Hi,
Darius Blaszyk wrote 9 lines
Again I would like to ask some support on a piece C code that I need to
translate to pascal. The code in question is:
#define MEMNEXT(x) ((MyStruct *)(((char *) x) - ((char *) & (((MyStruct *)0)->next))))
What exactly is going on here? Where does the result point to? It seems
like it starts with the pointer x and subtracts the size of the offset
of the beginning of MyStruct up to the "next" variable. At least that's
what I think is happening.
Yes, it looks like it. It simply substracts the offset of the "next" field from the value of X; This is a better solution than hardwiring the offset of next, which could change depending on the target platform's structure padding, or other reasons (and sometimes could be hard to calculate by hand anyway).
Does anyone has an idea how to translate this into working pascal code?
Well, not sure why one needs to do this abomination, without knowing the data structure it operates on, but nevertheless, you can do the same in Free Pascal, but you better turn the macro into a function:
type
PMyStruct = ^TMyStruct;
TMyStruct = record
some, fields: integer;
next: PMyStruct;
end;
function MEMNEXT(x: Pointer): PMyStruct;
begin
MEMNEXT:=PMyStruct(PByte(x)-PByte(@(PMyStruct(nil)^.next)));
end;
If this function is used often, you can mark it as inline; So the compiler can inline it, whenever necessary. If you want to avoid pointermath for whatever reason, the same works if you cast the pointers to PtrUInt instead of a PByte.
If the input value of X is other than a pointer, you can either overloaded methods, or define a var parameter without type, for example.
Charlie
