Hello,

I'm converting apache 1.3 headers to pascal, and I found something I don't know how to convert. It's these lines:

static const handler_rec hw_handlers[] = {
{ "hw-app", hw_handle_req },
{ NULL }
};

Where handler_req is declared as:

  handler_t = function (param1: Prequest_rec): Integer; cdecl;
  handler_rec = record
    content_type: PChar;	{ MUST be all lower case }
    handler: handler_t;
  end;

And hw_handle_req is a function of type handler_t

I don't understand what exactly that constant is. I looks like a dynamic array with 2 elements, but the second is NULL. How can a structure be NULL ?

Also, the static modifier here plays any role? Looks redundant to me.

thanks,

Am Samstag, den 23.09.2006, 12:39 -0300 schrieb Felipe Monteiro de Carvalho:

Typically such structure arrays are iterated by the part using them. This way one can add new items simply by writing the name and associated handler into the code, if needed.

Think of hardware detection, one driver may know different models it supports. When detection is active it simply iterates over the model describing struct and checks presence until the item read from the struct is NULL.

I think, a NIL pointer in pascal will do, although I have no idea what this is good for in apache:

const
	first_handler_rec = (("hw_app"), (@hw_handle_rec));
type
	Phandler_rec = ^handler_rec
var
	hw_handlers: array of [0..1] Phandler_rec = (
		first_handler_rec,
		NIL
	);

That should be close to the pascal syntax for this construction - not checked, only typed away.

Maybe you can put the constant value into the array directly, omitting to have it separated and maybe you can omit the array size (0..1) too, you'll have to try.

HTH,
Marc

This is what almost works:

var
hw_handlers: array[0..1] of handler_rec =
(
(content_type: 'hw_app'; handler: @DefaultHandler), nil

  );

But the compiler will complain on the nil:

mod_hello.lpr(108,5) Fatal: Syntax error, "(" expected but "NIL" found

Marc Santhoff wrote

Think of hardware detection, one driver may know different models it supports. When detection is active it simply iterates over the model describing struct and checks presence until the item read from the struct is NULL.

I still don't know how what is the inner working of this. Does that c statement really means a static array with size two? A record cannot be null. When you access a static record you go directly to it's memory position of Initial_Pos + X * Record_Size, where X is the record number. Then you cannot have a null record. The best you can have is record whose content is null.

Or NULL could represent that the array is of size 1 not 2.

Am Sonntag, den 24.09.2006, 11:37 -0300 schrieb Felipe Monteiro de Carvalho:

That's why I suggested making it an array of pointers. A pointer can be NIL. The main question is if apache can still access it in the right way ...

Have you tried something stupid, like:

hw_handlers: array[0..1] of handler_rec =
(
(content_type: 'hw_app'; handler: @DefaultHandler), (,)
)

Yes it does.

There is the problem, pascal is sort of stronger typed than C.

Or NULL could represent that the array is of size 1 not 2.

It is an element that can have a NULL value, however the compiler interprets it. Mostly variables in C do not have anything attached but can be viewed as a block of memory. You can do with it what you like. Pascal is lacking some common base type compatible with any record. One solution for a few cases would be to use an array of byte and do the housekeeping yourself.

How is the source using this array defined, can you get any hints from that C code? Is it happy with a record having a NIL string and a NIL pointer in it?

HTH,
Marc

Maybe you can create the array dynamically at run-time, in the unit's initialization section...

type
  p_handler_rec=^handler_rec;
  pp_handler_rec=^p_handler_rec;
var
  hw_handlers:pp_handler_rec;
initialization
  hw_handlers:=getmem(sizeof(p_handler_rec)*2);
  new(hw_handlers[0]);
  hw_handlers[0]^.content_type:='hw-app';
  ;
  hw_handlers[1]:=nil;
end.

- Jeff

Ok, after a lot of headaches and making questions on ##c freenode channel I found out what that is.

The structure is a NULL-terminated array / table. It was declared like this:

static const handler_rec hw_handlers[] = {
{ "hw-app", hw_handle_req },
{ NULL }
};

But that is just the short declaration for something like this:

static const handler_rec hw_handlers[] = {
{ "hw-app", hw_handle_req },
{ NULL, NULL }
};

So it´s an array of size 2, where the second element is filled with NULLs, but it will be interpreted as an array of size 1, because the last element marks the end of the array.

Am Samstag, den 30.09.2006, 11:48 -0300 schrieb Felipe Monteiro de Carvalho:

Thanks for making the explanation public,
Marc