Lazarus/Delphi: UnicodeString in self-allocated record data type causes access violations - dynamic-memory-allocation

Question

I guess, my problem is caused by the concept how UnicodeStrings are implemented, but I cannot solve this problem.
I'm trying to scan a directory tree on disk recursively and build a treeview which should show all files and subfolders. Plus, I want to store additional information for each tree node. The TTreeNode object has only a "Data" property (type Pointer) for this purpose, so I allocate memory manually, store the information and assign the allocated pointer to my data property. Everything seems to work fine unless I include usage of a UnicodeString field within my data record.
So, here is my custom data record definition:
type
TFileInformation = record
AbsoluteFileName: UnicodeString;
FileSize: Int64;
FileAttributes: LongInt;
CreationTime, ModificationTime: TDateTime;
end;
And here is my code for directory recusion:
const NO_ERROR = 0;
procedure ScanDirectory(Folder: UnicodeString; Node: TTreeNode);
var
Details: Pointer;
NewNode: TTreeNode;
SearchAttributes: LongInt;
SearchMask: UnicodeString;
SearchRecord: TUnicodeSearchRec;
begin
if (Folder <> '') and (Folder[Length(Folder)] <> DirectorySeparator) then begin
Folder += DirectorySeparator;
end;
SearchMask := Folder + '*'{$IFDEF WINDOWS} + '.*'{$ENDIF};
SearchAttributes := faReadOnly or faHidden or faSysFile or faDirectory or faArchive or faSymLink;
if FindFirst(SearchMask, SearchAttributes, SearchRecord) = NO_ERROR then begin
repeat
if ((SearchRecord.Attr and faDirectory) <> faDirectory) or
((SearchRecord.Name <> '.') and (SearchRecord.Name <> '..')) then begin
Details := MemAlloc(SizeOf(TFileInformation));
//TFileInformation(Details^).AbsoluteFileName := Folder + SearchRecord.Name;
TFileInformation(Details^).FileAttributes := SearchRecord.Attr;
TFileInformation(Details^).FileSize := SearchRecord.Size;
TFileInformation(Details^).CreationTime := SearchRecord.Time;
//TFileInformation(Details^).ModificationTime := -1;
if Node = nil then begin
NewNode := self.trvOrigin.Items.AddNode(nil, nil, ansistring(SearchRecord.Name), Details, naAdd);
end else begin
NewNode := self.trvOrigin.Items.AddNode(nil, Node, ansistring(SearchRecord.Name), Details, naAddChild);
end;
if (SearchRecord.Attr and (faDirectory or faSymLink)) = faDirectory then begin
// only recurse folders which are NOT SymLink:
ScanDirectory(Folder + SearchRecord.Name, NewNode);
end;
end;
until FindNext(SearchRecord) <> NO_ERROR;
end;
FindClose(SearchRecord);
end;
When I uncomment the line containing .AbsoluteFileName :=, I get an Access Violation (SIGSEGV-Exception in Unix).
I'm currently using Lazarus in objfpc mode on Debian Linux, but I guess it's the same with Delphi on Windows. The Treeview.Data property value is stored in the "Details" variable in my example code, self.trvOrigin is my treeview control.

Answer 1

When you allocate the Details record, the memory is not defined.
AbsoluteFileName is a managed type and must be properly initialized before use.
You need to clear the memory after the allocation:
FillChar(Details^, SizeOf(TFileInformation), #0);
As an alternative, use New(Details) in combination with Dispose(Details).
They will correctly initialize/finalize the record.
Note: Details must be a typed pointer, PFileInformation.