DLL not loaded with LoadLibrary() function

[2kaud]

Looking at the output from dumpbin, it doesn't seem quite right.

Dump of file asm.dll

File Type: DLL

  Section contains the following exports for ERRORMSG.DLL

Why is it referring to ERRORMSG.DLL ? Has the .dll been renamed?

[binary2]

When testing on a new console app, the code works, on my other project GetyLastError outputs error 193 (0xC1) %1 is not a valid Win32 application. In Build -> Configuration Manager -> Active Solution Platform x64 is selected. In Project -> Properties -> Configuration Properties -> Linker -> Command Line -> Aditional Options "%(AdditionalOptions) /machine:x64" is specified.

In Active Solution Platform when changing to X86 and on Aditional Options "%(AdditionalOptions) /machine:x86" I'm getting these errors:


Severity Code Description Project File Line Suppression State
Error A2013 .MODEL must precede this directive cpuid C:\xmrig-master\src\3rdparty\libcpuid\masm-x64.asm 2
Error A2034 must be in segment block : exec_cpuid cpuid C:\xmrig-master\src\3rdparty\libcpuid\masm-x64.asm 5
Error A2034 must be in segment block cpuid C:\xmrig-master\src\3rdparty\libcpuid\masm-x64.asm 6

[2kaud]

You can't mix x64 .exe and x86 dll. They have to be the same. My test was for a x86 exe which worked with asm.dll so asm.dll is also x86.

For x86, Active Solution Platform should be WIN32. You don't need to specify any linker additional options. Setting the Active Solution Platform in Configuration Manager sets the required solution properties. Note that if you change the Active Solution Configuration (release/debug), you'll need to ensure that the Active Solution Platform is correct and that the solution properties are also correct as these are stored separately per solution configuration.

The errors shown are assembler errors. I thought you were using fasm as the assembler - not the VS masm assembler? Have you included your asm code as part of the solution? Also note that x64 asm is different in many ways to x86 asm and usually a x86 asm file cannot be just assembled as x64 asm without source changes.

[binary2]

I'm using Fasm for dll compilation and VS2017 for the c++ code

I made a 64bit Dll with fasm, when running the program I'm getting this error:

"asm.dll is either not designed to run on Windows or it contains an error"

[2kaud]

I'm using Fasm for dll compilation and VS2017 for the c++ code

I made a 64bit Dll with fasm, when running the program I'm getting this error:

"asm.dll is either not designed to run on Windows or it contains an error"

There's a problem with the 64bit dll. As fasm has a message board, I'd ask there. https://board.flatassembler.net/

[binary2]

Thank you, I solved the issue with the dll file. I have one more question, how do I reference in a header file a function from the loaded dll to be used somewhere else

#include "App.h"
#include <windows.h>
#include <iostream>
#include "a.h"

using namespace std;

typedef int(__stdcall *f_funci)(unsigned int, unsigned int);

int main(int argc, char **argv) {
    
	HMODULE ll = LoadLibrary(L"asm.dll");

	if (ll == NULL) {
		cout << "asm.dll not loaded. Error code is " << GetLastError() << endl;
	}
	cout << "asm.dll loaded ok\n";

	f_funci assemblerMode = (f_funci)GetProcAddress(ll, "asm");

	if (!assemblerMode) {
		cout << "could not locate the function " << GetLastError() << endl;
	}
	
	App app(argc, argv);
    return app.exec();
}

"a.h" is my header

[2kaud]

In the header file, you just specify the function declaration as _declspec(dllimport). See https://docs.microsoft.com/en-us/cpp...port-dllimport You also need to inform the linker into which .dll this function resides - so you need to specify the name of the appropriate .lib file in the linker configuration. Linker/General/Additional Library Directories/ specifies where the .lib file(s) reside(s) and Linker/Input/Additional Dependencies/ specifies the name(s) of the lib file(s). Or use a #pragma statement in the program. See lib within https://msdn.microsoft.com/en-us/library/7f0aews7.aspx

If there is no generated .lib file when the .dll file is created, then you can't reference the exported .dll function as a 'normal' function, but will have to continue to use loadlibrary/getprocaddress or use the 'old fashioned way' of creating a .def file and specifying the .def file as a resource file in the solution. If when creating the .dll you don't get a .def file then you can create one manually. It's just a text file. See https://docs.microsoft.com/en-us/cpp...sing-def-files .

PS You'll probably also need to specify extern "c" as well to avoid 'name-mangling' issues.See https://docs.microsoft.com/en-us/cpp...pecify-linkage

So to specify a function, say int myfunc(int a) as import from a .dll,

extern "C" __declspec(dllimport) int myfunc(int a);

Note that usually in c++, the same header file is used for the .dll source and the .exe that uses the .dll. Thus the header file often has the structure

#pragma once

#ifdef INDLL
   #define DLL __declspec(dllexport)
#else
   #define DLL __declspec(dllimport)
#endif

#ifdef __cplusplus
    extern "c" {
#endif

DLL int myfunc(int a);
// More declarations here

#ifdef __cplusplus
   }
#endif

If the header is used in a .exe source then it imports the functions. If used in the .dll source then INDLL needs to be defined before the #include so that the functions are exported.

[binary2]

I added the def file to my project "asm.def" with "asm" as the function name specified in the dll

LIBRARY   asm  
EXPORTS  
   asm   @1

The header file

#pragma once

#ifdef INDLL
#define DLL __declspec(dllexport)
#else
#define DLL __declspec(dllimport)
#endif

#ifdef __cplusplus
extern "c" __declspec(dllimport) int assemblerMode(unsigned int, unsigned int);

#endif

#pragma once

#ifdef INDLL
#define DLL __declspec(dllexport)
#else
#define DLL __declspec(dllimport)
#endif

#ifdef __cplusplus
extern "c" {
#endif

	DLL int assemblerMode(unsigned int, unsigned int);
	// More declarations here

#ifdef __cplusplus
}
#endif

In both cases when compiling I'm getting the error:

error C2537: 'c': illegal linkage specification

[2kaud]

Sorry. My bad. Finger trouble. extern "c" should be extern "C" (capital C).

[2kaud]

It looks like with VS2017 you can't just use a .def with a .dll to import - you actually do need the .lib file.

To create the .lib file from the .def file from a VS developer command prompt, chnage to the directory in which the .def is located then

lib /def:asm.def

[or other file name instead of asm.def]

This will produce a 32bit .lib file (asm.lib in this case) which can be used to import the functions from the .dll. To produce a 64 bit lib

lib /def:asm.def /machine:x64

Note that asm is not allowed as a name. My revised .def file is

LIBRARY   asm
EXPORTS
   asm1=asm   @1
   ShowErrorMessage @2
   ShowLastError @3

The c++ code that compiles OK is

#pragma comment (lib, "asm")

extern "C" __declspec(dllimport) int asm1(unsigned int, unsigned int);

int main()
{
	asm1(1, 2);
}

[binary2]

Thank you