using free() with structs

[P3nGu1N]

I'm new to the board, so first - hi.

I'm also fairly new to C programming, but have a good background in Java and a little C++.

There's part of a program that I'm stuck on how to do:

struct SymTab {
	int Size;
	struct SymEntry **Contents;
};

I'm fairly certain my constructor works, so I'm not posting that. For compilation I'm just trying to create a SymTab of size 25 with a bunch of NULL pointers instead of SymEntrys. Now, to destroy it I've tried

free((struct SymTab) ATable);

and

free(ATable);

neither of which will compile... can someone please help?

[itCbitC]

Can't free() memory that hasn't been allocated and from your post it looks like you are trying to free() NULL pointers which is undefined.

[Meldreth]

Now, to destroy it I've tried

free((struct SymTab) ATable);

and

free(ATable);

neither of which will compile... can someone please help?

if it doesn't compile then ATable probably isn't a pointer. free wants a void*, and any pointer except a function pointer can be converted to a void* without a cast.

it looks like you are trying to free() NULL pointers which is undefined.

freeing a null pointer is defined. it just doesn't do anything. think of it like this.

void free(void *p)
{
    if (p)
    {
        // free p
    }
}

[P3nGu1N]

alright, maybe I should post my constructor to clarify things:

struct SymTab *   CreateSymTab(int Size) {
	int i;
	
	if( Size > 0){
		tableSize = Size;
	}
	
	struct SymTab *theTable;
	
	theTable = (struct SymTab*)malloc(sizeof(struct SymTab));
	
	theTable->Size = tableSize;
	
	theTable->Contents = (struct SymEntry**)malloc(sizeof(struct SymEntry*) * tableSize);
	
	for (i = 0; i < theTable->Size; i++) {
		theTable->Contents[i] = NULL;
	}
	
	return theTable;
};

The SymEntrys are NULL in the SymTable, but I do assign a value to Size.

[MK27]

I think you are doing something more incorrect than that. Clue #1:

For compilation I'm just trying to create a SymTab of size 25

You create a struct or you don't. You don't pick a size for it! You should post this constructor you are so confident in, perhaps.

Then you try to free the whole struct

free((struct SymTab) ATable);
free(ATable);

I'm presuming that ATable is an instance of struct SymTab, since you don't say. Even if ATable was originally a pointer that you decided would enjoy 25 bytes:

struct SymTab *ATable=malloc(25);

(which as I said is the wrong way to proceed, but anyway...) -- you are not freeing ATable->Contents by freeing ATable. *ATable and (struct SymEntry**)ATable->Contents must be allocated and freed seperately -- and if you free ATable first, you won't be able to free Contents so you have inadvertently leaked that memory.

[MK27]

I apologize, our posts crossed. You constructor looks okay to me and I probably just explained some stuff you already understand. When you write "doesn't compile", do we get some kind of message with this from the actual compiler?

[itCbitC]

freeing a null pointer is defined. it just doesn't do anything. think of it like this.

void free(void *p)
{
    if (!p)
    {
        // free p
    }
}

Ah! yes it is a nop and in the code above you probably meant if (p is NULL).

[Meldreth]

Ah! yes it is a nop and in the code above you probably meant if (p is NULL).

i meant what i wrote. you broke it.

[Snafuist]

Where's the problem? If you assigned some value to X with malloc(), simply pass X to free() to free the memory, i.e.

free(theTable->Contents);
free(theTable);

Did you try that? If not, post your code. It's hard to deduce the actual situation from what you're saying.

from your post it looks like you are trying to free() NULL pointers which is undefined.

Citing from the standard: "The free function causes the space pointed to by ptr to be deallocated, that is, made available for further allocation. If ptr is a null pointer, no action occurs."

Greets,
Philip

[Sly]

I may be completely off here, but it is my understanding that you cannot free() something that has not been allocated with malloc().

[MK27]

I may be completely off here, but it is my understanding that you cannot free() something that has not been allocated with malloc().

Very true Sly.

[P3nGu1N]

odd, it compiled this time with:

free(ATable);

Might have just been the iMac version of gcc that didn't like it? I just tried on my Window's downloaded version of gcc... Well, thanks for looking at my code anyway