Re: C interface confusion about char codes, lists and atoms...

[Daniel Diaz]

Le 03/07/2013 16:44, emacstheviking a écrit :

Hi,

I have been experimenting with the foreign interface and managed to confuse myself.

Given this foreign declaration:

%% :- foreign( foo( +string ), [fct_name( foo )]). %% +string => atom at call site

%% :- foreign( foo( +codes ), [fct_name( foo )]).

:- foreign( foo( +codes ), [fct_name( foo )]).

and this implementation:

// +string :: atom                ==>  char*, works

// +codes  :: character-code list ==>  char*, works as foo("Hello").

// +chars  :: character list      ==>  char*, fails on call foo("HI"), foo([64,65]). Why?

PlBool

foo(char* s)

{

  printf("FOO: ==> %s\n", s);

  return PL_TRUE;

}

My problem is that when I use "+chars" I can't then say:

    foo("Hello")

as I get a type_error so please can anybody explain to me what "character list" is and how it looks in a command line session? The flag that controls the interpretation of double-quoted strings is at its default value as I have not altered it but I am aware of its existence and the fact that "" is syntactic sugaring of lists of codes.

Hi,

+atom means you pass an atom (e.g. abc) and you get the internal reference of the atom in C (which is an integer)
+chars means you pass a list of characters (e.g. [a,b,c]) and you get a NULL terminated string in C, ie. a char * (e.g. "abc")
+code means you pass a list of char codes (e.g. [97,98,99] which is the same a "abc" in Prolog) an dyou get the same char * as above.

See http://gprolog.univ-paris1.fr/manual/html_node/gprolog068.html

Daniel

Thanks.

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