Question

I wrote a simple counter structure in C:

typedef struct{
    int value;
}Counter;

then, I wrote some simple implementations:

void createCounter(Counter *dCount)
{ 
    dCount = (Counter*)malloc(sizeof(Counter));
    dCount->value = 0;
}

void FreeResource(Counter *dCount)
{
  free(dCount);
}

now in the main, i wanted to free the pointer i created and it complained that the pointer being freed was not allocated.I am looking at the code and I thought I allocated memory for it when I called the createCounter() function?

 int main()
  {
    Counter m;
    CreateCounter(&m);
    FreeResource(&m); //run time error given here..

    return 0;
 }

Answer 1

You are trying to pass the address of a variable allocated in stack and then trying to assign an address allocated by malloc to it which won’t get reflected in the caller. So, when you try to free it, you are effectively passing a stack variable’s address to free due to which you get undefined behavior.

Change the function

void createCounter(Counter *dCount) 
{      
    dCount = (Counter*)malloc(sizeof(Counter));    
    dCount->value = 0; 
} 

as

void createCounter(Counter **dCount) 
{      
   *dCount = (Counter*)malloc(sizeof(Counter));     
   (*dCount)->value = 0; 
} 

In your case, the pointer gets passed by value and the new memory address allocation doesn’t reflect in the caller.

The main function must be changed as:

int main()     
{       
  Counter *m;       
  CreateCounter(&m);       
  FreeResource(m); //run time error given here..          
  return 0;    
}