Freeing a pointer by dereferencing a doublepointer in C-CodePudding

I'm trying to write a function which will free a dynamically allocated string by sending the address of the string (char*) to the function. (Sending char**)

I have tried doing it like this:

void dstring_delete(DString* stringToDelete)
{
    // Precondition: stringToDelete is not NULL

    assert(stringToDelete != NULL);

    free(*stringToDelete);

Where DString is defined like this:

typedef char* DString;

The call to the function looks like this:

dstring_delete(&str1);

Where str1 is of type DString:

DString str1, str2;

I cant seem to figure out what is going wrong but this assert below the function call fails:

assert(str1 == NULL);

So my question is how I properly free the memory in this instance.

I have tried different arguments sent to the free function and also tried to find the same issues here.

CodePudding user response：

I assume that str1 is pointing to an allocated block that is to be freed and that the code sequence to free the block is like this:

    dstring_delete(&str1);
    assert(str1 == NULL);

OP's implementation of dstring_delete seems to be as follows, but OP did not post the full function definition including the closing brace:

void dstring_delete(DString* stringToDelete)
{
    // Precondition: stringToDelete is not NULL

    assert(stringToDelete != NULL);

    free(*stringToDelete);
}

It is not very clear from OP's question, but I think their code contains the following sequence, with str1 pointing to a previously allocated block to be freed:

    dstring_delete(&str1);
    assert(str1 == NULL);

OP wrote that the assert(str1 == NULL); was failing. This is because dstring_delete does not change the value of *stringToDelete (which aliases the str1 variable) so *stringToDelete (and the str1 variable) will still contain a non-null value on return from the function. However, the assert(str1 == NULL); implies that it is expecting dstring_delete to set the pointer value to NULL. That is easily fixed by changing the function body of dstring_delete to set the pointer to NULL after freeing the allocated block:

void dstring_delete(DString* stringToDelete)
{
    // Precondition: stringToDelete is not NULL

    assert(stringToDelete != NULL);

    free(*stringToDelete);

    *stringToDelete = NULL;
}

Now, after the function call dstring_delete(&str1);, str1 will be NULL so the assert(str1 == NULL); call will pass.

CodePudding user response：

The variable name of a string is itself a memory reference to the string; the type is char pointer and so the variable contains a pointer, which is a memory address. So when you call free() you only have to pass it the name of the variable.
(If you use the "&" you are passing the memory address where the string's memory address is located, and you only need the latter.)