I'm trying to share a std::map<std::string, std::chrono::system_clock::time_point> map: each string is a hostname identifying a site, and the time_point is the last time a process visited that site.
I was trying with mmap but each process still see its own copy of the map.
Here's my code (I took away all the methods and variables not concerning my problem):
#include <sys/mman.h>
#include <unistd.h>
#include <iostream>
#include <map>
#include <string>
#include <chrono>
typedef std::map<std::string, std::chrono::system_clock::time_point> mymap;
typedef mymap::iterator iter;
typedef mymap* mapPointer;
class MmapManager {
private:
MmapManager() {
frequency = (mapPointer) mmap(NULL, sizeof(frequency), PROT_READ | PROT_WRITE, MAP_SHARED | MAP_ANONYMOUS, -1, 0);
if (frequency == MAP_FAILED) {
std::cout << "mapping failed" << std::endl;
}
};
~MmapManager() {
std::cout << "~MmapManager()" << std::endl;
}
public:
// my class was designed with the singleton pattern
static MmapManager& getInstance() {
static MmapManager instance;
return instance;
}
private:
// pointer to my map
mapPointer frequency;
public:
// check if the process already visited site "host"
bool isHostAlreadyVisited(std::string host) {
return frequency->find(host) != frequency->end();
}
// add new visited site and time of the visit
void addHost(std::string host) {
(*frequency)[host] = std::chrono::system_clock::now();
std::cout << "PROC " << getpid() << " added " << host << std::endl;
}
// get time of the visit for site "host"
std::chrono::system_clock::time_point getElement(std::string host) {
return (*frequency)[host];
}
// print the map
void showMap(void) {
std::cout << "PROC " << getpid() << " prints map keys" << std::endl;
for (auto it = frequency->begin(); it != frequency->end(); ++it) {
std::cout << it->first << std::endl;
}
}
};
int main(void) {
// simulate the processes
for (int i=0; i<5; i++) {
// child process
if (fork() == 0) {
// if child never visited this site...
if (! MmapManager::getInstance().isHostAlreadyVisited("www.google.com")) {
std::cout << "PID " << getpid() << " www.google.com is new" << std::endl;
// ...add it to the map
MmapManager::getInstance().addHost("www.google.com");
}
else {
// if child already visited it, calculate
// how much time passed since last visit
auto now = std::chrono::system_clock::now();
auto before = MmapManager::getInstance().getElement("www.google.com");
std::chrono::duration<double> diff = now-before;
std::cout << "PID " << getpid() << " visited www.google.com " << diff.count() << " seconds ago" << std::endl;
}
MmapManager::getInstance().showMap();
_exit(EXIT_SUCCESS);
}
}
return 0;
}
Here's one of the possible outputs:
PID 12457 www.google.com is new
PID 12459 www.google.com is new
PID 12458 www.google.com is new
PID 12460 www.google.com is new
PID 12461 www.google.com is new
I can't use other external libraries like Boost or use threads: I know they share memory, but the program was designed this way (with child processes doing stuff) and I can't modify it (original code is not mine).
Why does each process still see its own copy of the map?
Edit: I think I did all the things you suggested me:
insertion in map is protected with a lock mechanism (thanks kfsone);
created a custom allocator for string and another for map (thanks Maxim Egorushkin for these two);
map is allocated before forking (thanks Zan Lynx).
The output is not different and map is still not shared:
MmapManager()
printMap
map empty
PID 5085 www.google.com is new
PID 5086 www.google.com is new
PROC 5086 added www.goole.com
PROC 5085 added www.goole.com
PID 5087 www.google.com is new
PROC 5087 added www.goole.com
You suggested me to use Boost but I'd like to use it after my code will work: I'm not reinventing the wheel, just learning the hard way.
Here follows my new code:
#include <sys/mman.h>
#include <unistd.h>
#include <sys/shm.h> /* shmat(), IPC_RMID */
#include <semaphore.h> /* sem_open(), sem_destroy(), sem_wait().. */
#include <fcntl.h> /* O_CREAT, O_EXEC */
#include <stdlib.h>
#include <iostream>
#include <map>
#include <string>
#include <chrono>
#include <cstddef>
#include <vector>
#include <limits>
#include <memory>
template<typename T> class stringAllocator {
public :
typedef T value_type;
typedef size_t size_type;
typedef ptrdiff_t difference_type;
typedef T* pointer;
typedef T const * const_pointer;
typedef T& reference;
typedef T const & const_reference;
template<typename U> struct rebind {
typedef stringAllocator<U> other;
};
pointer address (reference value ) const {
return &value;
}
const_pointer address (const_reference value) const {
return &value;
}
size_type max_size () const throw() {
return std::numeric_limits <size_type>::max() / sizeof(T);
}
stringAllocator () throw () {}
stringAllocator (stringAllocator const &) throw () {}
template <typename U>
stringAllocator(stringAllocator <U> const &) throw () {}
~stringAllocator() throw () {}
pointer allocate (size_type n) {
pointer ptr = (pointer)malloc(n * sizeof(value_type));
return ptr;
}
void deallocate (pointer p, size_type n) {
free(p);
}
void construct (pointer p, const_reference value) {
new(p) T(value);
}
void destroy (pointer p) {
p->~T();
}
};
template <class T1, class T2>
bool operator==(const stringAllocator<T1>&, const stringAllocator<T2>&) throw() {
return true;
}
template <class T1, class T2>
bool operator!=(const stringAllocator<T1>&, const stringAllocator<T2>&) throw() {
return false;
}
typedef std::basic_string<
char,
std::char_traits<char>,
stringAllocator<char>
> myString;
/*************************************** map allocator ****************************************/
template<typename T> class mapAllocator{
public :
typedef T value_type;
typedef value_type* pointer;
typedef const value_type* const_pointer;
typedef value_type& reference;
typedef const value_type& const_reference;
typedef std::size_t size_type;
typedef std::ptrdiff_t difference_type;
template<typename U>
struct rebind {
typedef mapAllocator<U> other;
};
mapAllocator() throw() {}
mapAllocator (mapAllocator const &) throw () {}
~mapAllocator() throw () {}
template<typename U>
mapAllocator(mapAllocator<U> const&) {}
pointer address(reference r) { return &r; }
const_pointer address(const_reference r) { return &r; }
pointer allocate(size_type cnt, typename std::allocator<void>::const_pointer = 0) {
pointer new_memory = reinterpret_cast<pointer>(::operator new(cnt * sizeof (T)));
return new_memory;
}
void deallocate(pointer p, size_type n) {
::operator delete(p);
}
size_type max_size() const {
return std::numeric_limits<size_type>::max() / sizeof(T);
}
void construct(pointer p, const T& t) {
new(p) T(t);
}
void destroy(pointer p) {
p->~T();
}
};
template <class T1, class T2>
bool operator==(const mapAllocator<T1>&, const mapAllocator<T2>&) throw() {
return true;
}
template <class T1, class T2>
bool operator!=(const mapAllocator<T1>&, const mapAllocator<T2>&) throw() {
return false;
}
/*************************************** end map allocator ****************************************/
// class compare for map with std::string as Key
class strless {
public:
bool operator() (const myString first, const myString second ) const {
return first.compare(second) < 0;
}
};
template<typename Key, typename T>
using Map = std::map<
Key, // class Key
T, // class T
strless, // class Compare = std::less<Key>
mapAllocator<std::pair<const Key, T> // class Allocator = std::allocator<std::pair<const Key, T> >
>
>;
// typedef for the actual map I need to share between processes
typedef Map<myString, std::chrono::system_clock::time_point> frequencyMap;
class MmapManager {
private:
MmapManager() {
std::cout << "MmapManager()" << std::endl;
semMmap = sem_open("semaphore", O_CREAT|O_EXCL, 0644, 1);
sem_unlink("semaphore");
};
~MmapManager() {
std::cout << "~MmapManager()" << std::endl;
}
public:
static MmapManager& getInstance() {
static MmapManager instance;
return instance;
}
private:
frequencyMap fmap;
sem_t *semMmap;
public:
void start(void) {}
bool isHostAlreadyVisited(myString host) {
return fmap.find(host) != fmap.end();
}
void addHost(myString host) {
sem_wait(semMmap);
fmap[host] = std::chrono::system_clock::now();
sem_post(semMmap);
std::cout << "PROC " << getpid() << " added " << host << std::endl;
}
// get time of the visit for site "host"
std::chrono::system_clock::time_point getElement(myString host) {
return fmap[host];
}
void printMap(void) {
std::cout << "printMap" << std::endl;
if (!fmap.empty()) {
for (auto it : fmap) {
std::cout << it.first << ' ';
}
std::cout << std::endl;
} else {
std::cout << "map empty" << std::endl;
}
}
};
int main(void) {
MmapManager::getInstance().start();
for (int i=0; i<3; i++) {
if (fork() == 0) {
if (!MmapManager::getInstance().isHostAlreadyVisited("www.google.com")) {
std::cout << "PID " << getpid() << " www.google.com is new" << std::endl;
MmapManager::getInstance().addHost("www.goole.com");
}
else {
// if child already visited it, calculate
// how much time passed since last visit
auto now = std::chrono::system_clock::now();
auto before = MmapManager::getInstance().getElement("www.google.com");
std::chrono::duration<double> diff = now-before;
std::cout << "PID " << getpid() << " visited www.google.com " << diff.count() << " seconds ago" << std::endl;
}
_exit(EXIT_SUCCESS);
}
}
MmapManager::getInstance().printMap();
return 0;
}
This does not work because although you placed the container object into the shared memory, the elements are still allocated from the heap and thus they are not accessible by other processes.
You need a custom allocator that allocates elements in the shared memory. See Creating maps in shared memory for how it is done.
Note that the string class you use must also allocate memory from the shared memory.
In other words, you cannot have pointers to heap memory in the shared memory, because heap memory is not shared between processes. std classes have an allocator template argument, the default one allocates memory from the heap. This needs to be changed to a shared memory allocator to be able to share such objects via shared memory.
Another reason your code doesn't work is that you only create the maps after you called fork().
If you want your MAP_SHARED|MAP_ANONYMOUS map to be seen by all the children then you have to call mmap() before forking.
Related
class test
{
void thread1()
{
int i = 0;
while(true){
for(unsigned int k = 0;k < mLD.size(); k++ )
{
mLD[k] = i++;
}
}
}
void thread2()
{
std::cout << "thread2 address : " << &mLD << "\n";
C();
}
void B()
{
std::cout << "B address : " << &mLD << "\n";
for(unsigned int k = 0;k < mLD.size(); k++ )
{
if(mLD[k]<=25)
{
}
}
}
void C()
{
B();
std::cout << "C address : " << &mLD << "\n";
double distance = mLD[0]; // <---- segmetation fault
}
std::array<double, 360> mLD;
};
cout result --->
thread2 address : 0x7e807660
B address : 0x7e807660
C address : 0x1010160 (sometimes 0x7e807660 )
Why mLD's address changed ....?
even i change std::array to std::array<std::atomic<double>360>, the result is the same.
Most probably, the object you referred is destroyed at the point of call to C, which points to a synchronization issue. You need to extend the lifetime of the object referred by thread(s), until the threads done executing their routine. To accomplish this, you can have something like this;
#include <thread>
#include <array>
#include <iostream>
struct foo{
void callback1(){
for(auto & elem: storage){
elem += 5;
}
}
void callback2(){
for(const auto & elem: storage){
std::cout << elem << std::endl;
}
}
std::array<double, 300> storage;
};
int main(void){
foo f;
std::thread t1 {[&f](){f.callback1();}};
std::thread t2 {[&f](){f.callback2();}};
// wait until both threads are done executing their routines
t1.join();
t2.join();
return 0;
}
The instance of foo, f lives in scope of main() function, so its' lifetime is defined by from the line it defined to end of the main's scope. By joining both threads, we block main from proceeding further until both threads are done executing their callback functions, hence the lifetime of f extended until callbacks are done.
The second issue is, the code needs synchronization primitives, because storage variable is shared between two independent execution paths. The final code with proper synchronization can look like this;
#include <thread>
#include <array>
#include <iostream>
#include <mutex>
struct foo{
void callback1(){
// RAII style lock, which invokes .lock() upon construction, and .unlock() upon destruction
// automatically.
std::unique_lock<std::mutex> lock(mtx);
for(auto & elem: storage){
elem += 5;
}
}
void callback2(){
std::unique_lock<std::mutex> lock(mtx);
for(const auto & elem: storage){
std::cout << elem << std::endl;
}
}
std::array<double, 300> storage;
// non-reentrant mutex
mutable std::mutex mtx;
};
int main(void){
foo f;
std::thread t1 {[&f](){f.callback1();}};
std::thread t2 {[&f](){f.callback2();}};
// wait until both threads are done executing their routines
t1.join();
t2.join();
return 0;
}
I am trying to construct a std::thread with a member function that takes no arguments and returns void. I can't figure out any syntax that works - the compiler complains no matter what. What is the correct way to implement spawn() so that it returns a std::thread that executes test()?
#include <thread>
class blub {
void test() {
}
public:
std::thread spawn() {
return { test };
}
};
#include <thread>
#include <iostream>
class bar {
public:
void foo() {
std::cout << "hello from member function" << std::endl;
}
};
int main()
{
std::thread t(&bar::foo, bar());
t.join();
}
EDIT:
Accounting your edit, you have to do it like this:
std::thread spawn() {
return std::thread(&blub::test, this);
}
UPDATE: I want to explain some more points, some of them have also been discussed in the comments.
The syntax described above is defined in terms of the INVOKE definition (ยง20.8.2.1):
Define INVOKE (f, t1, t2, ..., tN) as follows:
(t1.*f)(t2, ..., tN) when f is a pointer to a member function of a class T and t1 is an object of type T or a reference to an object of
type T or a reference to an object of a type derived from T;
((*t1).*f)(t2, ..., tN) when f is a pointer to a member function of a class T and t1 is not one of the types described in the previous
item;
t1.*f when N == 1 and f is a pointer to member data of a class T and t 1 is an object of type T or a
reference to an object of type T or a reference to an object of a
type derived from T;
(*t1).*f when N == 1 and f is a pointer to member data of a class T and t 1 is not one of the types described in the previous item;
f(t1, t2, ..., tN) in all other cases.
Another general fact which I want to point out is that by default the thread constructor will copy all arguments passed to it. The reason for this is that the arguments may need to outlive the calling thread, copying the arguments guarantees that. Instead, if you want to really pass a reference, you can use a std::reference_wrapper created by std::ref.
std::thread (foo, std::ref(arg1));
By doing this, you are promising that you will take care of guaranteeing that the arguments will still exist when the thread operates on them.
Note that all the things mentioned above can also be applied to std::async and std::bind.
Since you are using C++11, lambda-expression is a nice&clean solution.
class blub {
void test() {}
public:
std::thread spawn() {
return std::thread( [this] { this->test(); } );
}
};
since this-> can be omitted, it could be shorten to:
std::thread( [this] { test(); } )
or just (deprecated)
std::thread( [=] { test(); } )
Here is a complete example
#include <thread>
#include <iostream>
class Wrapper {
public:
void member1() {
std::cout << "i am member1" << std::endl;
}
void member2(const char *arg1, unsigned arg2) {
std::cout << "i am member2 and my first arg is (" << arg1 << ") and second arg is (" << arg2 << ")" << std::endl;
}
std::thread member1Thread() {
return std::thread([=] { member1(); });
}
std::thread member2Thread(const char *arg1, unsigned arg2) {
return std::thread([=] { member2(arg1, arg2); });
}
};
int main(int argc, char **argv) {
Wrapper *w = new Wrapper();
std::thread tw1 = w->member1Thread();
std::thread tw2 = w->member2Thread("hello", 100);
tw1.join();
tw2.join();
return 0;
}
Compiling with g++ produces the following result
g++ -Wall -std=c++11 hello.cc -o hello -pthread
i am member1
i am member2 and my first arg is (hello) and second arg is (100)
#hop5 and #RnMss suggested to use C++11 lambdas, but if you deal with pointers, you can use them directly:
#include <thread>
#include <iostream>
class CFoo {
public:
int m_i = 0;
void bar() {
++m_i;
}
};
int main() {
CFoo foo;
std::thread t1(&CFoo::bar, &foo);
t1.join();
std::thread t2(&CFoo::bar, &foo);
t2.join();
std::cout << foo.m_i << std::endl;
return 0;
}
outputs
2
Rewritten sample from this answer would be then:
#include <thread>
#include <iostream>
class Wrapper {
public:
void member1() {
std::cout << "i am member1" << std::endl;
}
void member2(const char *arg1, unsigned arg2) {
std::cout << "i am member2 and my first arg is (" << arg1 << ") and second arg is (" << arg2 << ")" << std::endl;
}
std::thread member1Thread() {
return std::thread(&Wrapper::member1, this);
}
std::thread member2Thread(const char *arg1, unsigned arg2) {
return std::thread(&Wrapper::member2, this, arg1, arg2);
}
};
int main() {
Wrapper *w = new Wrapper();
std::thread tw1 = w->member1Thread();
tw1.join();
std::thread tw2 = w->member2Thread("hello", 100);
tw2.join();
return 0;
}
Some users have already given their answer and explained it very well.
I would like to add few more things related to thread.
How to work with functor and thread.
Please refer to below example.
The thread will make its own copy of the object while passing the object.
#include<thread>
#include<Windows.h>
#include<iostream>
using namespace std;
class CB
{
public:
CB()
{
cout << "this=" << this << endl;
}
void operator()();
};
void CB::operator()()
{
cout << "this=" << this << endl;
for (int i = 0; i < 5; i++)
{
cout << "CB()=" << i << endl;
Sleep(1000);
}
}
void main()
{
CB obj; // please note the address of obj.
thread t(obj); // here obj will be passed by value
//i.e. thread will make it own local copy of it.
// we can confirm it by matching the address of
//object printed in the constructor
// and address of the obj printed in the function
t.join();
}
Another way of achieving the same thing is like:
void main()
{
thread t((CB()));
t.join();
}
But if you want to pass the object by reference then use the below syntax:
void main()
{
CB obj;
//thread t(obj);
thread t(std::ref(obj));
t.join();
}
now I'm Having problem in repeating the loop after it finished doing the first and i want to try it again without exiting the program? I've been using while loop to do it but still no joy. so i decided to do the if statement. But the Array only accept 4 strings then it exit. Any one who can help? TIA.
#include "stdafx.h"
#include <iostream>
#include <string>
#include <algorithm>
#include <vector>
using namespace std;
template <typename T>
void GetContents(T& Input);
template <typename T>
void DisplayContents(const T& Input);
int main()
{
int PASS = 0;
// To Display the unsorted and sorted Book Titles
std::vector<std::string> books;
GetContents(books);
std::cout << "\nMy original library (number of books: " << books.size() << "):\n\n";
DisplayContents(books);
std::sort(books.begin(), books.end());
std::cout << "\nMy sorted library (number of books: " << books.size() << "):\n\n";
DisplayContents(books);
std::cout << "Press 1 to try again, else to quit: ";
std::cin >> PASS;
std::cout << "\n";
if (PASS == 1)
{
GetContents(books);
}
else
{
return 0;
}
// to input All book titles
template <typename T>
void GetContents(T& Input)
{
const int MAX = 5;
string bookName;
std::cout << "Enter a Book Titles:\n> ";
for (int i = 0; i < MAX; i++)
{
std::getline(std::cin, bookName);
Input.push_back(bookName);
std::cout <<">";
}
}
//Display All input book titles
template <typename T>
void DisplayContents(const T& Input)
{
for (auto iElement : Input)
{
std::cout << iElement << '\n';
}
std::cout << '\n';
system("pause");
}
Well, I have created a program which picks up the input signal from serial input. I can successfully receive the data transmitted from the device through UART. I want to terminate the thread after achieving certain conditions( such as receiving more than 5 bytes, etc.) I think the problem is how to terminate the thread in Qt correctly, but I couldn't find the way. The program seems falls into deadlock after calling the exec() in the sub function. Can anyone help with that problem? Thank you very much!
Here's my header file:
#ifndef SERIALTHREAD
#define SERIALTHREAD
#include <QtSerialPort/QSerialPort>
#include <QDebug>
#include <QString>
#include <QThread>
#include <QtCore>
#include <iostream>
#include <fstream>
class SerialControlThread : public QThread
{
Q_OBJECT
public:
explicit SerialControlThread(QString ComPort,QObject *parent = 0);
~SerialControlThread(); // Destructor
bool openSerialPort();
void closeSerialPort();
void run();
bool TelltoExit();
void StarttoRun();
private:
int DataCount;
QString ComPortNumber;
QSerialPort *serial;
int* VoltageStorage; // Total 3 channels, each channel takes 10 data
unsigned int Channel_A[10]; // Channel_A is for Phase Tx s
int DataCountIndexA; // This is how many data has been sent to the buffer;
int SentDataCount;
unsigned char StoreDataBuffer[2];
unsigned char TotalDataCounter;
std::ofstream write;
signals:
void BufferisFull(int*);
void TimeToQuit();
public slots:
private slots:
void readData();
void handleError(QSerialPort::SerialPortError error);
};
#endif // SERIALTHREAD
This is the.cpp
#include "serialcontrol.h"
#include <iostream>
SerialControlThread::SerialControlThread(QString ComPort,QObject *parent) :
QThread(parent),ComPortNumber(ComPort)
{
DataCountIndexA=0;
DataCount=0;
serial = new QSerialPort(this);
connect(this,SIGNAL(TimeToQuit()),this,SLOT(quit()));\
connect(serial, SIGNAL(readyRead()), this, SLOT(readData()));
connect(serial, SIGNAL(error(QSerialPort::SerialPortError)), this,
SLOT(handleError(QSerialPort::SerialPortError)));
for (int i=0;i<10;i++)
Channel_A[i]=0;
}
SerialControlThread::~SerialControlThread()
{
this->closeSerialPort();
delete serial;
}
bool SerialControlThread::openSerialPort()
{
// std::cout << "Hey I am in serial function" << std::endl;
serial->setPortName(ComPortNumber) ;
serial->setBaudRate(QSerialPort::Baud9600); //This can be set through menu in the future
serial->setDataBits(QSerialPort::Data8); // A packets contains 8 bits ( 3 for signature bits)
serial->setParity(QSerialPort::NoParity);
serial->setStopBits(QSerialPort::OneStop);
serial->setFlowControl(QSerialPort::NoFlowControl);
if (!(serial->open(QIODevice::ReadWrite))) {
return false; // return false when the device can't be opened
}else
{
return true;} // return true when the device is avalaible
}
void SerialControlThread::closeSerialPort()
{
if (serial->isOpen())
serial->close();
}
void SerialControlThread::handleError(QSerialPort::SerialPortError error)
{
}
void SerialControlThread::readData()
{
QByteArray data=serial->read(100);
const char *TempChar=data.data();
std::cout << TempChar << std::endl;
DataCount++;
if(DataCount>=4)
{
std::cout << "I am bigger than 4" << std::endl;
this->quit();
}
}
}
void SerialControlThread::run()
{
}
bool SerialControlThread::TelltoExit()
{
}
void SerialControlThread::StarttoRun()
{
// Sending the msp430 S to activate the following sequence
const char *temp="S";
serial->write(temp);
serial->waitForBytesWritten(30000);
this->exec();
}
This is the main.cpp
#include <QCoreApplication>
#include <QtSerialPort/QSerialPortInfo>
#include <QList>
#include <iostream>
#include <QString>
#include <QDebug>
#include <QSerialPort>
#include "serialcontrol.h"
using namespace std;
int main(int argc, char *argv[])
{
QCoreApplication a(argc, argv);
int AvailablePorts=QSerialPortInfo::availablePorts().count();
QList<QSerialPortInfo> SerialObject=QSerialPortInfo::availablePorts();
cout << "There are total: " << SerialObject.count() << " available ports " << endl << endl;
QString description;
for (int i=0;i<AvailablePorts;i++)
{
cout << "The " << i+1 << " com port is :";
qDebug() << SerialObject[i].portName();
qDebug() << "Description : " << SerialObject[i].description();
qDebug() << "Manufacturer: " << SerialObject[i].manufacturer();
cout << endl;
}
SerialControlThread *RunThread=new SerialControlThread(SerialObject[0].portName(),&a);
cout << RunThread->openSerialPort() << endl;
RunThread->StarttoRun();
cout << "I am out of here" << endl;
delete RunThread;
return a.exec();
}
I wish to close the thread( back to the main function) when the buffer has received more than 4 data, but it doesn't.
It is my output
There are total: 1 available ports
The 1 com port is :"COM8"
Description : "MSP430 Application UART"
Manufacturer: "Texas Instruments"
1
0
1
2
3
I am bigger than 4
4
I am bigger than 4
5
I am bigger than 4
6
I am bigger than 4
7
I am bigger than 4
8
I am bigger than 4
9
I am bigger than 4
Apparently, the program gets stuck in a loop. I have tried some solutions, but none of these worked.
StartToRun calls QThread::exec in the wrong thread: you call it in the main thread, but it's supposed to be called in the thread itself - from within run().
Alas, SerialControlThread doesn't have to be a thread. Making it a thread forces it to be used in a dedicated thread - that should be a choice left to its user. Perhaps the thread would be shared among other serial controllers, or perhaps it'll do just fine in the main thread. Thus, it should be an object that handles serial data, that has a thread-safe interface so that you can move it to another thread if you wish - but would still work fine in the main thread, and thus has to handle data asynchronously without blocking.
Considering whether one needs to control the worker thread's run status so tightly: an idle thread consumes no resources - its event loop is blocked waiting on new events, its stack eventually gets paged out if there's memory pressure. If one intends to "wake" the thread for each operation, there's no need to be explicit about it: the event loop in the thread behaves that way be default and by design: it wakes when there are new events, such as incoming data, otherwise it sleeps. One shouldn't be stopping the thread then.
The example below shows a very minimal implementation. On the whole it's not very useful other than to demonstrate brevity as a contrast to the length of code in the question - in spite of identical limited functionality. Presumably you have a more complex communications protocol that you wish to handle. You may wish to consider the use of QDataStream read transactions to make the reader code more expressive, and using a state machine to represent the protocol.
// https://github.com/KubaO/stackoverflown/tree/master/questions/serial-galore-42241570
#include <QtWidgets>
#include <QtSerialPort>
// See https://stackoverflow.com/q/40382820/1329652
template <typename Fun> void safe(QObject * obj, Fun && fun) {
Q_ASSERT(obj->thread() || qApp && qApp->thread() == QThread::currentThread());
if (Q_LIKELY(obj->thread() == QThread::currentThread() || !obj->thread()))
return fun();
struct Event : public QEvent {
using F = typename std::decay<Fun>::type;
F fun;
Event(F && fun) : QEvent(QEvent::None), fun(std::move(fun)) {}
Event(const F & fun) : QEvent(QEvent::None), fun(fun) {}
~Event() { fun(); }
};
QCoreApplication::postEvent(
obj->thread() ? obj : qApp, new Event(std::forward<Fun>(fun)));
}
class SerialController : public QObject {
Q_OBJECT
QSerialPort m_port{this};
QByteArray m_rxData;
void onError(QSerialPort::SerialPortError error) {
Q_UNUSED(error);
}
void onData(const QByteArray & data) {
m_rxData.append(data);
qDebug() << "Got" << m_rxData.toHex() << "(" << m_rxData.size() << ") - done.";
emit hasReply(m_rxData);
}
void onData() {
if (m_port.bytesAvailable() >= 4)
onData(m_port.readAll());
}
public:
explicit SerialController(const QString & port, QObject * parent = nullptr) :
QObject{parent}
{
m_port.setPortName(port);
connect(&m_port, static_cast<void(QSerialPort::*)(QSerialPort::SerialPortError)>(&QSerialPort::error),
this, &SerialController::onError);
}
~SerialController() { qDebug() << __FUNCTION__; }
bool open() {
m_port.setBaudRate(QSerialPort::Baud9600);
m_port.setDataBits(QSerialPort::Data8);
m_port.setParity(QSerialPort::NoParity);
m_port.setStopBits(QSerialPort::OneStop);
m_port.setFlowControl(QSerialPort::NoFlowControl);
return m_port.open(QIODevice::ReadWrite);
}
/// This method is thread-safe.
void start() {
safe(this, [=]{
m_port.write("S");
qDebug() << "Sent data";
});
}
Q_SIGNAL void hasReply(const QByteArray &);
void injectData(const QByteArray & data) {
onData(data);
}
};
QDebug operator<<(QDebug dbg, const QSerialPortInfo & info) {
dbg << info.portName();
if (!info.description().isEmpty())
dbg << " Description: " << info.description();
if (!info.manufacturer().isEmpty())
dbg << " Manufacturer: " << info.manufacturer();
return dbg;
}
// A thread that starts on construction, and is always safe to destruct.
class RunningThread : public QThread {
Q_OBJECT
using QThread::run; // final
public:
RunningThread(QObject * parent = nullptr) : QThread(parent) { start(); }
~RunningThread() { qDebug() << __FUNCTION__; quit(); wait(); }
};
int main(int argc, char *argv[])
{
QCoreApplication app(argc, argv);
auto const ports = QSerialPortInfo::availablePorts();
if (ports.isEmpty())
qFatal("No serial ports");
int n{};
qDebug() << "Available ports:";
for (auto & port : ports)
qDebug() << "port[" << n++ << "]: " << port;
SerialController ctl{ports.at(5).portName()};
if (!ctl.open())
qFatal("Open Failed");
// Optional: the controller will work fine in the main thread.
if (true) ctl.moveToThread(new RunningThread{&ctl}); // Owns its thread
// Let's pretend we got a reply;
QTimer::singleShot(1000, &ctl, [&ctl]{
ctl.injectData("ABCD");
});
QObject::connect(&ctl, &SerialController::hasReply, ctl.thread(), &QThread::quit);
QObject::connect(&ctl, &SerialController::hasReply, [&]{
qDebug() << "The controller is done, quitting.";
app.quit();
});
ctl.start();
return app.exec();
}
#include "main.moc"
I am trying to change the behavior of a future object based on user input.
#include <iostream>
#include <future>
//=======================================================================================!
struct DoWork
{
DoWork(int cycles, int restTime) : _cycles(cycles), _restTime(restTime), _stop(false)
{
}
void operator () ()
{
for(int i = 0 ; i < _cycles; ++i)
{
std::this_thread::sleep_for(std::chrono::milliseconds(_restTime));
if(_stop)break;
doTask();
}
}
void stop()
{
_stop = true;
}
private:
void doTask()
{
std::cout << "doing task!" << std::endl;
}
private:
int _cycles;
int _restTime;
bool _stop;
};
//=======================================================================================!
int main()
{
DoWork doObj(50, 500);
std::future<int> f = std::async(std::launch::async, doObj);
std::cout << "Should I stop work ?" << std::endl;
std::cout << "('1' = Yes, '2' = no, 'any other' = maybe)" << std::endl;
int answer;
std::cin >> answer;
if(answer == 1) doObj.stop();
std::cout << f.get() << std::endl;
return 0;
}
//=======================================================================================!
However this does not stop the execution of the future object. How do I change the behavior of the doObj after I have created the future object?
You have a few problems. First, your function object doesn't actually return int, so std::async will return a std::future<void>. You can fix this either by actually returning int from DoWork::operator(), or by storing the result from async in a std::future<void> and not trying to print it.
Second, std::async copies its arguments if they aren't in reference wrappers, so the doObj on the stack is not going to be the same instance of DoWork that is being used by the asynchronous thread. You can correct this by passing doObj in a reference wrapper a la std::async(std::launch::async, std::ref(doObj)).
Third, both the main thread and the asynchronous thread are simultaneously accessing DoWork::_stop. This is a data race and means the program has undefined behavior. The fix is to protect accesses to _stop with a std::mutex or to make it a std::atomic.
Altogether, program should look like (Live at Coliru):
#include <iostream>
#include <future>
//=======================================================================================!
struct DoWork
{
DoWork(int cycles, int restTime) : _cycles(cycles), _restTime(restTime), _stop(false)
{
}
int operator () ()
{
for(int i = 0 ; i < _cycles; ++i)
{
std::this_thread::sleep_for(std::chrono::milliseconds(_restTime));
if(_stop) return 42;
doTask();
}
return 13;
}
void stop()
{
_stop = true;
}
private:
void doTask()
{
std::cout << "doing task!" << std::endl;
}
private:
int _cycles;
int _restTime;
std::atomic<bool> _stop;
};
//=======================================================================================!
int main()
{
DoWork doObj(50, 500);
std::future<int> f = std::async(std::launch::async, std::ref(doObj));
std::cout << "Should I stop work ?" << std::endl;
std::cout << "('1' = Yes, '2' = no, 'any other' = maybe)" << std::endl;
int answer;
std::cin >> answer;
if(answer == 1) doObj.stop();
std::cout << f.get() << std::endl;
}
//=======================================================================================!