CODEWITHSUNDEEP
c++memoryvalgrind
Matt Pennington
Matt Pennington

Reputation: 582

Invalid read of size 8

I have written a class for a sequence of doubles in c++ using a dynamically allocated array.. When running the program, it completes successfully, but valgrind is finding an error. I receive Invalid read of size 8 when my resize function is called.

  void sequence::resize(size_type new_capacity){
    if (new_capacity == capacity){
      return;
    }else {
      if (new_capacity < used)
          used = new_capacity;
      capacity = new_capacity;
      value_type* new_vals;
      new_vals = new value_type[capacity];
      for (int i=0;i<used;i++){
          new_vals[i] = data[i];
      }
      cout<<endl;
      delete [] data;
      data = new_vals;
    }
  }

Resize is being called by attach:

  void sequence::attach(const value_type& entry){
    //Behaivoir for empty sequence
    if(used == 0){
      current_index = 0;
      used++;
      if (used > capacity)
        resize(capacity*2);
      data[current_index] = entry;     
    } 
    //Behaivoir for no current_index
    else if (!is_item()){
      current_index = used;
      used++;
      if (used > capacity)
        resize(capacity*2);
      data[current_index] = entry;
    }
    //Default behaivoir
    else {
      used++;
      if (used > capacity)
        resize(capacity*2);
      for(int i = used-1; i>current_index+1;i--)
        data[i] = data[i-1];
      advance();
      data[current_index] = entry;
    }
  }

Here is the error I am receiving in my test program:

==1919== Invalid read of size 8
==1919==    at 0x400DB3: main_savitch_4::sequence::resize(unsigned long) (sequence2.cxx:44)
==1919==    by 0x401091: main_savitch_4::sequence::attach(double const&) (sequence2.cxx:95)
==1919==    by 0x403232: test5() (sequence_exam2.cxx:538)
==1919==    by 0x40414E: run_a_test(int, char const*, int (*)(), int) (sequence_exam2.cxx:744)
==1919==    by 0x404321: main (sequence_exam2.cxx:775)
==1919==  Address 0x5a1ae50 is 0 bytes after a block of size 240 alloc'd
==1919==    at 0x4C2C037: operator new[](unsigned long) (in /usr/lib/valgrind/vgpreload_memcheck-amd64-linux.so)
==1919==    by 0x400C33: main_savitch_4::sequence::sequence(unsigned long) (sequence2.cxx:17)
==1919==    by 0x4030AC: test5() (sequence_exam2.cxx:520)
==1919==    by 0x40414E: run_a_test(int, char const*, int (*)(), int) (sequence_exam2.cxx:744)
==1919==    by 0x404321: main (sequence_exam2.cxx:775)
==1919==

I have tried running valgrind with --leak-check=full and --read-var-info=yes and cannot determine why I am getting this error. Line 45 of resize is the one that reads: new_vals[i] = data[i];

Thanks!!!

Upvotes: 0

Views: 2785

Answers (2)

Santosh Sahu
Santosh Sahu

Reputation: 2244

if (used > capacity)
 resize(capacity*2);     //here current index=capacity i.e maxm size allocated to new_vals
  data[current_index] = entry; //current_index > capacity, this is out of reach memory for new vals and data(since data=new_vals), so you are getting invalid read.

Upvotes: 0

Monroe_27
Monroe_27

Reputation: 31

The issue is that you are setting used to new_capacity without checking it so if it is less than new_capacity it causes a problem.

Upvotes: 1

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