Re: Glib and memory allocation

[John Vetterli <jvetterli linux ca>]

On Tue, 8 Feb 2005 18:19:03 +0100
Cristiano Ghirardi <cristiano ghirardi gmail com> wrote:
> it's not clear to me what happens when memory has been allocate with
> g_new or g_realloc and not freed. I mean: obviously  this memory
> remains in the user space of the process as an infamous memory leak 
> but it seems to me that under linux 2.4.26 this memory is not freed
> also when the process has died! I think that is a problem in vm in
> this (quite old) release of Linux but I'd like to know if someone has
> already found   this strange behaviour in glib/Linux.

When you call malloc (or realloc, g_malloc, etc.) the memory allocation
routines tries to find a large-enough chunk of available memory in the
process' data segment.  If no chunk exists, it calls the brk() system
call to enlarge the data segment.  When you call free (or g_free) the
memory deallocation routines simply make note that that chunk of memory
is now available to be re-allocated at a future time.  Very rarely will
the process' data segment shrink.  The kernel and vmm only know the size
of the process' data segment, they don't care how much of it has been
allocated with malloc or how much of it has been freed with free.  So
when your program ends, it doesn't matter if you've freed every block or
memory or not, the kernel will simply discard the whole data segment,
freeing up that much system memory.  Simply terminating a program
without freeing memory when an error condition is detected is a common
occurrance (e.g. g_new will terminate if it fails) and is often
harmless.  So if you think the kernel is not freeing memory when a
process dies, it probably has nothing to do with calling free or not. 
Not freeing memory is mostly a concert in long-lived programs, where
repeated calls to malloc without balancing calls to free will eat up
more and more memory over time.

I hope this info is helpful, or at least not fatally boring.  And to
answer your question, no, I haven't noticed any behaviour like
that.

JV
-- 
Remember, UNIX spelled backwards is XINU.
                -- Mt.