MESSAGE
| DATE | 2016-12-22 |
| FROM | ruben safir
|
| SUBJECT | Subject: [Learn] Fwd: Re: thread concurancy
|
From learn-bounces-at-nylxs.com Thu Dec 22 11:00:01 2016
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From: ruben safir
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-------- Forwarded Message --------
Subject: Re: thread concurancy
Date: Thu, 22 Dec 2016 12:32:15 +0200
From: Paavo Helde
Newsgroups: comp.lang.c++
References:
On 22.12.2016 6:48, ruben safir wrote:
>
> How is this fix ;)
Yes, this makes a bit more sense, at least the mutex is really used for
mutual exclusion. A couple of remarks:
The functions get_canvas_index() and get_source_index() are lacking the
mutex lock. Any access to data which is concurrently modified needs a
mutex lock.
The function get_canvas() returns a pointer to the shared array without
any synchronization. This means race conditions. Instead, it should wait
on a std::condition_variable until all threads have fulfilled the task;
upon completion, each thread should increment a counter of completed
tasks under another mutex lock and notify the condition variable. (In
this toy example this waiting could be replaced by just joining all the
threads, but in real life you typically don't want to terminate your
service threads after each task.)
The mutex is defined in another place (a global!) than the protected
data. This is insane. The mutex should be together with the protected
data, preferably in its own section of private variables, clearly
documented:
private: // data members protected by medco
std::mutex medco;
char * canvas_index;
char * source_index;
private: // other data
// ...
The thread array t is also global, with no need, no protection, etc. As
soon as you create another instance of PIC it gets garbled.
HTH
Paavo
>
> /*
> * =====================================================================================
> *
> * Filename: test.cpp
> *
> * Description: Threading Experiment
> *
> * Version: 1.0
> * Created: 12/18/2016 12:46:51 PM
> * Revision: none
> * Compiler: gcc
> *
> * Author: Ruben Safir (mn), ruben-at-mrbrklyn.com
> * Company: NYLXS Inc
> *
> * =====================================================================================
> */
>
> #include
> #include
> #include
> std::mutex medco;
> std::mutex master;
>
> namespace testing{
> std::thread t[10];
> class PIC
> {
> public:
> PIC():source_index{&source[0]}
> {
> canvas_index = canvas;
> std::cout << "Testing Source" << std::endl;
> for(int i = 0; i<100; i++)
> {
> std::cout << i << " " << source[i] << std::endl ;
> }
>
> for(int i = 0; i < 10;i++)
> {
> t[i] = std::thread([this]{ readin(); });
> std::cerr << i << ": Making a thread" << std::endl;
> }
> };
>
>
> ~PIC()
> {
> std::cerr << "In the destructor" << std::endl;
> for(int i=0; i<10; i++)
> {
> t[i].join();
> std::cerr << i << ": Joining a thread" << std::endl;
> }
>
> };
>
> void readin()
> {
> char * loc_canvas_index;
> char * loc_source_index;
> sync_canvas_and_input(loc_canvas_index, loc_source_index);
>
> for( int i = 9; i>=0; i-- )
> {
> *loc_canvas_index = loc_source_index[i];
> std::cerr << i << ": Copy " << loc_source_index[i] << std::endl;
> std::cerr << i << ": Copied to loc_canvas_index " << reinterpret_cast(*loc_canvas_index) << std::endl;
> loc_canvas_index++;
> }
>
> };
> void sync_canvas_and_input(char * &loc_canvas_index, char * &loc_source_index )
> {
> std::cout << "**LOCKING**" << std::endl;
> std::lock_guard turn(medco);
> loc_canvas_index = canvas_index;
> loc_source_index = source_index;
> source_index += 10;
> canvas_index += 10;
> };
>
> char * get_canvas()
> {
> return canvas;
> }
> char * get_canvas_index()
> {
> return canvas_index;
> }
> char * get_source_index()
> {
> return source_index;
> }
>
> private:
> char * canvas = new char[100]{
> 'z', 'z','z','z','z','z','z','z','z','z',
> 'z', 'z','z','z','z','z','z','z','z','z',
> 'z', 'z','z','z','z','z','z','z','z','z',
> 'z', 'z','z','z','z','z','z','z','z','z',
> 'z', 'z','z','z','z','z','z','z','z','z',
> 'z', 'z','z','z','z','z','z','z','z','z',
> 'z', 'z','z','z','z','z','z','z','z','z',
> 'z', 'z','z','z','z','z','z','z','z','z',
> 'z', 'z','z','z','z','z','z','z','z','z',
> 'z', 'z','z','z','z','z','z','z','z','z'
> };
> char * canvas_index;
> char source[100] = {
> 'a', 'b', 'c', 'd', 'e', 'f', 'g', 'h', 'i', 'j',
> 'a', 'b', 'c', 'd', 'e', 'f', 'g', 'h', 'i', 'j',
> 'a', 'b', 'c', 'd', 'e', 'f', 'g', 'h', 'i', 'j',
> 'a', 'b', 'c', 'd', 'e', 'f', 'g', 'h', 'i', 'j',
> 'a', 'b', 'c', 'd', 'e', 'f', 'g', 'h', 'i', 'j',
> 'a', 'b', 'c', 'd', 'e', 'f', 'g', 'h', 'i', 'j',
> 'a', 'b', 'c', 'd', 'e', 'f', 'g', 'h', 'i', 'j',
> 'a', 'b', 'c', 'd', 'e', 'f', 'g', 'h', 'i', 'j',
> 'a', 'b', 'c', 'd', 'e', 'f', 'g', 'h', 'i', 'j',
> 'a', 'b', 'c', 'd', 'e', 'f', 'g', 'h', 'i', 'j'
> };
> char * source_index;
> };
> }//end namespace
>
> int main(int argc, char** argv)
> {
> testing::PIC fido;
> for(int i = 0; i<100;i++)
> {
> std::cout << i << " Canvas Position " << fido.get_canvas()[i] << std::endl;
> }
>
> return 0;
> }
>
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