SRL Archive - Lape

I bumped into a case where I get exceptions from TFileStream.readBuffer() depending on how I set the length of the string I'm using for the buffer.

If I use a local integer or longint for setLength() all goes well.

If I use TFileStream.getSize() in the setLength() it ends up with an access violation when you call readBuffer. There are some oddities with this case:

Other operations on the string seem to work ok in this scenario. You can assign a string to it, print it, but don't call readBuffer().

HOWEVER, if you use s[1] directly in between the setLength() and the readBuffer(), now it works!

Example:

Manually create file test.txt with test data in folder that script is saved in.

program filestreamerror;

const
   fmCreate        = $FFFF;
   fmOpenRead      = 0;
   fmOpenWrite     = 1;
   fmOpenReadWrite = 2;

procedure thisWorks();
var
   testFile : TFileStream;
   s: string;
   i: integer;
   li: longint;
begin
   testFile.init(scriptPath+'test.txt', fmOpenRead);
   i := testFile.getSize();   // longint -> integer
   setLength(s, i);           // works ok
   testFile.readbuffer(s[1], testFile.getSize());
   writeln(length(s), ' ', s);
   testFile.free();
   
   testFile.init(scriptPath+'test.txt', fmOpenRead);
   li := testFile.getSize();
   setLength(s, li);          // longint -> setLength() works ok too
   testFile.readbuffer(s[1], testFile.getSize());
   writeln(length(s), ' ', s);
   testFile.free();
end;

procedure thisDoesntWork();
var
   testFile : TFileStream;
   s: string;
begin
   try
      testFile.init(scriptPath+'test.txt', fmOpenRead);
      
      // using result of getSize() causes problems
      setLength(s, testFile.getSize());

      // if you uncomment this the function starts working!!
      // writeln(s[1]);

      testFile.readbuffer(s[1], testFile.getSize());
      writeln(length(s), ' ', s);
   finally
      testFile.free();
   end;
end;

var
   ans: integer;
 
begin  
   ans := messagebox('Would you like an exception?', 'Choose', 4 {yn box});
   if (ans = 6 {yes}) then
   begin
      writeln('You asked for it');
      thisDoesntWork();
   end
   else
   begin
      thisWorks();
   end;
end.

Use

setLength(s, testFile.getSize() + 1);

and problem solved.

WriteLn(testFile.getSize()); // 10
writeln(length(s)); // 11

Not sure what could be causing this, but it seems to work in the latest Simba build.