Why do I get an access violation when I call FillChar?

Question

Consider my sample code:

var p512Sector:PByte;
.....
getmem(p512Sector, 262144);
FillChar( p512Sector,262144 ,0);

When I run the program, Delphi gives me an violation access error. Why?

Answer 1

Use FillChar(p512Sector^, 262144, 0) (note the dereferencing ^). Otherwise you are overwriting the pointer and the stuff behind in memory, not the allocated buffer.

Answer 2

FillChar expects an untyped variable. You should dereference the pointer:

FillChar(p512Sector^, ...);