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digitalmars.D.learn - Accessing D globals in C

reply "Thad" <tswoskowiak gmail.com> writes:
Hello,
I am mixing some of my existing C code with D via static linking. 
I can access C globals from D using __gshared but I cannot seem 
to be able to access a D global from within the C code. Keep in 
mind I am a novice programmer. Everything is built with gcc and 
gdc under Linux.

e.g.
//D code
import std.stdio;

extern (C) void print_global();

__gshared int global = 5;

void main(){
     writeln("The global value is: ", global);
     print_global(); //Call our C code
}

//C code
#include <stdio.h>
int global;

void print_global(){
	printf("Global value: %d\n", global);
	return;
}

If I compile and link the above two object files, the 
print_global function prints "Global value: 0". But of course, 
the writeln in the D code prints 5. If I put "extern int global;" 
or remove "int global" in the C file, gdc exits with:
staticdC.o: In function `print_global':
staticdC.c:(.text+0xa5): undefined reference to `global'

Am I missing something? Or is accessing D globals from C not 
possible?
Oct 28 2014
parent reply "Marc =?UTF-8?B?U2Now7x0eiI=?= <schuetzm gmx.net> writes:
On Tuesday, 28 October 2014 at 15:24:03 UTC, Thad wrote:

 Hello,
 I am mixing some of my existing C code with D via static 
 linking. I can access C globals from D using __gshared but I 
 cannot seem to be able to access a D global from within the C 
 code. Keep in mind I am a novice programmer. Everything is 
 built with gcc and gdc under Linux.

 e.g.
 //D code
 import std.stdio;

 extern (C) void print_global();

 __gshared int global = 5;

 void main(){
     writeln("The global value is: ", global);
     print_global(); //Call our C code
 }

 //C code
 #include <stdio.h>
 int global;

 void print_global(){
 	printf("Global value: %d\n", global);
 	return;
 }

 If I compile and link the above two object files, the 
 print_global function prints "Global value: 0". But of course, 
 the writeln in the D code prints 5. If I put "extern int 
 global;" or remove "int global" in the C file, gdc exits with:
 staticdC.o: In function `print_global':
 staticdC.c:(.text+0xa5): undefined reference to `global'

 Am I missing something? Or is accessing D globals from C not 
 possible?


There are only two small things you need to change:

// D code
// this is necessary to get the name mangling right
extern(C) __gshared int global = 5;

// C code
// `extern` to declare that it should not reserve space
// for the variable in this compilation unit
extern int global;
Oct 28 2014
parent "Thad" <tswoskowiak gmail.com> writes:
On Tuesday, 28 October 2014 at 16:42:20 UTC, Marc Schütz wrote:

 On Tuesday, 28 October 2014 at 15:24:03 UTC, Thad wrote:

 Hello,
 I am mixing some of my existing C code with D via static 
 linking. I can access C globals from D using __gshared but I 
 cannot seem to be able to access a D global from within the C 
 code. Keep in mind I am a novice programmer. Everything is 
 built with gcc and gdc under Linux.

 e.g.
 //D code
 import std.stdio;

 extern (C) void print_global();

 __gshared int global = 5;

 void main(){
    writeln("The global value is: ", global);
    print_global(); //Call our C code
 }

 //C code
 #include <stdio.h>
 int global;

 void print_global(){
 	printf("Global value: %d\n", global);
 	return;
 }

 If I compile and link the above two object files, the 
 print_global function prints "Global value: 0". But of course, 
 the writeln in the D code prints 5. If I put "extern int 
 global;" or remove "int global" in the C file, gdc exits with:
 staticdC.o: In function `print_global':
 staticdC.c:(.text+0xa5): undefined reference to `global'

 Am I missing something? Or is accessing D globals from C not 
 possible?


 There are only two small things you need to change:

 // D code
 // this is necessary to get the name mangling right
 extern(C) __gshared int global = 5;

 // C code
 // `extern` to declare that it should not reserve space
 // for the variable in this compilation unit
 extern int global;


Ah! That solved it. Thank you for the clarification.
Oct 28 2014