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digitalmars.D - Importing global variables from a library

reply "Lionello Lunesu" <lionello.lunesu crystalinter.remove.com> writes:
Hi...

I've made an attempt to port disp.h (from dmc) to D (see a previous post of 
mine). The functions work fine, but the varbiables don't get imported. 
Specifically:

extern (C)  int
  disp_numrows,
  disp_numcols,
  disp_cursorrow,
  disp_cursorcol;

I get no linker errors when I try to use these variables, but their value is 
always 0. The name doesn't even matter. It seems "extern (C) int something;" 
actually declares that variable instead of importing it from a library.

What am I doing wrong?

-- 
L.

-- Get the root certificate at https://www.cacert.org/
Dec 09 2004
parent reply John Reimer <brk_6502 yahoo.com> writes:
On Thu, 09 Dec 2004 16:28:52 +0200, Lionello Lunesu wrote:


 Hi...
 
 I've made an attempt to port disp.h (from dmc) to D (see a previous post of 
 mine). The functions work fine, but the varbiables don't get imported. 
 Specifically:
 
 extern (C)  int
   disp_numrows,
   disp_numcols,
   disp_cursorrow,
   disp_cursorcol;
 
 I get no linker errors when I try to use these variables, but their value is 
 always 0. The name doesn't even matter. It seems "extern (C) int something;" 
 actually declares that variable instead of importing it from a library.
 
 What am I doing wrong?


First declare them in a their own module, then import that module into
your project, but don't compile and link that module.  If you compile the
declaring module, you end up making a new set of global variables and the
external library ones won't be accessible.

So do something like this:

// file: disp.d

module disp;

extern(C) {
	disp_numrows,
	disp_numcols,
	disp_cursorrow,
	disp_cursorcol,
}

// end of disp.d

-------------------------------

// file: project.d

module project.d;

import disp;

int main() 
{
// do whatever
}

// end of project.d

-------------------------------

Now compile project.d.  Do not compile disp.d.  It merely provides the
names of the external symbols it needs to access.  link in the appropriate
library that contains the actual global variables you need to access.  It
should now work as planned.  This is actually much the same as the
"interface" module idea discussed in another newsgroup post.

Have a look at phobos for another example of this
(std.c.linux.linuxextern).  I was confused about this same issue at one
point.

Later,

John

John
Dec 09 2004
next sibling parent reply John Reimer <brk_6502 yahoo.com> writes:
On Thu, 09 Dec 2004 14:28:03 -0800, John Reimer wrote:


 extern(C) {
 	disp_numrows,
 	disp_numcols,
 	disp_cursorrow,
 	disp_cursorcol,
 }


Okay... a typed a bit hastily.  Pardon me!

I meant to say:

extern(C) {
	int disp_numrows;
	int disp_numcols;
	int disp_cursorrow;
	int disp_cursorcol;
}

Your way of declaring extern with commas should work as well.
Dec 09 2004
parent reply "Lionello Lunesu" <lionello.lunesu crystalinter.remove.com> writes:
Thanks for your advice. It helps if I don't compile the disp.d. I guess it's 
acting like a C header file now :-/


 extern(C) {
 int disp_numrows;
 int disp_numcols;
 int disp_cursorrow;
 int disp_cursorcol;
 }


I still can't get it to import those 4 integers. I'm starting to think 
they're simply not available in the library, because in the same disp.d 
there's a declaration of an extern structure:

// from disp.h: extern disp_t __cdecl disp_state;
extern (C) disp_t  disp_state;

This one is working and correctly imported. Even if I put it in my compiled 
module: it doesn't declare a new variable but (correctly) imports it from 
the C library. However, if I change the name, it doesn't import the variable 
but declares it.

Lionello.
Dec 10 2004
parent reply kshmirko <kshmirko gmail.com> writes:
On Friday, 10 December 2004 at 10:03:24 UTC, Lionello Lunesu 
wrote:

 Thanks for your advice. It helps if I don't compile the disp.d. 
 I guess it's acting like a C header file now :-/

  > extern(C) {

 int disp_numrows;
 int disp_numcols;
 int disp_cursorrow;
 int disp_cursorcol;
 }


 I still can't get it to import those 4 integers. I'm starting 
 to think they're simply not available in the library, because 
 in the same disp.d there's a declaration of an extern structure:

 // from disp.h: extern disp_t __cdecl disp_state;
 extern (C) disp_t  disp_state;

 This one is working and correctly imported. Even if I put it in 
 my compiled module: it doesn't declare a new variable but 
 (correctly) imports it from the C library. However, if I change 
 the name, it doesn't import the variable but declares it.

 Lionello.


For your question, just add __gshared attribute
extern __gshared int a, b, c;
Sep 28 2021
parent max haughton <maxhaton gmail.com> writes:
On Tuesday, 28 September 2021 at 09:52:35 UTC, kshmirko wrote:

 On Friday, 10 December 2004 at 10:03:24 UTC, Lionello Lunesu 
 wrote:

 [...]


 For your question, just add __gshared attribute
 extern __gshared int a, b, c;


I was three years old when the message you are replying to was 
posted, please be careful.
Sep 28 2021
prev sibling parent "Lionello Lunesu" <lionello.lunesu crystalinter.remove.com> writes:
Isn't it a bug that "extern (C) int something;" actually declares the 
variable, instead of importing it?

Come to think of it, something is missing in the language definition:

* With functions it's obvious that "extern (C) int func();" is a 'forward' 
and the function should be imported.

* With variables, "extern (C) int bla;" it's not clear whether the variable 
should be imported or is declared (and should be exported with C linkage).

True? Or nonsense?

Lionello.
Dec 10 2004