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

[Udo Giacomozzi]

Hello,

I encountered a problem using AnsiStrings. It isn't a real bug, but
can make problems, like it does for me.

I noticed it with Free Pascal version 1.1, snapshot 17/11/2000 under
Linux (i386).

The problem is that when altering AnsiStrings the greatest available
memory block is becoming smaller and smaller, but much smaller than it
is really needed.

For example, I made the following example program that creates a
string of 5000 chars by adding one char at a time. I gave the program
1 mb of heap memory but it runs out of memory.

Here is this program:

-----------------8<------------------8<-------------------------------

{$MODE Delphi}       // I prefer Delphi programming style
{$M 16387, 1000000}  // 16 kb stack, 1 mb heap


program StringTest;

var
  S        : AnsiString;  // test-string
  i        : Integer;     // count-variables
  StartMem : Integer;     // total free memory at startup
  StartMax : Integer;     // greatest free memory block atstartup
  c        : Char;

begin
  { This disables the automatic heap growing when more memory is needed. 
    As with the $M switch defined, the program has 1 mb of heap available,
    that should be enough for a 5,000 chars string... }
  HeapError := NIL; 
 

  writeln('ANSISTRINGS TEST');
  writeln;
  writeln('Please note heap griwing is disabled. ',
          'The program may only use 1 mb of heap.');
  
  StartMem := MemAvail;
  StartMax := MaxAvail;
  writeln('Memory at program start: ', StartMem, ' bytes free, ',
           StartMax, ' in largest block.');
  
  writeln('Hit RETURN to continue...');
  ReadLn;
  
  c:='.';
  
  S:='start';
  For i:=1 to 5000 do
  begin
    S := S + c; // simple instruction to add one single character to a string
      
    writeln('String length = ', Length(S),' chars;  memory: ',
            MemAvail,' free, ',MaxAvail,' largest block.');
  end;

  writeln('memory loss: ',StartMem-MemAvail, ' bytes, ',
          StartMax-MaxAvail, ' in largest block');
  
  writeln('done.');
  writeln('S="',S,'"');
  writeln('That is, ', Length(S),' chars.');
  writeln('Memory at program end: ', MemAvail, ' bytes free, ',
          MaxAvail, ' in largest block.');
  
end.

-----------------8<------------------8<-------------------------------


And on my machine it generates the following output:

ANSISTRINGS TEST

Please note heap griwing is disabled. The program may only use 1 mb of heap.
Memory at program start: 999920 bytes free, 999920 in largest block.
Hit RETURN to continue...
String length = 6 chars;  memory: 999856 free, 999856 largest block.
String length = 7 chars;  memory: 999856 free, 999824 largest block.
String length = 8 chars;  memory: 999856 free, 999824 largest block.
String length = 9 chars;  memory: 999856 free, 999824 largest block.
String length = 10 chars;  memory: 999856 free, 999824 largest block.
[...]
String length = 3958 chars;  memory: 995904 free, 7904 largest block.
String length = 3959 chars;  memory: 995904 free, 3984 largest block.
String length = 3960 chars;  memory: 995904 free, 7904 largest block.
String length = 3961 chars;  memory: 995904 free, 3984 largest block.
String length = 3962 chars;  memory: 995904 free, 7904 largest block.
String length = 3963 chars;  memory: 995904 free, 3984 largest block.
String length = 3964 chars;  memory: 995904 free, 7904 largest block.
String length = 3965 chars;  memory: 995904 free, 3984 largest block.
String length = 3966 chars;  memory: 995904 free, 7904 largest block.
String length = 3967 chars;  memory: 995904 free, 3984 largest block.
Runtime error 203 at 0x080503B0
  0x080503B0
  0x0804FF40
  0x0804C1F6
  0x0804C696
  0x0804C379
  0x080539BF
  0x080480B0
  0x00000000


As you can see, the used memory begins to grow exponencially (well, to
be exact, the free memory sinks correctly).

Can you correct this behavior? I have a big problem with a web server
application that has to process large strings and in some cases it
runs out of memory, even with 15 mb heap.

I restricted the maximum heap size to prevent the program use too much
memory. So, if there is a way to restrict the maximum heap size in
another way, let me know. But I don't want to give 50 mb heap at
program startup. Best would be a growing heap, but with a maximum
value.

Best regards,
Udo