pointer being freed was not allocated error? - c++

Question

I have seen many posts for this error. But I'm not reserving memory dynamically or doing anything in destructor:
This program is SSJF algorithm for selecting cylinder in operating system.
I have a simple class called IO:
class IO
{
public:
IO();
IO(int,int);
void setIO(int,int);
~IO();
int trackNo;
int arrival;
int start;
int end;
bool finished;
};
Here is the implementation of the class::
IO::IO(int arr, int tNum)
{
this->arrival = arr;
this->trackNo = tNum;
this->start = 0;
this->end = 0;
}
IO::IO()
{
}
IO::~IO()
{
}
void IO::setIO(int t1, int t2)
{
this->trackNo = t1;
this->arrival = t2;
}
And finally here is part of main program:
list<IO> myList;
....
myList.push_back(tmpIO); //Add to the list
...
list<IO> wt_list;
And later I'm trying to do some operations. I have deleted some of the part which is not related.
//list<IO>::iterator itMin;
while(myList.size()>0)
{
//If it is the first input just get it
if(f)
{
IO selected = myList.front();
curr_time += selected.arrival + selected.trackNo;
f=false;
cout << selected.arrival<<endl;
lastPos = selected.trackNo;
myList.pop_front();
}
//Check if there is any item to add to queue
while(myList.front().arrival < curr_time)
{
wt_list.push_back(myList.front());
myList.pop_front(); //Error is coming from this line
}
while(wt_list.size()>0)
{
}
Error message:
malloc: * error for object 0x10f68b3e0: pointer being freed was not allocated
* set a breakpoint in malloc_error_break to debug
Anyone can help me and explain why I get this error and how can I skip it?

Answer 1

The simplest code I can come up with to reproduce this error looks like this:
#include <list>
int main()
{
std::list<int> mylist;
mylist.pop_front();
}
I can prevent the error by doing:
#include <list>
int main()
{
std::list<int> mylist;
if (!mylist.empty())
{
mylist.pop_front();
}
}
You're calling:
myList.pop_front();
...within a while-loop, which in turn is within a while-loop that also calls myList.pop_front().
I can only suggest that you debug your code to see how many times pop_front() is invoked for mylist. My money is on it being more than mylist.size() times, hence my question in the comments (with new emphasis):
How many items are in myList when the error is thrown?
Perhaps the simplest fix will be to replace...
//Check if there is any item to add to queue
while(myList.front().arrival < curr_time)
{
wt_list.push_back(myList.front());
myList.pop_front(); //Error is coming from this line
}
while(wt_list.size()>0)
{
}
...with...
while (!mylist.empty() && myList.front().arrival < curr_time)
{
wt_list.push_back(myList.front());
myList.pop_front();
}
while (!wt_list.empty())
{
}
...but it's hard to tell from the snippet you've provided.