free memomy of link list

bibhukalyana (4)
Hi everyone,
I have two structure A & B.
1
2
3
4
5
6
7
8
9
10
11
 
struct B
{
 int h,w;
 double d;
 struct B *next,*prev;
};

struct A
{
 struct B *st;
};


In my main code

1
2
 
 struct A str;
 allocate_mem(&str);


In allocate_mem

1
2
3
4
 
 if(str->st == NULL)
 {
  str->st = (B*)malloc(sizeof(B);
 }


Now I want to free the memory.
I tried free(str.st) But it is not working. Memory leakage wasw there.
Please tell me how to free the list.

thanks.
SIK (167)
It seems like your allocate_mem function is checking if the member varibale pointer [st] of struct A is NULL before performing a malloc - you however don't seem to do anything to ensure that this value is NULL when an instance of [A] is created. This then implies that function allocate_mem most likely saw that str->st was not NULL (had some unitialized value) and consequently did no malloc for str->st.

The call then to free(str.st) probably called free on some illegal memory (the unitialized value of st).

Maybe you should add a constructor to struct A that initializes st to NULL.

1
2
3
4
5
6
 
struct A
{
   A() : st(NULL) {}
 
   struct B *st;
};


Even better - add a destructor for struct A to free st automatically.


1
2
3
4
5
6
7
8
9
10
11
12
13
14
 
struct A
{
   A() : st(NULL) {}
   
   ~A()
   {
      if (st)  
      {
         free(st);
      }
   }
 
   struct B *st;
};
Peter87 (3908)
I suspect bibhukalyana is writing in C so a constructor and destructor is not possible. You don't need to check if a pointer is null before calling free on it. free(NULL); is safe and will do nothing.

How do you know there is a memory leak? If str->st->next and str->st->prev is pointing to something are freeing that memory somewhere?
Gaminic (1541)
Or even better: both. A wild pointer should always be assigned a 0 [in my opinion], both when initialized and after delete is called.
SIK (167)
Please correct me if I am wrong here - I haven't worked in pure c for a long time now:

If we create a new instance of struct [A], then what will the initial value of member [st] be?

From my experience, I would have thought that st would have some garbage value.

free(NULL) may be safe but free(abc)

where abc is pointing to something unitialized (ie garbage) may not be.

If [st] indeed has some garbage value, then would it still be safe to call free on it?
hanst99 (2845)
Indeed, it will be garbage. Also, I'm not even sure if free checks for 0 or not. delete does though.
Topic archived. No new replies allowed.

C++

Forum