Wrapping a C function that takes a variable number of arguments

Wrapping a C function that takes a variable number of arguments

[Joel Reymont]

I'm trying to figure out how to wrap Cocoa's NSLog [1].

It's basically just like fprintf in that the signature looks like this

void NSLog(NSString *format, ...);

There must be an idiomatic way of creating a wrapper for such a  
function, probably with a bit of dark magic. Would someone please  
show me how?

	Thanks, Joel

[1] http://www.cocoadev.com/index.pl?NSLog

--
http://wagerlabs.com

Re: [Caml-list] Wrapping a C function that takes a variable number of arguments

[skaller]

On Fri, 2007-09-07 at 00:24 +0100, Joel Reymont wrote:
> I'm trying to figure out how to wrap Cocoa's NSLog [1].
> 
> It's basically just like fprintf in that the signature looks like this
> 
> void NSLog(NSString *format, ...);
> 
> There must be an idiomatic way of creating a wrapper for such a  
> function, probably with a bit of dark magic. Would someone please  
> show me how?

Variants. In Caml you pass

	NSLog(fmt, [Int 1; Double 2.0])

The wrapper can then decode the 'fmt' and check that the corresonding
variant has a suitable type.

It is not possible, however, to call the C function in a conforming way.
Instead you have to cheat. If there is a 

	vNSLog(fmt, va_args)

function you're cool, the cheat is easy. If you're using gcc as your
C compiler, read up on 

	 __builtin_va_list

and how gcc implements varargs.

The best cheat is this one: scan the fmt, and break it into
separate %codes like:

	"Hello %s today %d\n"

will be (say) Str.split on % to give

	"Hello "
	"s today "
	"d\n"

Now call NSLog 3 times, adding the % back to the second and third
strings:

	NSLog("Hello ");
	NSLog("%s today ",vs);
	NSLog("%d\n",vi);

in a loop, obviously, which sets vs if the variant is a string,
vi if it is an integer, vd if it is a double, etc etc.

Note this will NOT work if NSLog isn't functorial, that is:

	NSLog (x ^ y) = NSLog (x); NSlog (y)

eg if it adds an 'end of line' or 'date and time stamp' to the
log message, then this technique won't work so well.

-- 
John Skaller <skaller at users dot sf dot net>
Felix, successor to C++: http://felix.sf.net

Re: [Caml-list] Wrapping a C function that takes a variable number of arguments

[Daniel Bünzli]

Le 7 sept. 07 à 04:34, skaller a écrit :

> Variants.

Alternatively, on the caml side of things, you can use functional  
unparsing [1].

Best,

Daniel

[1] Olivier Danvy. Functional unparsing. J. of functional programing,  
1998.
http://dx.doi.org/10.1017/S0956796898003104

Also available here :
http://www.brics.dk/RS/98/12/