How to compile a 64-bit dll written in C?

Question

I have a Java program which makes use of some native function calls to speed up video encoding. It requires a DLL, which I will write in C (I have just a test one right now).

When I compile the DLL with cl /I "java-path/include" /"java-path/include/win32" -DL -ML Main.c -FeTest.dll it compiles, but I get a 32-bit DLL. After I did some research on the internet, I found out that I would need a 64-bit DLL instead.

After more research, I have found this post which is the only one for C (even C++ was hard to find), but this only works if you are writing/building via Visual Studio 2010. I am using Elipse for the Java, CLion for the C, and compiling via the "Developer Command Prompt." so this does not work for me. How might I recompile as a 64-bit DLL?

EDIT: I am using the cl.exe that comes with Visual Studio 2017

UPDATE: I found the 64-bit cl.exe under C:\Program Files (x86)\Microsoft Visual Studio\2017\Community\VC\Tools\MSVC\14.13.26128\bin\Hostx64\x64\cl.exe, however when running it, I get an error that the library machine type (x86) conflicts with the target type (x64). How do I change the library machine type?

Answer 1

As I explained at the beginning of [SO]: How to build a DLL version of libjpeg 9b? (@CristiFati's answer) (bullets from 1. Prepare the ground section), there are different ways to deal with building from command line in VStudio.
I'm going to focus on vcvarsall.bat. More details on [MSDN]: Setting the Path and Environment Variables for Command-Line Builds (It's VStudio2015 as VStudio2017 link is broken).
I prepared a dummy example.

code.c:

#include <stdio.h>
#include "jni.h"


__declspec(dllexport) int func() {
    JavaVMInitArgs args;
    printf("Pointer size: %lld bits\n", sizeof(void*) * 8);
    printf("JNI_GetDefaultJavaVMInitArgs returned: %d\n", JNI_GetDefaultJavaVMInitArgs(&args));
    return 0;
}

Build:

e:\Work\Dev\StackOverflow\q050164687>"c:\Install\x86\Microsoft\Visual Studio Community\2017\VC\Auxiliary\Build\vcvarsall.bat" amd64
**********************************************************************
** Visual Studio 2017 Developer Command Prompt v15.6.6
** Copyright (c) 2017 Microsoft Corporation
**********************************************************************
[vcvarsall.bat] Environment initialized for: 'x64'

e:\Work\Dev\StackOverflow\q050164687>dir /b
code.c

e:\Work\Dev\StackOverflow\q050164687>cl /nologo /LD /I"c:\Install\x64\Oracle\Java\jdk1.8.0_152\include" /I"c:\Install\x64\Oracle\Java\jdk1.8.0_152\include\win32" /DWIN64 /DWIN32 code.c /link /LIBPATH:"c:\Install\x64\Oracle\Java\jdk1.8.0_152\lib" /OUT:dummy.dll jvm.lib
code.c
   Creating library code.lib and object code.exp

e:\Work\Dev\StackOverflow\q050164687>dir /b
code.c
code.exp
code.lib
code.obj
dummy.dll

Notes:

• My vcvarsall path is custom, because I installed VStudio2017 under "C:\Install\x86\Microsoft\Visual Studio Community\2017". Default path is "%SystemDrive%\Program Files (x86)\Microsoft Visual Studio\2017\Community"
• After running vcvarsall, I don't have to specify to cl.exe (or link.exe):
  • The full path
  • Build options (architecture specific, including paths)
  • I still have to specify things that it doesn't know about (like Java stuff)
• In order to test the newly built .dll, I'm going to use Python, as it's easier than writing another .c program that uses it
• Since I linked the .dll to jvm.lib, at runtime it will need jvm.dll, so I'm adding its path into %PATH%
• I built my code with VStudio2017 (VCRuntime14.0), but jvm.dll is linked to VCRuntime10.0 (VStudio2010), meaning that there will be (at least) 2 VCRuntimes loaded in my program. That is to be avoided as it could lead to all kinds of nasty problems

e:\Work\Dev\StackOverflow\q050164687>set PATH=%PATH%;c:\Install\x64\Oracle\Java\jdk1.8.0_152\jre\bin\server

e:\Work\Dev\StackOverflow\q050164687>"e:\Work\Dev\VEnvs\py35x64_test\Scripts\python.exe"
Python 3.5.4 (v3.5.4:3f56838, Aug  8 2017, 02:17:05) [MSC v.1900 64 bit (AMD64)] on win32
Type "help", "copyright", "credits" or "license" for more information.
>>> import ctypes
>>> dummy = ctypes.CDLL("dummy.dll")
>>> dummy.func()
Pointer size: 64 bits
JNI_GetDefaultJavaVMInitArgs returned: -1
0
>>>