World: r3wp

ah yes... they are allocated on the stack, so memory its not cleared 
by default.

Updated the snippet accordingly:
https://gist.github.com/0c6305d7c25e40742b31

actually, in Olde's code its:   

 s.index = 0;

actually... its:    arg.index = 0;

we could propose a usefull macro to not get bitten by  this  again:

something like:

#define NEW_RXIARG(v) RXIARG v; v.index=0

CLEARS(&a);

RXIARG a;
CLEARS(&a);

is the most sensible way. but then you'll better #include <string.h> 
(at least until some include woes in the hostkit headers are straightened 
out).

however we do it, the problem is not in how, but that we forget to 
do it in the first place.

that's C for you :)

which is why a standard macro which takes care of that would be nice... 
in the same idea than the RL_MAKE_BLOCK (which I know isn't exactly 
the same since its just a wrapper around the RL vector)

you have to make sure to initialise all stack- and heap-allocated 
memory

yep.

Ah.. thanks.. so I made another small step in learning extensions:) 
Thanks. I'm sure I will have more questions later. But I must work 
now for a while.

When I have command like:
	test: command [str [string!]]
And on C side I need a pointer to this string, how to get it?

str_arg = RXA_SERIES(frm, 1);

with:
 char *str_arg;
?

sorry..

REBSER * str_arg = RXA_SERIES(frm, 1);

That's not working, the function I'm trying to call is:

char **MagickQueryConfigureOptions(const char *pattern, unsigned 
long *number_options)
And I would like to use REBOL input for the *pattern.

Does it mean that I must use FOR loop with RL_GET_CHAR or I can get 
the pointer directly?

loop with RL_GET_CHAR

to get the length of the REBOL string you need to use this:

RL_Series(str_arg, RXI_SER_TAIL)

with RL_Series I have this error:  undefined reference to `_imp__RL_Series'

This works:
            REBSER *ser;
            i32 len;
            
            ser = RXA_SERIES(frm, 1);

            len = RL_SERIES (ser, RXI_SER_TAIL) - RXA_INDEX(frm, 1);

            char pattern[len]; 
            for(n = 0; n < len; ++n)   
                pattern[n] = RL_GET_CHAR(ser, n);

yeah... sorry... I have my own defines, which include the required 
files.

What I must include to be able use RL_Print?

AFAIK, you should just need 

#include "reb-host.h"

That I have, but I get the similar error: undefined reference to 
`_imp__RL_Print'

It would be much more easier to play with extensions with the ability 
to trace it.

is your file a .c or .cpp  ?

.c

you can try adding 

extern  void RL_Print(char *fmt, ...);

at the begining of your file.

That does not works.

are you using gcc of VS ?

of = or

GCC

it's not working with Andreas' minimal example as well - http://www.curecode.org/rebol3/ticket.rsp?id=1727

(wrong link, this one https://gist.github.com/0c6305d7c25e40742b31
)

ah, I did a little bit of searching and I think I understand... 

try using RL_PRINT() instead.

its strange that I've never had a problem with this though cause 
I use RL_Print in all my files, but I realize that it might just 
be luck, that in my setup I don't have the linking problem you are 
having.

That works, but only with 2 args.

RL_PRINT should work in any uses of the host_kit

ok... that is easy to fix.

open reb-lib.h

and replace:
#define RL_PRINT(a,b)               RL->print(a,b)

with:
#define RL_PRINT(a,b)               RL->print(a,__VA_ARGS__)

can't test it right now, but that should work.

the function itself is defined with variable arguments, its just 
the #define which isn't ... the above redefines it so it matches 
the function declaration.

__VA_ARGS__ can only appear in the expansion of a C99 variadic macro
Never mind... I really have to do real work now :)

what?  I'm using it my own gcc compiled version of the host-kit... 
I don't understand... Andreas will have to step in... I don't know 
why you're having that error.

Actually it's:

../src/include/reb-lib.h:270:49: warning: __VA_ARGS__ can only appear 
in the expansion of a C99 variadic macro

wand.c:74:43: macro "RL_PRINT" passed 3 arguments, but takes just 
2
wand.c: In function `RX_Call':

wand.c:74: error: `RL_PRINT' undeclared (first use in this function)

ok, well, I guess you'll have to use just two args for now  ;-)