Re: What is the best way to define the Imported C function

["Jeffrey R. Carter" <spam.jrcarter.not@acm.nospam.org>]

qunying wrote:
> 
> I am learning Ada and try to test the interface with C.

I would suggest that you learn Ada before trying to interface it to C. But if 
you are going to do it, use the types in Interfaces.C and its children, or types 
declared Convention C, not String and Integer; and don't pass explicit pointer 
values when Ada's interfacing rules will do the right thing for you.

> for this function, what is the best way to define the function in Ada?
> 
> int xcb_parse_display(const char *name, char **host, int *display, int
> *screen);

I can't help you (and really, neither can those who have replied to you) because 
the C specification isn't a specification; it doesn't tell us how the function 
uses its parameters. Without that information, we can't tell how to interface to it.

The problem is that C uses the same construct for many conceptually different 
things. For example:

void f (char* c);

may represent something that is conceptually equivalent to Ada's

procedure F (C : in Character); [unlikely, but possible]
procedure F (C : in out Character);
procedure F (C : out Character);
procedure F (C : in String);
procedure F (C : in out String);
procedure F (C : out String);

Given the common practice of using "char" to represent what Ada calls a 
Storage_Element, and "char[]" or "char*" for Storage_Array, this single C 
declaration may mean any of 12 different things.

-- 
Jeff Carter
"C++ is like jamming a helicopter inside a Miata
and expecting some sort of improvement."
Drew Olbrich
51