Question

double free or corruption

Asked by: chsalvia

I'm getting this error at runtime:

*** glibc detected *** double free or corruption (fasttop): 0x0804a008 ***
Aborted

This happens when an instance of one class encapsulates and assigns an instance of another class.

I tried to narrow down the problem using a debugger, and it seems the error is produced because somehow I call delete[] on an already deleted pointer in the class destructor.  At least, I think that's the problem - but I'm not sure HOW that's happening.  I was finally able to create a very simplified test program that reproduces the exact problem so I could post it here without posting hundreds of lines of code.  Here it is:

// BEGIN CODE

class Buffer
{
      char* buffer;
      int len;

      void init(char* str)
      {
            len = strlen(str);
            buffer = new char[len+1];
            memcpy(buffer,str,len);
      }

      public:
            Buffer(char* str)      { init(str); }

            ~Buffer() { delete[] buffer; } // <--- this destructor is causing the problem, but why?
      
};

class Class2
{
       Buffer buf;

      public:
            Class2(Buffer& buffer) : buf(buffer) { }
            ~Class2() { };
};

int main()
{
      Buffer buf = "abcdefghijklmnopqrstuvwxyz";
      Class2 c2(buf);
}

// END CODE

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Asked On
2006-03-08 at 19:08:51ID21766511
Tags

free

,

double

,

corruption

Topic

C++ Programming Language

Participating Experts
4
Points
500
Comments
7

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Answers

 

by: amit_gPosted on 2006-03-08 at 19:24:08ID: 16141206

How are the copy constructor and =operator written for Buffer?

 

by: migoEXPosted on 2006-03-08 at 19:27:50ID: 16141218

The problem is that your Buffer class does not have copy constructor. So when you initialize Class2 - it's "buf" member uses the default copy constructor, which just copies all the values from original class.

As result, both "Buffer buf" and "Buffer Cluss2::buf" point to same memory allocated by "init" of the first Buffer. And they both try to release the memory when destructed.

You should add a copy constructor to Buffer class, which will keep a copy of string in "buffer" - for example, use strdup function.

 

by: rajeev_devinPosted on 2006-03-08 at 19:28:14ID: 16141220

Hey you don't have a copy constructor here
>> Class2(Buffer& buffer) : buf(buffer) { }
buffer and buf are pointing to the same data.

 

by: rajeev_devinPosted on 2006-03-08 at 19:29:45ID: 16141227

That means you have two objects of Buffer pointing to the same data.
You are doing shallow copy

 

by: rajeev_devinPosted on 2006-03-08 at 19:30:28ID: 16141232

Write one assignment operator. Or, use some reference counting mechanish to resolve this problem

 

by: jkrPosted on 2006-03-08 at 19:30:48ID: 16141235

That works fine here - 'as is'. No runtime error. Nothing. And I fail to spot an error in the code.

 

by: rajeev_devinPosted on 2006-03-08 at 19:32:11ID: 16141244

Write this copy constructor in your function
Buffer(const Buffer& b) {
   init(b.buffer);
}

20120131-EE-VQP-002

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