Re: [gnome-love] How to know when to free memory.

[Marcus Brubaker <jghrfa home com>]

Good question.  Well, there is basically two answers to your question here:

1) The answer to the specific question about sheet_get_extent() is no.

2) Of coure, you want to know why. :)  Well, in this particular case 
sheet_get_extent returns a struct (Range) not a pointer.  (So for one, you'd 
want: Range range = sheet_get_extent;)  You never free() anything that is not a 
pointer.  Why?

Well, the computer divides memory in to a stack and a heap.  Variables that you 
alloc/free are located in the heap.  Variables that are just defined (eg, an 
int) are located on the stack.  Variables that are located on the stack have a 
lifespan limited to the duration of the function.  Once a function returns, that 
memory is no longer valid, and so it doesn't need freeing.  However, variables 
that are *allocated* on the heap will have the lifespan of the program (when the 
OS will reclaim the memory) or until you tell the OS that it can take that 
memory back.

So general rule of thumb: if you don't see a * in the type declaration, you will 
never free it.

The more interesting case is when it DOES return a pointer.  Then, well, whether 
you free it or not is entirely based on the function.  In most cases you will 
probably be required to free memory that is returned to you.  The exceptions are 
important though:
	1) If a pointer is returned in to a block of memory (eg from a strchr()) that you 
already own, don't free it.  You'll want to free the block of memory directly 
when you're done with the whole thing.
	2) We have the source.  Use it.  If a function that returns something keeps a 
reference to it beyond the lifespan of the function (eg. in a global variable or 
in a structure somewhere) then you will probably not want to free it, or at 
least, not right away.


Thats about all I can think of right now.  Feel free to ask more questions. 
Pointers and memory management is an important and powerful tool in C/C++ but it 
can also be hard to understand. :)

Regards,
Marcus Brubaker

Adrian Custer wrote:

> Hey all,
>
> There is some crucial piece of info that I don't get about the gtk
> object system. I'm working on a plugin to gnumeric and am trying to
> figure out when the return value from some call into the libraries needs
> to be freed. As the maintainer Jody says: "you need to understand the
> management semantics of all results and arguments." So here's an example
> with questions throughout.
>
>
>
> GList *sheets;
> Range *range;
>
> The range is defined at: 
> http://cvs.gnome.org/lxr/source/gnumeric/src/gnumeric.h#99
>
> Consider this call:
> sheets = workbook_sheets (wb);
> http://cvs.gnome.org/lxr/source/gnumeric/src/workbook.c#915             
> which needs to be freed with:
> g_list_free (sheets);                                                   
>
> Similarly consider:
> range = sheet_get_extent (aSheet, FALSE);
> http://cvs.gnome.org/lxr/source/gnumeric/src/sheet.c#1200
> does this need to be freed, and more importantly why/why not? How do I
> figure this out?
>
> Thanks for any pointers,
> adrian
> acuster@nature.berkeley.educational
>
>
>
>
>
>
>
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