FastMM4 says “The block header has been corrupted”

Question

I had this nasty bug that disappeared in the past but now after quite some time it returned.

I have two TSam objects (derived from TPersistent) created and loaded into a

Answer 1

This error means that your code corrupted internal memory manager's structures. Your call stack represents point, when MM detected this. This is not error path or anything related to it. The actual error happens BEFORE this moment. It may or may be not related to mentioned classes.

You should try to use "Range check errors" option (don't forget to make Build, not Compile) and FastMM in full debug mode (with CheckHeapForCorruption, CatchUseOfFreedInterfaces и DetectMMOperationsAfterUninstall options enabled).

You can also turn on FullDebugModeScanMemoryPoolBeforeEveryOperation global variable, to get an error almost immediately after problem occurs, but this option slows down your execution A LOT.

Probably the best choice is call ScanMemoryPoolForCorruptions periodically. Call it in one place. Got an error? Call it sooner. Still got an error? Call it sooner again. No error? Your problem sits somewhere between those last calls. Now you can use FullDebugModeScanMemoryPoolBeforeEveryOperation variable to get precise location. Just turn it on only on this code's area and turn it off right after it.

There is a very similar error: "FastMM detected that a block has been modified after being freed". In this case your code modifies not internal structures, but other memory, which isn't used at all ("free memory").

BTW, your error is NOT double-free! If this is a double-free call, FastMM will tell you that explicitly (it is easy to detect, as you are trying to free not-used or not-existed memory block): "An attempt has been made to free/reallocate an unallocated block".