[fpc-devel]Problems with AnsiStrings (memory leak)

[Udo Giacomozzi]

Hello Florian,

Sunday, November 19, 2000, 5:12:47 PM, you wrote:

FK> This is nearly impossible to fix because the problem is the memory 
FK> fragmentation:
FK> there are a lot of memory blocks allocated and deallocated but the
FK> new blocks never fit into the available free blocks because there
FK> is new data concatenated.


I already supposed something like that and I already implemented your
solution in some parts of the program, I just hoped there was a better
solution.
Anyway, I don't understand completely why this problem comes up. As
far as I understood, when I add something to a string, the new string
is placed to a new memory block (because it wouldn't fit in the
original memory block). So far so good, but I think the memory loss
shouldn't be so much.
Have a look at this example (works like my sample program I posted):

First the string is set to "start" then a point "." is added in each step.
AAAA, BBBB etc. marking previously used blocks (can be allocated):

"start"
"AAAAAstart."
"AAAAABBBBBBstart.."

shouldn't now fit the next block in the previously used blocks?

"start...BBBCCCCCCC"
"DDDDDDDDstart....C" (new additional memory needed!)
"DDDDDDDDEEEEEEEEEstart....."
"start......EEEEEEFFFFFFFFFF" (again, no more memory needed)
"GGGGGGGGGGGstart.......FFFF" (same here)

You see, never more memory than 3 times the real string is used (15 kb
in my example and not >1 mb). Of course I don't know enough about
memory organisation in fpc, but this seems quite logical to me. After
two steps there should be always enough space left to re-use old
blocks.

What makes the problem? Do you have problems re-using previously
allocated (and then freed) blocks?

-- 
Best regards,
 Udo