URGENT: Segfault Problem

Ratham (33)
I'm writing a queue/scheduler program. The problem is after it prints out the high queue correctly, it segfaults. I've spent hours trying to fix this code and cant find a solution.

main
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Job j1("low", 25, "3rd");
  Job j2("high", 100, "1st");
  Job j3("high", 25, "2nd");
  Job j4("low", 25, "4th");
  
  Scheduler<Job> sch(0,50);
  
  sch.schedule(j1);
  sch.schedule(j2);
  sch.schedule(j3);
  sch.schedule(j4);
  
  sch.print_scheduler(cout);
  cout << endl <<endl;
  Job j;
 
 while(!sch.empty()){
   j = sch.run();
   cout << "Running Job, "<<j <<endl << endl;
   sch.print_scheduler(cout);
   cout << endl;
  } 
}


scheduler.cpp
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template<typename T>
Scheduler<T>::Scheduler(int time, int late){ //Scheduler constructor
    
    time_ = time;
    late_ = late;
}


template<typename T>
void Scheduler<T>::schedule(T &job){ //Adds job to appropriate queue
    
    job.arrival_ = this->time_; // Sets the arrival time of the job

    
    if(job.priority_ == "low")
    {
        low_.push_back(job);
    }
    
    if(job.priority_ == "high")
    {
        high_.push_back(job);
    }
}

template<typename T>
ostream& Scheduler<T>::print_scheduler(ostream& out){ //Prints scheduler
    
    out << "Scheduler, " << "Time:" << this->time_;
    out << ", Late:" << this->late_ << endl;
    
    out << "low queue" << endl;
    
    for(int i = 0; i < low_.size(); i++) //Prints low priority queue
    {
        out << "        ";
        out << low_[i] << endl;
    }
    
    out << "high queue" << endl;
    
    for(int i = 0; i < high_.size(); i++) //Prints high priority queue
    {
        out << "        ";
        out << high_[i] << endl;
    }
            
    return out;
}


template<typename T>
bool Scheduler<T>::empty(){
    if(high_.size() == 0 && low_.size() == 0)
    {
        return true;
    }
    
    else
    {
        return false;
    }
}


template<typename T>
T Scheduler<T>::run(){

    T selected;
    
    if(time_ - high_.front().arrival_ > late_) //High priority late, takes next high priority.
    {
      selected = high_.front();
      time_ = time_ + selected.duration_;
      high_.pop_front();
     //THIS WORKS FINE
    }
    
    else if(time_ - high_.front().arrival_ < late_ && time_ - low_.front().arrival_ >= late_) //Low priority late, takes next low priority.
    {
        selected = low_.front();
        time_ = time_ + selected.duration_;
        low_.pop_front();
    }
    
    else if(time_ - high_.front().arrival_ < late_ && time_ - low_.front().arrival_ < late_) //Neither late, takes next high priority.
    {
       selected = high_.front();
       time_ = time_ + selected.duration_;
       high_.pop_front(); 
       //THIS WORKS FINE
    }
    
    else if(high_.empty() == true) //High empty, picks next low priority.
    { 
        selected = low_.front();
        time_ = time_ + selected.duration_;
        low_.pop_front();
        //I THINK ERROR IS IN THIS HIGH EMPTY STATEMENT SOMEHOW <------
    }
    
   else if(low_.empty() == true) //Low empty, picks next high priority.
    {
      selected = high_.front();
      time_ = time_ + selected.duration_;
      high_.pop_front(); 
    }
    
   else if(high_.empty() == true && low_.empty() == true)
    {
       
    }
    
    return selected;
}
#endif	/* SCHEDULER_H */


I have 2 deques, one low and one high priority. Currently the code correctly goes through and prints the jobs inside the high_ queue. When it gets past that point it dies with a segfault. How do I get it to stop dying at this point and go through with printing out the low queue? There is also a job.cpp and job.h but I didnt feel that it was necessary to post (yet?).

OUTPUT
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Running Job, Name:1st Arrival:0 Duration:100 

Scheduler, Time:100, Late:50
low queue
        Name:3rd Arrival:0 Duration:25 
        Name:4th Arrival:0 Duration:25 
high queue
        Name:2nd Arrival:0 Duration:25 

Running Job, Name:2nd Arrival:0 Duration:25 

Scheduler, Time:125, Late:50
low queue
        Name:3rd Arrival:0 Duration:25 
        Name:4th Arrival:0 Duration:25 
high queue


RUN FINISHED; Segmentation fault;

Last edited on
histrungalot (272)
Not having seen all of the code, my best guess is that after you finish running the high priority jobs you are calling high_.front() on a deque that is empty.

First line of code in run()
if(time_ - high_.front().arrival_ > late_) // <--- What happens if high_ is empty?

http://www.cplusplus.com/reference/deque/deque/front/
"Calling this function on an empty container causes undefined behavior."

Try checking if the the deques are empty() before you call front().

Ratham (33)
Will look into it.
Last edited on
Ratham (33)
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if(high_.size() == 0)
{
}


^ Added this as the first line of Run().

It doesnt segfault anymore but it still doesn't print the contents of the low_ queue, it just infinitely spits out "Running Job" with all the blank/default values.
Last edited on
Ratham (33)
Asked 2 other classmates, neither one said they have a "if the deque is empty" checker before other stuff in Run(). Guess that's not the issue :(
histrungalot (272)
run() is always returning selected even if you fail all of you if checks which is what must be happenning.
Post your latest run() code.
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