#63 FastMM doesn't detect double call to FreeMem

[Igor]

The code below produces AV instead of report about double call to FreeMem.

    var
      P: Pointer;
    begin
      GetMem(P, 50);
      FreeMem(P);
      FreeMem(P);
    end;

[Pierre le Riche]

Hi Igor,

In normal mode (i.e. not "FullDebugMode"), the address space surrounding a block may be released to the operating system when a block is freed, so it is not always possible for FastMM to catch such a double free "gracefully". That said, double frees should almost always crash your application (either through above A/V or another exception raised by FastMM) so the end result is the same.

Best regards,
Pierre

[Igor]

Here is exception log in FullDebugMode:

---

Date: 14 Nov 2013 09:51:02
Address: 00408D96
ClassName: EAccessViolation
Message: Access violation at address 00408D96 in module 'BatPost.exe'. Read of address 00000002
IsOSException: 1
Modules:
00400000 20131114075057 C:\ritserver\EXE\BatPost.exe
Stack:
00408D96 [00400000][FastMM4][FastMM4.GetAvailableSpaceInBlock][3310]
0040AD1A [00400000][FastMM4][FastMM4.CheckBlockBeforeFreeOrRealloc][8810]
0040AD95 [00400000][FastMM4][FastMM4.DebugFreeMem][8857]
004030B4 [00400000][System][System.@FreeMem][2699]
005507C5 [00400000][BatPost][BatPost.BatPost][436]

---

AllocatedByRoutine can be used in CheckBlockBeforeFreeOrRealloc to determine that memory block is already freed before calling GetAvailableSpaceInBlock.