endif
#ifndef ALA_LUA_CO_H
#define ALA_LUA_CO_H
#include<vector>
#include<time.h>
#include<stdio.h>
#include<cstring>
#include<lua.hpp>
std::vector<lua_State*>co_threads;
std::vector<time_t*>co_threads_delay;
static const size_t CO_NPOS=-1;
bool co_update();
size_t co_new(lua_State*);
size_t co_get(lua_State*);
void co_resume(lua_State*);
void co_report(lua_State*);
void co_rem(lua_State*,size_t);
void co_sleep(lua_State*,time_t);
void co_remifdead(lua_State*,size_t);
void co_remifdead(lua_State*handle,size_t index=CO_NPOS){
switch(lua_status(handle)){
case LUA_OK:{
if(lua_gettop(handle)==0)co_rem(handle,index);
return;
};
case LUA_ERRRUN:{
co_rem(handle,index);
return;
};
};
};
void co_rem(lua_State*handle,size_t index=CO_NPOS){
if(index==CO_NPOS){
index=co_get(handle);
};
if(index!=CO_NPOS){
co_threads.erase(co_threads.begin()+index);
delete[]co_threads_delay[index];
co_threads_delay.erase(co_threads_delay.begin()+index);
};
};
bool co_update(){
if(co_threads.empty())return false;
size_t i=co_threads.size();
while(i>0){
--i;
lua_State*handle=co_threads[i];
if(lua_status(handle)==LUA_YIELD){
if(*co_threads_delay[i]<=clock()){
lua_resume(handle,NULL,0);//here maybe
co_remifdead(handle,i);
};
}else{
co_remifdead(handle,i);
};
};
return!co_threads.empty();
};
void co_resume(lua_State*handle){
switch(lua_status(handle)){
case LUA_YIELD:{
lua_resume(handle,NULL,0);
co_remifdead(handle);
return;
};
case LUA_OK:{
if(lua_gettop(handle)!=0){
size_t index=co_new(handle);
lua_resume(handle,NULL,0);//here maybe
co_remifdead(handle,index);
return;
}else{return;};
};
default:{return;};
};
};
void co_sleep(lua_State*handle,time_t slp){
size_t index=co_get(handle);
if(index!=CO_NPOS){
*co_threads_delay[index]=slp+clock();
lua_yield(co_threads[index],0);
};
};
void co_report(lua_State*handle){
if(lua_status(handle)==LUA_ERRRUN){
const char*error=lua_tostring(handle,-1);
#ifdef ALA_LUA_ERRLOG
FILE*file=fopen(ALA_LUA_ERRLOG,"a");
fputs(error,file);
fclose(file);
#else
puts(error);
#endif
lua_pop(handle,-1);
};
};
size_t co_get(lua_State*handle){
if(co_threads.empty())return CO_NPOS;
const size_t l=co_threads.size();
for(size_t i=0;i<l;++i){
if(co_threads[i]==handle)return i;
};
return CO_NPOS;
};
size_t co_new(lua_State*handle){
if(lua_status(handle)==LUA_OK&&lua_gettop(handle)!=0){
time_t*tm=new time_t[1];
co_threads.push_back(handle);
co_threads_delay.push_back(tm);
return co_threads.size()-1;
};
return CO_NPOS;
};
#endif

Lua has no built-in support for multithreading at the OS level. Lua threads are coroutines, not OS threads. You cannot use Lua threads of the same mother state in different OS threads. You can use separate Lua states in different OS threads but any data exchange between two states must be managed manually.
On the other hand, you can build Lua to support OS-level multithreading by defining lock and unlock macros but these will be called every time the app goes into and leaves Lua.

Related

Wait for thread queue to be empty

I am new to C++ and multithreading applications. I want to process a long list of data (potentially several thousands of entries) by dividing its entries among a few threads. I have retrieved a ThreadPool class and a Queue class from the web (it is my first time tackling the subject). I construct the threads and populate the queue in the following way (definitions at the end of the post):
ThreadPool *pool = new ThreadPool(8);
std::vector<std::function<void(int)>> *caller =
new std::vector<std::function<void(int)>>;
for (size_t i = 0; i < Nentries; ++i)
{
caller->push_back(
[=](int j){func(entries[i], j);});
pool->PushTask((*caller)[i]);
}
delete pool;
The problem is that only a number of entries equaling the number of created threads are processed, as if the program does not wait for the queue to be empty. Indeed, if I put
while (pool->GetWorkQueueLength()) {}
just before the pool destructor, the whole list is correctly processed. However, I am afraid I am consuming too many resources by using a while loop. Moreover, I have not found anyone doing anything like it, so I think this is the wrong approach and the classes I use have some error. Can anyone find the error (if present) or suggest another solution?
Here are the classes I use. I suppose the problem is in the implementation of the destructor, but I am not sure.
SynchronizeQueue.hh
#ifndef SYNCQUEUE_H
#define SYNCQUEUE_H
#include <list>
#include <mutex>
#include <condition_variable>
template<typename T>
class SynchronizedQueue
{
public:
SynchronizedQueue();
void Put(T const & data);
T Get();
size_t Size();
private:
SynchronizedQueue(SynchronizedQueue const &)=delete;
SynchronizedQueue & operator=(SynchronizedQueue const &)=delete;
std::list<T> queue;
std::mutex mut;
std::condition_variable condvar;
};
template<typename T>
SynchronizedQueue<T>::SynchronizedQueue()
{}
template<typename T>
void SynchronizedQueue<T>::Put(T const & data)
{
std::unique_lock<std::mutex> lck(mut);
queue.push_back(data);
condvar.notify_one();
}
template<typename T>
T SynchronizedQueue<T>::Get()
{
std::unique_lock<std::mutex> lck(mut);
while (queue.empty())
{
condvar.wait(lck);
}
T result = queue.front();
queue.pop_front();
return result;
}
template<typename T>
size_t SynchronizedQueue<T>::Size()
{
std::unique_lock<std::mutex> lck(mut);
return queue.size();
}
#endif
ThreadPool.hh
#ifndef THREADPOOL_H
#define THREADPOOL_H
#include "SynchronizedQueue.hh"
#include <atomic>
#include <functional>
#include <mutex>
#include <thread>
#include <vector>
class ThreadPool
{
public:
ThreadPool(int nThreads = 0);
virtual ~ThreadPool();
void PushTask(std::function<void(int)> func);
size_t GetWorkQueueLength();
private:
void WorkerThread(int i);
std::atomic<bool> done;
unsigned int threadCount;
SynchronizedQueue<std::function<void(int)>> workQueue;
std::vector<std::thread> threads;
};
#endif
ThreadPool.cc
#include "ThreadPool.hh"
#include "SynchronizedQueue.hh"
void doNothing(int i)
{}
ThreadPool::ThreadPool(int nThreads)
: done(false)
{
if (nThreads <= 0)
{
threadCount = std::thread::hardware_concurrency();
}
else
{
threadCount = nThreads;
}
for (unsigned int i = 0; i < threadCount; ++i)
{
threads.push_back(std::thread(&ThreadPool::WorkerThread, this, i));
}
}
ThreadPool::~ThreadPool()
{
done = true;
for (unsigned int i = 0; i < threadCount; ++i)
{
PushTask(&doNothing);
}
for (auto& th : threads)
{
if (th.joinable())
{
th.join();
}
}
}
void ThreadPool::PushTask(std::function<void(int)> func)
{
workQueue.Put(func);
}
void ThreadPool::WorkerThread(int i)
{
while (!done)
{
workQueue.Get()(i);
}
}
size_t ThreadPool::GetWorkQueueLength()
{
return workQueue.Size();
}
You can push tasks saying "done" instead of setting "done" via atomic variable.
So that each thread will exit by itself when seeing "done" task, and no earlier. In destructor you only need to push these tasks and join threads. This is called "poison pill".
Alternatively, if you insist on your current design with done variable, you can wait on the same condition you already have:
std::unique_lock<std::mutex> lck(mut);
while (!queue.empty())
{
condvar.wait(lck);
}
But then you'll need to change your notify_one to notify_all, and this may be sub-optimal.
I want to process a long list of data (potentially several thousands of entries) by dividing its entries among a few threads.
You can do that with parallel algorithms, like tbb::parallel_for:
#include <tbb/parallel_for.h>
#include <vector>
void func(int entry);
int main () {
std::vector<int> entries(1000000);
tbb::parallel_for(size_t{0}, entries.size(), [&](size_t i) { func(entries[i]); });
}
If you need sequential thread ids, you can do:
void func(int element, int thread_id);
template<class C>
inline auto make_range(C& c) -> decltype(tbb::blocked_range<decltype(c.begin())>(c.begin(), c.end())) {
return tbb::blocked_range<decltype(c.begin())>(c.begin(), c.end());
}
int main () {
std::vector<int> entries(1000000);
std::atomic<int> thread_counter{0};
tbb::parallel_for(make_range(entries), [&](auto sub_range) {
static thread_local int const thread_id = thread_counter.fetch_add(1, std::memory_order_relaxed);
for(auto& element : sub_range)
func(element, thread_id);
});
}
Alternatively, there is std::this_thread::get_id.

How to prematurely kill std::async threads before they are finished *without* using a std::atomic_bool?

I have a function that takes a callback, and used it to do work on 10 separate threads. However, it is often the case that not all of the work is needed. For example, if the desired result is obtained on the third thread, it should stop all work being done on of the remaining alive threads.
This answer here suggests that it is not possible unless you have the callback functions take an additional std::atomic_bool argument, that signals whether the function should terminate prematurely.
This solution does not work for me. The workers are spun up inside a base class, and the whole point of this base class is to abstract away details of multithreading. How can I do this? I am anticipating that I will have to ditch std::async for something more involved.
#include <iostream>
#include <future>
#include <vector>
class ABC{
public:
std::vector<std::future<int> > m_results;
ABC() {};
~ABC(){};
virtual int callback(int a) = 0;
void doStuffWithCallBack();
};
void ABC::doStuffWithCallBack(){
// start working
for(int i = 0; i < 10; ++i)
m_results.push_back(std::async(&ABC::callback, this, i));
// analyze results and cancel all threads when you get the 1
for(int j = 0; j < 10; ++j){
double foo = m_results[j].get();
if ( foo == 1){
break; // but threads continue running
}
}
std::cout << m_results[9].get() << " <- this shouldn't have ever been computed\n";
}
class Derived : public ABC {
public:
Derived() : ABC() {};
~Derived() {};
int callback(int a){
std::cout << a << "!\n";
if (a == 3)
return 1;
else
return 0;
};
};
int main(int argc, char **argv)
{
Derived myObj;
myObj.doStuffWithCallBack();
return 0;
}
I'll just say that this should probably not be a part of a 'normal' program, since it could leak resources and/or leave your program in an unstable state, but in the interest of science...
If you have control of the thread loop, and you don't mind using platform features, you could inject an exception into the thread. With posix you can use signals for this, on Windows you would have to use SetThreadContext(). Though the exception will generally unwind the stack and call destructors, your thread may be in a system call or other 'non-exception safe place' when the exception occurs.
Disclaimer: I only have Linux at the moment, so I did not test the Windows code.
#if defined(_WIN32)
# define ITS_WINDOWS
#else
# define ITS_POSIX
#endif
#if defined(ITS_POSIX)
#include <signal.h>
#endif
void throw_exception() throw(std::string())
{
throw std::string();
}
void init_exceptions()
{
volatile int i = 0;
if (i)
throw_exception();
}
bool abort_thread(std::thread &t)
{
#if defined(ITS_WINDOWS)
bool bSuccess = false;
HANDLE h = t.native_handle();
if (INVALID_HANDLE_VALUE == h)
return false;
if (INFINITE == SuspendThread(h))
return false;
CONTEXT ctx;
ctx.ContextFlags = CONTEXT_CONTROL;
if (GetThreadContext(h, &ctx))
{
#if defined( _WIN64 )
ctx.Rip = (DWORD)(DWORD_PTR)throw_exception;
#else
ctx.Eip = (DWORD)(DWORD_PTR)throw_exception;
#endif
bSuccess = SetThreadContext(h, &ctx) ? true : false;
}
ResumeThread(h);
return bSuccess;
#elif defined(ITS_POSIX)
pthread_kill(t.native_handle(), SIGUSR2);
#endif
return false;
}
#if defined(ITS_POSIX)
void worker_thread_sig(int sig)
{
if(SIGUSR2 == sig)
throw std::string();
}
#endif
void init_threads()
{
#if defined(ITS_POSIX)
struct sigaction sa;
sigemptyset(&sa.sa_mask);
sa.sa_flags = 0;
sa.sa_handler = worker_thread_sig;
sigaction(SIGUSR2, &sa, 0);
#endif
}
class tracker
{
public:
tracker() { printf("tracker()\n"); }
~tracker() { printf("~tracker()\n"); }
};
int main(int argc, char *argv[])
{
init_threads();
printf("main: starting thread...\n");
std::thread t([]()
{
try
{
tracker a;
init_exceptions();
printf("thread: started...\n");
std::this_thread::sleep_for(std::chrono::minutes(1000));
printf("thread: stopping...\n");
}
catch(std::string s)
{
printf("thread: exception caught...\n");
}
});
printf("main: sleeping...\n");
std::this_thread::sleep_for(std::chrono::seconds(2));
printf("main: aborting...\n");
abort_thread(t);
printf("main: joining...\n");
t.join();
printf("main: exiting...\n");
return 0;
}
Output:
main: starting thread...
main: sleeping...
tracker()
thread: started...
main: aborting...
main: joining...
~tracker()
thread: exception caught...
main: exiting...

thread safe boost intrusive list is slow

I wrapped a boost intrusive list with mutex to make it thread safe, to be used as a producer/consumer queue.
But on windows (MSVC 14) it's really slow, after profiling, 95% of time is spent idle, mainly on push() and waint_and_pop() methods.
I only have 1 producer and 2 producer/consumer threads.
Any suggestions to make this faster?
#ifndef INTRUSIVE_CONCURRENT_QUEUE_HPP
#define INTRUSIVE_CONCURRENT_QUEUE_HPP
#include <thread>
#include <mutex>
#include <condition_variable>
#include <boost/intrusive/list.hpp>
using namespace boost::intrusive;
template<typename Data>
class intrusive_concurrent_queue
{
protected:
list<Data, constant_time_size<false> > the_queue;
mutable std::mutex the_mutex;
std::condition_variable the_condition_variable;
public:
void push(Data * data)
{
std::unique_lock<std::mutex> lock(the_mutex);
the_queue.push_back(*data);
lock.unlock();
the_condition_variable.notify_one();
}
bool empty() const
{
std::unique_lock<std::mutex> lock(the_mutex);
return the_queue.empty();
}
size_t unsafe_size() const
{
return the_queue.size();
}
size_t size() const
{
std::unique_lock<std::mutex> lock(the_mutex);
return the_queue.size();
}
Data* try_pop()
{
Data* popped_ptr;
std::unique_lock<std::mutex> lock(the_mutex);
if(the_queue.empty())
{
return nullptr;
}
popped_ptr= & the_queue.front();
the_queue.pop_front();
return popped_ptr;
}
Data* wait_and_pop(const bool & exernal_stop = false)
{
Data* popped_ptr;
std::unique_lock<std::mutex> lock(the_mutex);
the_condition_variable.wait(lock,[&]{ return ! ( the_queue.empty() | exernal_stop ) ; });
if ( exernal_stop){
return nullptr;
}
popped_ptr=&the_queue.front();
the_queue.pop_front();
return popped_ptr;
}
intrusive_concurrent_queue<Data> & operator=(intrusive_concurrent_queue<Data>&& origin)
{
this->the_queue = std::move(the_queue);
return *this;
}
};
#endif // !INTRUSIVE_CONCURRENT_QUEUE_HPP
Try removing the lock from the methods and lock the whole data structure when you do things with it.

Implementing boost::barrier in C++11

I've been trying to get a project rid of every boost reference and switch to pure C++11.
At one point, thread workers are created which wait for a barrier to give the 'go' command, do the work (spread through the N threads) and synchronize when all of them finish. The basic idea is that the main loop gives the go order (boost::barrier .wait()) and waits for the result with the same function.
I had implemented in a different project a custom made Barrier based on the Boost version and everything worked perfectly. Implementation is as follows:
Barrier.h:
class Barrier {
public:
Barrier(unsigned int n);
void Wait(void);
private:
std::mutex counterMutex;
std::mutex waitMutex;
unsigned int expectedN;
unsigned int currentN;
};
Barrier.cpp
Barrier::Barrier(unsigned int n) {
expectedN = n;
currentN = expectedN;
}
void Barrier::Wait(void) {
counterMutex.lock();
// If we're the first thread, we want an extra lock at our disposal
if (currentN == expectedN) {
waitMutex.lock();
}
// Decrease thread counter
--currentN;
if (currentN == 0) {
currentN = expectedN;
waitMutex.unlock();
currentN = expectedN;
counterMutex.unlock();
} else {
counterMutex.unlock();
waitMutex.lock();
waitMutex.unlock();
}
}
This code has been used on iOS and Android's NDK without any problems, but when trying it on a Visual Studio 2013 project it seems only a thread which locked a mutex can unlock it (assertion: unlock of unowned mutex).
Is there any non-spinning (blocking, such as this one) version of barrier that I can use that works for C++11? I've only been able to find barriers which used busy-waiting which is something I would like to prevent (unless there is really no reason for it).
class Barrier {
public:
explicit Barrier(std::size_t iCount) :
mThreshold(iCount),
mCount(iCount),
mGeneration(0) {
}
void Wait() {
std::unique_lock<std::mutex> lLock{mMutex};
auto lGen = mGeneration;
if (!--mCount) {
mGeneration++;
mCount = mThreshold;
mCond.notify_all();
} else {
mCond.wait(lLock, [this, lGen] { return lGen != mGeneration; });
}
}
private:
std::mutex mMutex;
std::condition_variable mCond;
std::size_t mThreshold;
std::size_t mCount;
std::size_t mGeneration;
};
Use a std::condition_variable instead of a std::mutex to block all threads until the last one reaches the barrier.
class Barrier
{
private:
std::mutex _mutex;
std::condition_variable _cv;
std::size_t _count;
public:
explicit Barrier(std::size_t count) : _count(count) { }
void Wait()
{
std::unique_lock<std::mutex> lock(_mutex);
if (--_count == 0) {
_cv.notify_all();
} else {
_cv.wait(lock, [this] { return _count == 0; });
}
}
};
Here's my version of the accepted answer above with Auto reset behavior for repetitive use; this was achieved by counting up and down alternately.
/**
* #brief Represents a CPU thread barrier
* #note The barrier automatically resets after all threads are synced
*/
class Barrier
{
private:
std::mutex m_mutex;
std::condition_variable m_cv;
size_t m_count;
const size_t m_initial;
enum State : unsigned char {
Up, Down
};
State m_state;
public:
explicit Barrier(std::size_t count) : m_count{ count }, m_initial{ count }, m_state{ State::Down } { }
/// Blocks until all N threads reach here
void Sync()
{
std::unique_lock<std::mutex> lock{ m_mutex };
if (m_state == State::Down)
{
// Counting down the number of syncing threads
if (--m_count == 0) {
m_state = State::Up;
m_cv.notify_all();
}
else {
m_cv.wait(lock, [this] { return m_state == State::Up; });
}
}
else // (m_state == State::Up)
{
// Counting back up for Auto reset
if (++m_count == m_initial) {
m_state = State::Down;
m_cv.notify_all();
}
else {
m_cv.wait(lock, [this] { return m_state == State::Down; });
}
}
}
};
Seem all above answers don't work in the case the barrier is placed too near
Example: Each thread run the while loop look like this:
while (true)
{
threadBarrier->Synch();
// do heavy computation
threadBarrier->Synch();
// small external calculations like timing, loop count, etc, ...
}
And here is the solution using STL:
class ThreadBarrier
{
public:
int m_threadCount = 0;
int m_currentThreadCount = 0;
std::mutex m_mutex;
std::condition_variable m_cv;
public:
inline ThreadBarrier(int threadCount)
{
m_threadCount = threadCount;
};
public:
inline void Synch()
{
bool wait = false;
m_mutex.lock();
m_currentThreadCount = (m_currentThreadCount + 1) % m_threadCount;
wait = (m_currentThreadCount != 0);
m_mutex.unlock();
if (wait)
{
std::unique_lock<std::mutex> lk(m_mutex);
m_cv.wait(lk);
}
else
{
m_cv.notify_all();
}
};
};
And the solution for Windows:
class ThreadBarrier
{
public:
SYNCHRONIZATION_BARRIER m_barrier;
public:
inline ThreadBarrier(int threadCount)
{
InitializeSynchronizationBarrier(
&m_barrier,
threadCount,
8000);
};
public:
inline void Synch()
{
EnterSynchronizationBarrier(
&m_barrier,
0);
};
};

Differences between POSIX threads on OSX and LINUX?

Can anyone shed light on the reason that when the below code is compiled and run on OSX the 'bartender' thread skips through the sem_wait() in what seems like a random manner and yet when compiled and run on a Linux machine the sem_wait() holds the thread until the relative call to sem_post() is made, as would be expected?
I am currently learning not only POSIX threads but concurrency as a whole so absoutely any comments, tips and insights are warmly welcomed...
Thanks in advance.
#include <stdio.h>
#include <stdlib.h>
#include <semaphore.h>
#include <fcntl.h>
#include <unistd.h>
#include <pthread.h>
#include <errno.h>
//using namespace std;
#define NSTUDENTS 30
#define MAX_SERVINGS 100
void* student(void* ptr);
void get_serving(int id);
void drink_and_think();
void* bartender(void* ptr);
void refill_barrel();
// This shared variable gives the number of servings currently in the barrel
int servings = 10;
// Define here your semaphores and any other shared data
sem_t *mutex_stu;
sem_t *mutex_bar;
int main() {
static const char *semname1 = "Semaphore1";
static const char *semname2 = "Semaphore2";
pthread_t tid;
mutex_stu = sem_open(semname1, O_CREAT, 0777, 0);
if (mutex_stu == SEM_FAILED)
{
fprintf(stderr, "%s\n", "ERROR creating semaphore semname1");
exit(EXIT_FAILURE);
}
mutex_bar = sem_open(semname2, O_CREAT, 0777, 1);
if (mutex_bar == SEM_FAILED)
{
fprintf(stderr, "%s\n", "ERROR creating semaphore semname2");
exit(EXIT_FAILURE);
}
pthread_create(&tid, NULL, bartender, &tid);
for(int i=0; i < NSTUDENTS; ++i) {
pthread_create(&tid, NULL, student, &tid);
}
pthread_join(tid, NULL);
sem_unlink(semname1);
sem_unlink(semname2);
printf("Exiting the program...\n");
}
//Called by a student process. Do not modify this.
void drink_and_think() {
// Sleep time in milliseconds
int st = rand() % 10;
sleep(st);
}
// Called by a student process. Do not modify this.
void get_serving(int id) {
if (servings > 0) {
servings -= 1;
} else {
servings = 0;
}
printf("ID %d got a serving. %d left\n", id, servings);
}
// Called by the bartender process.
void refill_barrel()
{
servings = 1 + rand() % 10;
printf("Barrel refilled up to -> %d\n", servings);
}
//-- Implement a synchronized version of the student
void* student(void* ptr) {
int id = *(int*)ptr;
printf("Started student %d\n", id);
while(1) {
sem_wait(mutex_stu);
if(servings > 0) {
get_serving(id);
} else {
sem_post(mutex_bar);
continue;
}
sem_post(mutex_stu);
drink_and_think();
}
return NULL;
}
//-- Implement a synchronized version of the bartender
void* bartender(void* ptr) {
int id = *(int*)ptr;
printf("Started bartender %d\n", id);
//sleep(5);
while(1) {
sem_wait(mutex_bar);
if(servings <= 0) {
refill_barrel();
} else {
printf("Bar skipped sem_wait()!\n");
}
sem_post(mutex_stu);
}
return NULL;
}
The first time you run the program, you're creating named semaphores with initial values, but since your threads never exit (they're infinite loops), you never get to the sem_unlink calls to delete those semaphores. If you kill the program (with ctrl-C or any other way), the semaphores will still exist in whatever state they are in. So if you run the program again, the sem_open calls will succeed (because you don't use O_EXCL), but they won't reset the semaphore value or state, so they might be in some odd state.
So you should make sure to call sem_unlink when the program STARTS, before calling sem_open. Better yet, don't use named semaphores at all -- use sem_init to initialize a couple of unnamed semaphores instead.

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