[PD] pd at startup creates 2 canvases, why?

Mathieu Bouchard matju at artengine.ca
Mon Dec 12 04:33:38 CET 2011

Le 2011-12-11 à 14:27:00, Ivica Ico Bukvic a écrit :
>> The OS releases all the memory allocated by the process when it
>> terminates, so no.
> OK, however, in pd-l2ork I am currently building infinite undo which
> will be a doubly-linked list linked to a canvas. So, if I am going to
> instantiate it dynamically, once the program exits are all these dynamic
> things taken care of? I think not. Otherwise, why would we need
> destructors in the first place if the os takes care of it all (other
> than eventually running out of memory)? Even vanilla canvas has
> dynamically allocated list that is destructed upon closing the patch but
> this is not the case with the two invisible canvases...

Memory allocation using getbytes(), malloc(), operator new or [NSObject 
alloc], are all the same : they work within the framework of sbrk(), 
mmap() and such things. For small amounts, malloc organises the memory 
given by sbrk() as a big data structure, in which pointers say where are 
the parts in use, the parts not in use, how big they are, and how new 
space can be found efficiently. For big amounts malloc just delegates to 
mmap(). In either case, those sbrk() and mmap() allocations are registered 
in a table in the kernel, and when the kernel deletes the sbrk and mmap 
regions, you don't need to do free() anymore.

The problem with destructors, is that they may do things other than 
freeing memory directly or indirectly... for example, if an object creates 
an empty file named «my_database.busy» to mean that the database should 
not be opened by anyone else, then this file has to be deleted when the 
object is deleted.

It's possible to reserve RAM that the OS won't free automatically, but the 
means to do so are a bit unusual and limited. For example, shmget() offers 
certain ways of creating leak bugs that have to be fixed by rebooting, but 
there's a shmctl() option for auto-release which is usually used just 

| Mathieu BOUCHARD ----- téléphone : +1.514.383.3801 ----- Montréal, QC

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