| Re: Memory allocation problem: [message #11010 is a reply to message #11009] |
Sat, 21 February 1998 00:00   |
Dr. G. Scott Lett
Messages: 14
Registered: February 1998
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Junior Member |
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I haven't yet checked this problem on all platforms, but IDL 5.1 beta frees
memory on Linux and Windows. I'll let you know what I find out about
other platforms next week.
Regards,
Scott
I~nigo Garcia wrote:
> 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
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