I've come up with the following blocking queue implementation, with std::vector as the container for the elements stored in the queue and using Boost for threading/synchronization. I also referred to a similar post here.
template<typename T>
class BlockingQueue
{
public:
explicit BlockingQueue(const std::vector<T>& buf):
buffer(buf)
{}
explicit BlockingQueue(): buffer()
{}
void push(const T& elem);
T pop();
~BlockingQueue()
{}
private:
boost::mutex mutex; // mutex variable
boost::condition_variable_any notEmptyCond; // condition variable, to check whether the queue is empty
std::vector<T> buffer;
};
template<typename T>
void BlockingQueue<T>::push(const T& elem)
{
boost::mutex::scoped_lock lock(mutex);
buffer.push_back(elem);
notEmptyCond.notify_one(); // notifies one of the waiting threads which are blocked on the queue
// assert(!buffer.empty());
}
template<typename T>
T BlockingQueue<T>::pop()
{
boost::mutex::scoped_lock lock(mutex);
notEmptyCond.wait(lock,[&](){ return (buffer.size() > 0); }); // waits for the queue to get filled and for a notification, to resume consuming
T elem = buffer.front();
buffer.erase(buffer.begin());
return elem;
}
I've two threads (producer/consumer), one reading strings from a file and populating them into the BlockingQueue, the other to remove the strings from the BlockingQueue and print them. Both of these are initialized from a class whose definition is given below.
class FileProcessor
{
public:
explicit FileProcessor():bqueue(),inFile("random.txt")
{
rt = boost::thread(boost::bind(&FileVerifier::read, this));
pt1 = boost::thread(boost::bind(&FileVerifier::process, this));
}
volatile ~FileProcessor()
{
rt.interrupt();
pt1.interrupt();
rt.join();
pt1.join();
}
/* Read strings from a file, populate them in the blocking-queue */
void read()
{
std::ifstream file(inFile, std::ios_base::in | std::ios_base::binary);
boost::iostreams::filtering_istream in;
if (file.fail()) {
std::cout << "couldn't open the input file.. please check its name and read permissions\n";
return;
}
try {
in.push(file);
for(std::string inputStr; std::getline(in,inputStr);)
{
bqueue.push(inputStr);
std::cout << "inserted " << inputStr << "\n";
}
}
catch(std::exception& e) {
std::cout << "exception occurred while reading file\n" << e.what() << "\n";
}
}
/* Process the elements (dequeue and print) */
void process()
{
while (true)
{
std::string rstr = bqueue.pop();
std::cout << "consumed " << rstr << "\n";
}
}
private:
boost::mutex mutex;
boost::thread rt;
boost::thread pt1;
BlockingQueue<std::string> bqueue;
std::string inFile; // input file name from where the strings are read
};
I observe the following output (only a snapshot included):
Run 1:
inserted AZ
inserted yezjAdCeV
inserted icKU
inserted q
inserted b
inserted DRQL
inserted aaOj
inserted CqlNRv
inserted e
inserted XuDemby
inserted rE
inserted YPk
inserted dLd
inserted xb
inserted bSrZdf
inserted sCQiRna
...
Run 4:
consumed jfRnjSxrw
inserted INdmXSCr
consumed oIDlu
inserted FfXdARGu
consumed tAO
inserted mBq
consumed I
inserted aoXNhP
consumed OOAf
inserted Qoi
consumed wCxJXGWJu
inserted WZGYHluTV
consumed oIFOh
inserted kkIoFF
consumed ecAYyjHh
inserted C
consumed KdrBIixw
inserted Ldeyjtxe
...
My problem : The consumer thread is sometimes given control over the queue's resource (able to dequeue and print) and sometimes it is not. I'm not sure why this happens. Any hints on the queue's design-flaws would be greatly appreciated. Thanks!
Observations:
When the threads aren't initialized from the (FileProcessor) class' ctor, they behave as expected i.e. they access the BlockingQueue resource and do their read/write operations. Please refer to the snippets below for the changes made to have this behavior.
The producer-consumer threads don't take alternative turns, as @n.m noted the producer doesn't yield to the consumer explicitly. Following the above observation, their respective outputs were something like the one given below
inserted DZxcOw
consumed inserted DZxcOw
consumed robECjOp
robECjOp
inserted BaILFsVaA
inserted HomURR
inserted PVjLPb
consumed BaILFsVaA
consumed HomURR
consumed PVjLPb
inserted SHdBVSEyU
consumed SHdBVSEyU
consumed JaEH
inserted JaEH
inserted g
inserted MwEgOVB
inserted qlohoszv
consumed g
consumed MwEgOVB
consumed qlohoszv
consumed AsQgq
inserted AsQgq
inserted tbm
inserted iriADeEL
inserted Zoxs
consumed tbm
Initializing from outside a class ctor.
#include <iostream>
#include <threading/file_processor.h> //has the FileProcessor class declaration
int main()
{
FileProcessor fp; //previously, I had only this statement which called the class constructor, from where the threads were initialized.
boost::thread rt(boost::bind(&FileProcessor::read, &fp));
boost::thread pt1(boost::bind(&FileProcessor::process, &fp));
rt.join();
pt1.join();
return 0;
}
Modified FileProcessor class (removed the thread-initialization from its ctor)
#include <boost/iostreams/filtering_stream.hpp>
#include <threading/blocking_queue.h> //has the BlockingQueue class
using namespace boost::iostreams;
class FileProcessor
{
public:
explicit FileProcessor():bqueue(),inFile("random.txt")
{}
~FileProcessor()
{}
void read()
{
std::ifstream file(inFile, std::ios_base::in | std::ios_base::binary);
filtering_istream in;
if (file.fail()) {
std::cout << "couldn't open the input file.. please check its name and read permissions\n";
return;
}
try {
in.push(file);
for(std::string inputStr; std::getline(in,inputStr);)
{
bqueue.push(inputStr);
std::cout << "inserted " << inputStr << "\n";
}
}
catch(std::exception& e) {
std::cout << "exception occurred while reading file\n" << e.what() << "\n";
}
}
void process()
{
while (true)
{
std::string rstr = bqueue.pop();
std::cout << "consumed " << rstr << "\n";
}
}
private:
BlockingQueue<std::string> bqueue;
std::string inFile; // input file name from where the strings are read
};
Edits:
24 May 2017 : Removed the inaccurate comment "gets the whole file content into a buffer".
Indeed there's no design flaw, just flawed expectations of how threads are scheduled by the OS.
Here's a version that adds a maximum queue "depth" (capacity) and makes push block if the queue reached capacity. The demo then uses a capacity of 1 to show perfect turn-by-turn consumption (of course this is suboptimal, performance-wise).
I've replaced the _any conditions with regular ones, since you can. I've momentarily dropped iostreams use (note that the comment // gets the whole file content into an input stream buffer is completely inaccurate anyway).
#include <boost/thread.hpp>
#include <deque>
#include <fstream>
#include <iostream>
static boost::mutex s_iomutex;
template <typename T> class BlockingQueue {
public:
explicit BlockingQueue(size_t capacity) : _buffer(), _capacity(capacity) {
assert(capacity>0);
}
void push(const T &elem) {
boost::unique_lock<boost::mutex> lock(_mutex);
_pop_event.wait(lock, [&] { return _buffer.size() < _capacity; });
_buffer.push_back(elem);
_push_event.notify_one(); // notifies one of the waiting threads which are blocked on the queue
// assert(!_buffer.empty());
}
T pop() {
boost::unique_lock<boost::mutex> lock(_mutex);
_push_event.wait(lock, [&] { return _buffer.size() > 0; });
T elem = _buffer.front();
_buffer.pop_front();
_pop_event.notify_one();
return elem;
}
private:
boost::mutex _mutex;
boost::condition_variable _push_event, _pop_event;
std::deque<T> _buffer;
size_t _capacity;
};
class FileProcessor {
public:
explicit FileProcessor(size_t capacity = 10) : bqueue(capacity), inFile("random.txt") {}
/* Read strings from a file, populate them in the blocking-queue */
void read() {
try {
std::ifstream file(inFile, std::ios_base::in | std::ios_base::binary);
for (std::string inputStr; std::getline(file, inputStr);) {
bqueue.push(inputStr);
boost::lock_guard<boost::mutex> lock(s_iomutex);
std::cout << "inserted " << inputStr << "\n";
}
} catch (std::exception &e) {
std::cerr << "exception occurred while reading file\n" << e.what() << "\n";
}
}
/* Process the elements (dequeue and print) */
void process() {
while (true) {
std::string rstr = bqueue.pop();
boost::lock_guard<boost::mutex> lock(s_iomutex);
std::cout << "consumed " << rstr << "\n";
}
}
private:
BlockingQueue<std::string> bqueue;
std::string inFile; // input file name from where the strings are read
};
int main() {
FileProcessor fp(1);
boost::thread rt(boost::bind(&FileProcessor::read, &fp));
boost::thread pt1(boost::bind(&FileProcessor::process, &fp));
rt.join();
pt1.interrupt();
pt1.join();
}
Prints
inserted 1 15786
inserted 2 2099
consumed 1 15786
consumed 2 2099
inserted 3 23963
consumed 3 23963
inserted 4 6928
consumed 4 6928
inserted 5 16279
consumed 5 16279
inserted 6 26787
consumed 6 26787
inserted 7 13463
consumed 7 13463
inserted 8 14099
consumed 8 14099
inserted 9 21808
consumed 9 21808
inserted 10 22618
consumed 10 22618
inserted 11 10618
consumed 11 10618
inserted 12 8211
consumed 12 8211
inserted 13 32033
consumed 13 32033
inserted 14 14512
consumed 14 14512
inserted 15 17734
consumed 15 17734
inserted 16 3632
consumed 16 3632
inserted 17 8265
consumed 17 8265
inserted 18 17922
consumed 18 17922
inserted 19 15753
consumed 19 15753
inserted 20 7474
consumed 20 7474
inserted 21 20136
consumed 21 20136
inserted 22 12334
consumed 22 12334
inserted 23 23299
consumed 23 23299
inserted 24 4066
consumed 24 4066
inserted 25 5173
consumed 25 5173
inserted 26 17640
consumed 26 17640
inserted 27 19218
consumed 27 19218
inserted 28 26387
consumed 28 26387
inserted 29 26357
consumed 29 26357
inserted 30 15206
consumed 30 15206
inserted 31 28714
consumed 31 28714
inserted 32 32648
consumed 32 32648
inserted 33 1500
consumed 33 1500
inserted 34 20941
consumed 34 20941
inserted 35 3838
consumed 35 3838
inserted 36 29680
consumed 36 29680
inserted 37 24626
consumed 37 24626
inserted 38 14824
consumed 38 14824
inserted 39 19690
consumed 39 19690
inserted 40 27815
consumed 40 27815
inserted 41 6760
consumed 41 6760
inserted 42 21322
consumed 42 21322
inserted 43 17966
consumed 43 17966
inserted 44 15292
consumed 44 15292
inserted 45 23321
consumed 45 23321
inserted 46 7437
consumed 46 7437
inserted 47 5444
consumed 47 5444
inserted 48 26785
consumed 48 26785
inserted 49 22430
consumed 49 22430
inserted 50 25417
consumed 50 25417
inserted 51 10408
consumed 51 10408
inserted 52 32096
consumed 52 32096
inserted 53 489
consumed 53 489
inserted 54 7083
consumed 54 7083
inserted 55 21555
consumed 55 21555
inserted 56 3759
consumed 56 3759
inserted 57 20811
consumed 57 20811
inserted 58 20176
consumed 58 20176
inserted 59 31305
consumed 59 31305
inserted 60 9894
consumed 60 9894
inserted 61 5515
consumed 61 5515
inserted 62 9978
consumed 62 9978
inserted 63 1981
consumed 63 1981
inserted 64 22286
consumed 64 22286
inserted 65 11081
consumed 65 11081
inserted 66 4392
consumed 66 4392
inserted 67 2252
consumed 67 2252
inserted 68 16714
consumed 68 16714
inserted 69 16003
consumed 69 16003
inserted 70 16695
consumed 70 16695
inserted 71 11288
consumed 71 11288
inserted 72 4788
consumed 72 4788
inserted 73 14454
consumed 73 14454
inserted 74 29920
consumed 74 29920
inserted 75 25154
consumed 75 25154
inserted 76 6206
consumed 76 6206
inserted 77 14444
consumed 77 14444
inserted 78 2921
consumed 78 2921
inserted 79 26908
consumed 79 26908
inserted 80 24148
consumed 80 24148
inserted 81 8487
consumed 81 8487
inserted 82 11371
consumed 82 11371
inserted 83 31047
consumed 83 31047
inserted 84 27749
consumed 84 27749
inserted 85 13548
consumed 85 13548
inserted 86 13807
consumed 86 13807
inserted 87 9411
consumed 87 9411
inserted 88 21999
consumed 88 21999
inserted 89 24386
consumed 89 24386
inserted 90 10190
consumed 90 10190
inserted 91 2472
consumed 91 2472
inserted 92 17149
consumed 92 17149
inserted 93 14288
consumed 93 14288
inserted 94 31625
consumed 94 31625
inserted 95 4732
consumed 95 4732
inserted 96 20273
consumed 96 20273
inserted 97 29036
consumed 97 29036
inserted 98 4425
consumed 98 4425
inserted 99 1563
consumed 99 1563
inserted 100 2796
consumed 100 2796
inserted 101 24374
consumed 101 24374
inserted 102 8151
consumed 102 8151
inserted 103 31361
consumed 103 31361
inserted 104 22466
consumed 104 22466
inserted 105 23365
consumed 105 23365
inserted 106 23762
consumed 106 23762
inserted 107 3616
consumed 107 3616
inserted 108 7711
consumed 108 7711
inserted 109 23178
consumed 109 23178
inserted 110 18791
consumed 110 18791
inserted 111 13371
consumed 111 13371
inserted 112 14553
consumed 112 14553
inserted 113 32026
consumed 113 32026
inserted 114 4567
consumed 114 4567
inserted 115 22178
consumed 115 22178
inserted 116 23947
inserted 117 5928
consumed 116 23947
consumed 117 5928
inserted 118 25606
consumed 118 25606
inserted 119 5141
consumed 119 5141
inserted 120 17681
consumed 120 17681
inserted 121 8024
consumed 121 8024
inserted 122 9094
consumed 122 9094
inserted 123 24878
consumed 123 24878
inserted 124 27800
consumed 124 27800
inserted 125 10225
consumed 125 10225
inserted 126 1157
consumed 126 1157
inserted 127 28217
consumed 127 28217
inserted 128 15144
consumed 128 15144
inserted 129 25692
consumed 129 25692
inserted 130 250
consumed 130 250
inserted 131 17432
consumed 131 17432
inserted 132 10055
consumed 132 10055
inserted 133 24279
consumed 133 24279
inserted 134 9445
consumed 134 9445
inserted 135 4149
consumed 135 4149
inserted 136 23240
consumed 136 23240
inserted 137 23146
consumed 137 23146
inserted 138 8576
consumed 138 8576
inserted 139 11469
consumed 139 11469
inserted 140 27250
consumed 140 27250
inserted 141 12203
consumed 141 12203
inserted 142 21730
consumed 142 21730
inserted 143 30824
consumed 143 30824
inserted 144 11197
consumed 144 11197
inserted 145 11076
consumed 145 11076
inserted 146 6960
consumed 146 6960
inserted 147 7313
consumed 147 7313
inserted 148 16701
consumed 148 16701
inserted 149 21044
consumed 149 21044
inserted 150 9934
consumed 150 9934
inserted 151 18562
consumed 151 18562
inserted 152 3559
consumed 152 3559
inserted 153 5541
consumed 153 5541
inserted 154 16024
consumed 154 16024
inserted 155 9877
consumed 155 9877
inserted 156 18443
consumed 156 18443
inserted 157 6312
consumed 157 6312
inserted 158 24237
consumed 158 24237
inserted 159 27685
consumed 159 27685
inserted 160 6154
consumed 160 6154
inserted 161 32723
consumed 161 32723
inserted 162 8358
consumed 162 8358
inserted 163 5518
consumed 163 5518
inserted 164 15857
consumed 164 15857
inserted 165 26383
consumed 165 26383
inserted 166 13179
consumed 166 13179
inserted 167 29919
consumed 167 29919
inserted 168 5135
consumed 168 5135
inserted 169 7147
consumed 169 7147
inserted 170 4383
consumed 170 4383
inserted 171 13147
consumed 171 13147
inserted 172 15658
consumed 172 15658
inserted 173 18478
consumed 173 18478
inserted 174 29793
consumed 174 29793
inserted 175 16003
consumed 175 16003
inserted 176 12804
consumed 176 12804
inserted 177 25713
consumed 177 25713
inserted 178 28108
consumed 178 28108
inserted 179 8518
consumed 179 8518
inserted 180 9874
consumed 180 9874
inserted 181 30731
consumed 181 30731
inserted 182 15582
consumed 182 15582
inserted 183 12589
consumed 183 12589
inserted 184 15839
consumed 184 15839
inserted 185 19505
consumed 185 19505
inserted 186 20543
consumed 186 20543
inserted 187 6331
consumed 187 6331
inserted 188 25289
consumed 188 25289
inserted 189 14877
consumed 189 14877
inserted 190 25571
consumed 190 25571
inserted 191 10873
consumed 191 10873
inserted 192 13568
consumed 192 13568
inserted 193 16319
consumed 193 16319
inserted 194 28590
consumed 194 28590
inserted 195 22303
consumed 195 22303
inserted 196 20685
consumed 196 20685
inserted 197 1528
consumed 197 1528
inserted 198 5200
consumed 198 5200
inserted 199 25689
consumed 199 25689
inserted 200 25140
consumed 200 25140