Thread: DLL Wrapper

Hi everybody!

I'm a newbie with C development, so have patience with me... ^^

Well... I´m building a DLL that calls another DLL (a DLL Wrapper to solve JNI problems... or better to say Oracle Webutil problems... but this don´t matter at this moment).

The problem occours when the method of target DLL is called. I got a Access Violation on MSVCRT.DLL. I believe that's some mistake on parameters passage or anywhere around it.

Please, a little help here... I promisse learn more about C language when I´ve more time (and improve my english too ^^).

The wrapper DLL code:
(wPCLink6.h)

Code:

#ifndef _wPCLINK6_H_
#define _wPCLINK6_H_

#if BUILDING_DLL
# define DLLIMPORT __declspec (dllexport)
#else /* Not BUILDING_DLL */
# define DLLIMPORT __declspec (dllimport)
#endif /* Not BUILDING_DLL */

DLLIMPORT void Display_Erro(int Ip_Setado, char * Message);

#endif /* _wPCLINK6_H_ */

(wPCLink6.c)

Code:

#include "wPCLink6.h"
#include <windows.h>
#include <stdlib.h> 
#include <stdio.h> 
#include <conio.h>

typedef char * (__cdecl *Display_ErroFn)(int Ip_Setado); 

DLLIMPORT void Display_Erro(int Ip_Setado, char *Message)
{
    DWORD Error = 0;
    HINSTANCE libInst = LoadLibrary("pclink6.dll"); 
    if (!libInst) 
    {
        Error = GetLastError();
        sprintf(Message, "Error: %d", Error);
        return; 
    }

    Display_ErroFn Display_ErroHnd = (Display_ErroFn)GetProcAddress(libInst, "Display_Erro"); 
    if (Display_ErroHnd == 0) 
    {
        Error = GetLastError();
        sprintf(Message, "Error: %d", Error);
        return; 
    }

    Message = Display_ErroHnd(const Ip_Setado);  // Crash happens
    sprintf(Message, "I´m here!");  // Unreachable code

    FreeLibrary(libInst); 
    return;
}

BOOL APIENTRY DllMain (HINSTANCE hInst     /* Library instance handle. */ ,
                       DWORD reason        /* Reason this function is being called. */ ,
                       LPVOID reserved     /* Not used. */ )
{
    switch (reason)
    {
      case DLL_PROCESS_ATTACH:
        break;

      case DLL_PROCESS_DETACH:
        break;

      case DLL_THREAD_ATTACH:
        break;

      case DLL_THREAD_DETACH:
        break;
    }

    /* Returns TRUE on success, FALSE on failure */
    return TRUE;
}

Target DLL Code:
(PCLink6.h)

Code:

#ifndef _PCLINK6_H_
#define _PCLINK6_H_

#if BUILDING_DLL
# define DLLIMPORT __declspec (dllexport)
#else /* Not BUILDING_DLL */
# define DLLIMPORT __declspec (dllimport)
#endif /* Not BUILDING_DLL */

DLLIMPORT char * Display_Erro(int Ip_Setado);

#endif /* _PCLINK6_H_ */

(PCLink6.c)

Code:

#include "teste.h"
#include <windows.h>
#include <stdlib.h> 
#include <stdio.h> 
#include <conio.h>

char * Message;

DLLIMPORT char * Display_Erro(int Ip_Setado)
{
    strcpy(Message,"Sucess!");
    return Message;
return 0;
}

BOOL APIENTRY DllMain (HINSTANCE hInst     /* Library instance handle. */ ,
                       DWORD reason        /* Reason this function is being called. */ ,
                       LPVOID reserved     /* Not used. */ )
{
    switch (reason)
    {
      case DLL_PROCESS_ATTACH:
        break;

      case DLL_PROCESS_DETACH:
        break;

      case DLL_THREAD_ATTACH:
        break;

      case DLL_THREAD_DETACH:
        break;
    }

    /* Returns TRUE on success, FALSE on failure */
    return TRUE;
}

Note that my only desire is recive "Sucess!" string through Message parameter.

Thank´s a lot vart.

Can I ask you to translate it to C code ? 8-S... I understand the problem, but I don´t know how to write it.

I try:

Code:

DLLIMPORT const char * Display_Erro(int Ip_Setado)
{
    strcpy(Message,"Sucess!");
    return Message;
return 0;
}

got the same error...

I try:

Code:

char * Message[255];

DLLIMPORT char * Display_Erro(int Ip_Setado)
{
    strcpy(*Message,"Success!");
    return *Message;
return 0;
}

The error:

Code:

#
# An unexpected error has been detected by Java Runtime Environment:
#
#  EXCEPTION_ACCESS_VIOLATION (0xc0000005) at pc=0x77c260c1, pid=3252, tid=3512
#
# Java VM: Java HotSpot(TM) Client VM (1.6.0_03-b05 mixed mode, sharing)
# Problematic frame:
# C  [msvcrt.dll+0x360c1]
#
# If you would like to submit a bug report, please visit:
#   http://java.sun.com/webapps/bugreport/crash.jsp
#

The PCLink6 is a emulator of the original one, that give me the same error. Because of this behavior I´ve guessed in a error on wPCLink6 call...

Code:

char * Message;

DLLIMPORT char * Display_Erro(int Ip_Setado)
{
    strcpy(Message,"Sucess!");
    return Message;
return 0;
}

Should be changed to:

Code:

char Message[255];

DLLIMPORT char * Display_Erro(int Ip_Setado)
{
    strcpy(Message,"Sucess!");
    return Message;
//return 0; // commented out this line since you are returning Message on the line above.
}

As vart mentioned, it would be better to pass in a buffer to be filled by the function.

Code:

DLLIMPORT void Display_Erro(int Ip_Setado, char* Message, int MessageLength)
{
    strncpy(Message,"Sucess!",MessageLength);
}

It´s done... now works fine, thank´s bithub... thank´s vart...