Re: [gtk-list] O.S. memory usage

[Sebastian Wilhelmi <wilhelmi ira uka de>]

Hi Derek,

>         This is offtopic; my apologies.  But it's a quickie, and I don't
> know who else to ask (suggestions welcome!).
> 
>         If I write a program that does
> 
> pointer = g_malloc(LOTS_OF_RAM);
> 
>         ...and then, later on, does
> 
> g_free(pointer);
> 
>         Does that memory immediately go back to the O.S.?  Will it be used
> for drive caching, other apps, etc. as my program continues to run?  Or
> does Linux (and/or other O.S.s) hold on to it, say, in case my program
> does another g_malloc or some such thing?

malloc (as used by g_malloc) returns memory from the so called heap. This is a
region in memory, that the allocation routines from libc take care of. It can
be expanded in one direction. when there is no more space left for a new
malloc, this region is expanded by a 'sbrk' system call (look at 'strace
<program-name>'). memory, returned to libc by free is in general not returned
to the system, but only appended to the list of free memory, which is
consulted first, when new memory is requested. Some systems and libc's however
support to give memory back to the OS. That is the case for Linux/glibc2.
Other combinations might also do it. Memory can however only be returned to
the system, if it is at the top of the heap. 

There are of course exceptions to the rule. glibc2 for example does have more
than one heap (one for every thread + big allocations end up on their own
heap), thus making it more complicated.

Bye,
Sebastian
-- 
Sebastian Wilhelmi                   |            här ovanför alla molnen
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