[fpc-devel]Small bugfix for SysUtils.CompareMem
michalis at camelot.homedns.org
Sat Nov 22 03:04:50 CET 2003
I just found a bug in CompareMem function in SysUtils.
CompareMem(p1, p2, 0) (third argument = zero) should always return true
(and ignore values p1 and p2). But, currently, it does not. CompareMem
is implemented like
function CompareMem(P1, P2: Pointer; Length: cardinal): Boolean;
for i := 0 to Length - 1 do
Result := True;
so it _looks_ like it's good - when Length=0 then we have
for i:=0 to -1 do ...
so this loop should never be executed even once. Right ? Wrong. Wrong
because Length and i are Cardinals (unsigned) and so calculating
Length-1 gives some extremely big positive value (namely, it's
High(Length) = $FFFFFFFF) (well, this bug would be catched at runtime by
compiling RTL with runtime checks for overflow... but RTL is usually
compiled without these checks).
So CompareMem(..., ..., 0) produces always false or EAccessViolation.
There are many ways to fix this - most elegant for me is to add a line
if Length = 0 then Exit(true);
at the beginning of this function.
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