Thread: free-ing un-malloc-ed memory

Probably. And what do you hope to achieve with that?

Originally Posted by Bladactania

It was a timing issue I was worried about. There was a chance that the way I had structured my main loop, that I might be freeing a variable that might not have been malloced during the iteration.

I wondered if I'd need to set a flag to make sure.

Another way to check this is to set those variable to NULL when they are declared or freed:

Code:

char *ptr=NULL;
[various things may or may not happen]
if (ptr) { free(ptr); ptr=NULL; }

Pointers are never set to NULL (either when initialized or freed) unless you do it, but if they are NULL then you know who did it and why (hopefully: they are already freed or unallocated*). This is easier than using a flag.

*remember that setting an allocated pointer to NULL does not free it, so would be a memory leak...

Originally Posted by Bladactania

I seem to remember seeing somewhere that you cannot allocate memory for a pointer that is already set to NULL. Is this true?

Huh? If what you mean by this is:

Code:

int *ptr = NULL;
ptr = malloc(...);

would be wrong, then you have misunderstood. This code-snippet is perfectly valid and correct.

--
Mats

Originally Posted by Bladactania

I seem to remember seeing somewhere that you cannot allocate memory for a pointer that is already set to NULL. Is this true?

Gotta concur with matsp, I do this all the time and so should you!

Originally Posted by Bladactania

Instead of NULL, would setting the prt to 0 work?

I guess; but that will become an obscurantist habit (because it is very normal to set a pointer to NULL for exactly these reasons).

Why not just use NULL????

Originally Posted by MK27

I guess; but that will become an obscurantist habit (because it is very normal to set a pointer to NULL for exactly these reasons).

It is not as obscure as you think: 0 is a null pointer constant.

Originally Posted by MK27

Why not just use NULL?

Personal choice to avoid the use of a macro.

NULL and 0 are essentially the same thing - serves the same purpose and behaves the same. Any particular reason you prefer to use 0?

--
Mats

Originally Posted by laserlight

It is not as obscure as you think: 0 is a null pointer constant.
Personal choice to avoid the use of a macro.

NULL being the macro I guess. That's a decent reason.

And I suppose if I saw this:

Code:

char *ptr=0;

I would immediately associate it with NULL, so not obscure at all. Sorry.

But then if you have a long function with something like this:

Code:

char* blah = 0;   // You can clearly see it's a pointer since you see the declaration.
...
...
blah = 0;   // Not so obvious 50 or more lines down since you can't see how blah is declared.  Is it a pointer or an int?

That's why I use NULL. It makes is clear that the variable it's being assigned to is a pointer.
And yes, since NULL is 0 you could assign NULL to an int, but that would just be dumb.

Although, usually the name of the variable itself gives you a pretty good idea about it's type since a lot of people prefix pointers with 'p'...

Originally Posted by cpjust

Not so obvious 50 or more lines down since you can't see how blah is declared.

Hence a function should do one thing and do it well

Originally Posted by cpjust

That's why I use NULL. It makes is clear that the variable it's being assigned to is a pointer.
And yes, since NULL is 0 you could assign NULL to an int, but that would just be dumb.

Unless a mistake is made for the same reason as you cite NULL being useful. The reader would then be misinformed unless he/she checked, but if he/she checked then the use of NULL would no longer be useful.