| Re: Memory allocation problem: [message #11014 is a reply to message #11010] |
Fri, 20 February 1998 00:00   |
Inigo Garcia
Messages: 13
Registered: June 1997
|
Junior Member |
|
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Well, I was afraid of something like this... I still find it a bug, whatever you
say, they should find a way of freeing that memory !!! Can it be done with
pointers ?? In a simple way, please, my brain is too small to fight with those
beings.
I~nigo.
David Fanning wrote:
> This is a result of IDL being written in C and using the C library
> functions (malloc and free) for memory allocation. In most C libraries,
> memory that is freed is NOT returned to the operating system. The C
> program retains this memory and will reuse it for future calls to malloc
> (assuming that the new allocation will fit in the freed block).
>
> Another way of considering it is in terms of how memory allocation is
> done under UNIX. New memory is allocated using brk() or sbrk() which
> control the size of the data segment. These routines are called by
> malloc().
>
> Suppose you allocate 3 1 MB regions of memory under C.
>
> char *p1=(char *)malloc(3*1024*1024);
> char *p2=(char *)malloc(3*1024*1024);
> char *p3=(char *)malloc(3*1024*1024);
>
> Here's what your data segment would look like assuming malloc had to call
> sbrk().
>
> ------------------------------------------------------------ ---
> prev stuff | overhead | 3MB | overhead | 3MB | overhead | 3MB |
> ------------------------------------------------------------ ---
> ^ ^ ^ ^
> p1 p2 p3 end of
> segment.
>
> Now we free(p1).
>
> ------------------------------------------------------------ ----
> prev stuff | overhead | free | overhead | 3MB | overhead | 3MB |
> ------------------------------------------------------------ ----
> ^ ^ ^
> p2 p3 end of
> segment
>
>
> Notice that the free memory is still in the data segment. If free had
> called brk to reduce the size of the segment, the 3MB pointed to my p3
> would be outside the data segment! SIGSEGV city! If free had moved the
> allocated memory to lower addresses so the segment size could be reduced
> without losing data, then p2 and p3 would point to invalid addresses, and
> we'd be forced to use handles rather than pointers and call
> GetPointerFromHandle() every time we wanted to access the memory. Ick!
> Just like Windows!
>
> Cheers,
>
> David
> -----------------------------------------------------------
> David Fanning, Ph.D.
> Fanning Software Consulting
> E-Mail: davidf@dfanning.com
> Phone: 970-221-0438
> Coyote's Guide to IDL Programming: http://www.dfanning.com/
--
\\|//
(o o)
+-----------------------oOOo-(_)-oOOo----------------------- --+
| I~nigo Garcia Ruiz |
| Kapteyn Instituut Phone: +31-(0)50-3634083 |
| Landleven 12 Fax: +31-(0)50-3636100 |
| 9747 AD GRONINGEN (Netherlands) e-mail: iruiz@astro.rug.nl |
+----------------------------------------------------------- --+
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