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?

@c0degreen Not duplicate, but related: https://stackoverflow.com/questions/2548138/how-to-compile-x64-code-with-visual-studio-in-command-line — Unmanned Player, May 03 '18 at 22:51
@jdv I must clarify, it is the Visual Studio compiler (cl.exe). Do you know of an alternative I could use? — c0degreen, May 03 '18 at 23:25
Please [edit] the question and be clear what toolchain you are using. — , May 03 '18 at 23:26
Are you using a developer command prompt, or an x64 native tools command prompt? — Retired Ninja, May 03 '18 at 23:40
On my system, there are several choices in Programs / Visual Studio Tools / Windows Desktop Command Prompts. — prl, May 04 '18 at 02:37
@prl, I found the 84-bit version of cl.exe uder C:\Program Files (x86)\Microsoft Visual Studio\2017\Community\VC\Tools\MSVC\14.13.26128\bin\Hostx64\x64, however when I run it i get what I added in the update — c0degreen, May 04 '18 at 14:02
*vcvarsall.bat* (`"C:\Program Files (x86)\Microsoft Visual Studio\2017\Community\VC\Auxiliary\Build\vcvarsall.bat" amd64` - the *VC2017* location is a bit odd) is the file that will set all the appropriate env vars for building on different architectures from command line. Check https://stackoverflow.com/questions/44450813/how-to-build-a-dll-version-of-libjpeg-9b/44469099#44469099 (in the 1st part I explain how to use it). Regarding the error in the last comment, just delete the *.obj* file and re-run the command. — CristiFati, May 04 '18 at 14:13
@CristiFati please add that as an answer, as it fixed my problems. Thanks sooo much! — c0degreen, May 04 '18 at 14:20
Glad to hear that! Just one sec, as I'm preparing a small (and dummy) example. — CristiFati, May 04 '18 at 14:24
Possible duplicate of [How to compile x64 code with Visual Studio in command line?](https://stackoverflow.com/questions/2548138/how-to-compile-x64-code-with-visual-studio-in-command-line) — Andrew Henle, May 04 '18 at 15:34

score 1 · Accepted Answer · answered May 04 '18 at 15:28

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
>>>

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

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