Question

I am having trouble freeing a pointer in a pointer array (values).

typedef struct MyStruct{  char** values;  }MyStruct;

In C, I create dynamic array.

JSDictionary **array = (JSDictionary **)malloc(sizeof(JSDictionary *) * numRows);

The resultSet should be an array of JSDictionary pointers. I create the struct like:

JSDictionary * newJSDictionaryWithSize(unsigned int size)

{
JSDictionary *new = malloc(sizeof(JSDictionary));
    printf("new %p\n", self);
new->_size = size;
new->count = 0;

new->values = (char **) malloc(sizeof(char *) * size);
for(unsigned int i = 0; i < size; i++){
    new->values[i] = (char *)malloc(sizeof(char *));
}

return new;
}

Everything creates and works fine. It’s the free that gives me a problem.

void deallocJSDictionary(struct JSDictionary *self)

{
printf("dealloc %p\n", self);
for(unsigned int i = 0; i < self->_size; i++){
    printf("free %p\n", &self->values[i]);
    free(self->values[i]);
}
free(self->values);
free(self);

}

I get a pointer being freed was not allocated error. The pointer being passed in shows the same memory address as the one that I created and added to the array. The values pointer (in debugger) shows the same memory address in the dealloc function as it did when I created it. The problem is trying to free the first point in the values array. Any ideas why the first point in the values pointer array is different?

Answer 1

I see two problems so far. First:

new->values[i] = (char *)malloc(sizeof(char *));

This line allocates a single pointer – probably not what you want. Should likely be something like:

new->values[i] = malloc( MAX_SIZE_OF_VALUE );

I.e. a buffer to hold the value. You can also allocate one big chuck to keep all values and just offset into that memory.

Second:

printf("free %p\n", &self->values[i]);
free(self->values[i]);

This prints and tries to free(3) two different pointers. First is the address of the second.

As for the actual deallocation error – do you have assignments like this in the code that manages the dictionary?

dict->values[i] = some_value;

If so – you are overriding pointers to (and leaking) allocated memory with pointers to probably stack or static memory. You have to copy the the value into the “value-buffer”. Something like:

strlcpy( dict->values[i], some_value, MAX_SIZE_OF_VALUE );

Hope this helps.