Re: how to export global variable in C dll?

On Thu, 10 Apr 2008 15:17:13 -0700 (PDT), Cyrfer <cyrfer@gmail.com> wrote:


Perhaps I've misunderstood your question. This seems to work and is quite
straightforward:

For the DLL:

*****************************

; DLLVAREXPORT.DEF
; export TESTVAR without name-mangling

NAME DLLVAREXPORT.DLL
    EXPORTS
        TESTVAR

*****************************

/* DLLVAREXPORT.H */

#include <windows.h>
#include <stdio.h>

// make TESTVAR "global"

extern DWORD TESTVAR;

*****************************

/* VAR.C */

#include "dllvarexport.h"

// make TESTVAR the same for all instances
// not necessary but not very useful otherwise

#pragma data_seg(".shared")
DWORD TESTVAR = 0;
#pragma data_seg()
#pragma comment(linker, "/SECTION:.shared,RWS")

***************************

/* DLLVAREXPORT.C */

#include "dllvarexport.h"

BOOL WINAPI DllMain(HANDLE hInstance, ULONG Command, LPVOID Reserved)
{
    printf("DLL loaded; TESTVAR = %lu\n", TESTVAR);
    return TRUE;
}

****************************

And now for a test:

/* DLLVAREXPORTTEST.C */

#include <windows.h>
#include <stdio.h>

INT main ( INT argc, CHAR **argv )
{
    HMODULE hMod = LoadLibrary
    (
        "g:\\projects\\dllvarexport\\release\\dllvarexport.dll"
    );

    DWORD *dwTESTVAR = (DWORD *) GetProcAddress(hMod, "TESTVAR");

    printf("TESTVAR = %lu\n", *dwTESTVAR);
    *dwTESTVAR += 1;
    printf("TESTVAR = %lu\n", *dwTESTVAR);

    Sleep(60000);
    FreeLibrary(hMod);

    return 0;
}

Run DLLVAREXPORTTTEST.EXE:

DLL loaded; TESTVAR = 0
TESTVAR = 0
TESTVAR = 1

Run the test again while the first test is still sleeping:

DLL loaded; TESTVAR = 1
TESTVAR = 1
TESTVAR = 2

And a question for the experts: Is "static" necessary on data is a shared
segment? For some reason I used to think so but in the test above it clashed
with "extern" so I left it out and all seems OK.
--
 - Vince