[Ecls-list] Re: Latest changes

[dan at sof.ch]

I have a FFI interface to SDL-TTF that work without problem, once my
little patch was applied.

On Tue, Jun 28, 2005 at 12:01:28PM +0000, Maciek Pasternacki wrote:
> Juan Jose Garcia Ripoll <jlr at ...> writes:
> 
> > Just to remaind what the problem is, you pass strings with fill pointers 
> > to the C world. It is the extra junk after the fill pointer which causes 
> > your problem, because the C code will see this junk. The FFI interface 
> > hence cannot simply get a pointer to the data in the string, but should 
> > create a new copy. However both cases have problems: if we do not make a 
> > copy, the C code gets junk, if we make a copy, the string that is passed 
> > to the C world is not the same one and we cannot use this string to get 
> > data from the C world.
> 
> #v+ ecldev.info
> 3.6 Strings
> ===========
> 
> A string, both in Common-Lisp and in ECL is nothing but a vector of
> characters. Therefore, almost everything mentioned in the section of
> arrays remains valid here. The only important difference is that ECL
> stores strings as a lisp object with a pointer to a zero terminated C
> string. Thus, if a string has N characters, ECL will reserve N+1 bytes
> for the string. This allows us to pass the string `self' pointer to any
> C routine.
> 
>    If X is a lisp object of type string, we can access the following
> fields:
> `x->string.dim'
>      Maximum number of characters that it can contain.
> 
> `x->string.fillp'
>      Actual number of characters in the string.
> 
> `x->string.self'
>      Pointer to the characters.
> #v-
> 
> #v+ src/c/string.d
> cl_object
> cl_alloc_simple_string(cl_index l)
> {
> 	cl_object x;
> 
> 	x = cl_alloc_object(t_string);
> 	x->string.hasfillp = FALSE;
> 	x->string.adjustable = FALSE;
> 	x->string.displaced = Cnil;
> 	x->string.dim = (x->string.fillp = l);
> 	x->string.self = (char *)cl_alloc_atomic(l+1);
> 	x->string.self[l] = x->string.self[0] = 0;
> 	return(x);
> }
> #v-
> 
> It seems there is (should be) *always* room for a terminating null, and said
> null, at x->string.self[x->string.dim], if not earlier; so if null is not set,
> something seems to be wrong at some point, but it should be possible just
> to set it without segfaulting.
> 
> 
> 
> 
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-- 
Courage, fuyons...