Realloc Double Free

Quote:

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Originally Posted by mingerso

This was mentioned earlier but heres an example of realloc and not destroying your previous pointer ( so you can avoid bad free()'s and memory leaks ):

Code:

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char * add_space(char ** str)
{
        int len = strlen(*str);
        char * tmp = (char *)realloc(*str, sizeof(char)*len+2);
        if(tmp)
        {
                /* allocation was ok so we have our new address to point to */
                *str = tmp;
        } else {
                /* out of memory, so do something... or not */
                free(*str);
        }

        /* now you can error check the function on NULL or not */
        return tmp;       
}

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Hope that helps.

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There's "*str=NULL;" missing after "free(*str);" - in case free() will be used, and return value of function ignored...

That is the typical method but depends on what you're doing. It is not required and I see your point, but if you're going to be using pointer allocations this way you might want to make a wrapper for free() that sets it for you:

Code:

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void * safe_free(void * ptr)
{
        if(ptr)
                free(ptr);
        return NULL;
}

var = safe_free(var);

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It is safe to free a null pointer, so one could write:

Code:

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void * safe_free(void * ptr)
{
    free(ptr);
    return NULL;
}

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But then since this requires one to remember to assign the return value of safe_free() back to the pointer, I see no benefit over just writing:

Code:

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free(var);
var = NULL;

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Quote:

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Originally Posted by mingerso

That is the typical method but depends on what you're doing. It is not required and I see your point, but if you're going to be using pointer allocations this way you might want to make a wrapper for free() that sets it for you:

Code:

---

void * safe_free(void * ptr)
{
        if(ptr)
                free(ptr);
        return NULL;
}

var = safe_free(var);

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Yes, but for me that's a 2-step operation (function code, and checking return value).
I just call a function and it sets pointer to NULL if something goes wrong, so checking return value is optional (and pointer even if free() got called, remains valid).

I would prefer something like this:

Code:

---

void safe_free(void **ptr)
{
  if (ptr)  // Check for stupid programmer errors
  { 
      free(*ptr);
      *ptr = NULL;
  }
}

// And to complete it...
#define free(x) safe_free(&x)

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--
Mats

Quote:

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Originally Posted by matsp

I would prefer something like this:

Code:

---

void safe_free(void **ptr)
{
  if (ptr)  // Check for stupid programmer errors
  { 
      free(*ptr);
      *ptr = NULL;
  }
}

// And to complete it...
#define free(x) safe_free(&x)

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--
Mats

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My style (preference?) exactly ;)

Yes all very good points of preference... and back to what was being said:

"... so do something... or not"

or better yet, you could just:

Code:

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assert(ptr);

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Rather than having an IF, in that case.

Quote:

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Originally Posted by iMalc

or better yet, you could just:

Code:

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assert(ptr);

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Rather than having an IF, in that case.

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Yes, I agree.

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Mats