[fpc-pascal] Variant record types in Sockates unit.

[Alexander Todorov]

Hello list,
I notice the following in socketsh.inc:

  TInetSockAddr = packed Record
    case boolean of
     false : (
  {$ifdef SOCK_HAS_SINLEN}
     sin_len     : cuchar;
  {$endif}
     sin_family  : sa_family_t;
     sin_port    : cushort;
     sin_addr    : in_addr;
     xpad         : array [0..7] of char; { to get to the size of sockaddr... }
      );
   true: (
  {$ifdef SOCK_HAS_SINLEN}
     len     : cuchar;
  {$endif}
     family  : sa_family_t;
     port    : cushort;
     addr    : cardinal;
     pad         : array [0..7] of char; { to get to the size of sockaddr... }
      );
    end;


Note that this structure differs in field names (sin_ omited in 2nd
case) and that field
addr : cardinal <--> sin_addr : in_addr differs in type.
Types cardinal and in_addr are compatible as far as I can see.


>From Delphi help:
type
TShapeList = (Rectangle, Triangle, Circle, Ellipse, Other);
TFigure = record
 case TShapeList of
  Rectangle: (Height, Width: Real);

  Triangle: (Side1, Side2, Angle: Real);
  Circle: (Radius: Real);
  Ellipse, Other: ();
end;

For each record instance, the compiler allocates enough memory to hold all
the fields in the largest variant. The optional tag and the constantLists
(like Rectangle, Triangle, and so forth in the last example) play no role in
the way the compiler manages the fields; they are there only for the
convenience of the programmer.

All fields exist simultaneously.
Why there are same fields with different names? Isn't this just
duplicating code?

I have tried working with sockets using either of the fields (with or
without sin_ prefix) and of course it works? Why is it there I am
wondering?

Greetings Alexander.