So i have been looking into JNI calls so i can interact with some pre written C++ programs, i dont know any C++ but am trying to learn some basics. I have just been trying to do a simple call to a method outside my JNI method but always get the following error:
error c3861 'myMethod': identifier not found
#include <stdio.h>
#include <string.h>
#include "StringFuncs.h"
JNIEXPORT jstring JNICALL Java_StringFuncs_changeWord(JNIEnv *env, jobject obj, jstring inStr, jint inLen)
{
const char *inString;
inString = env->GetStringUTFChars(inStr, NULL);
char otherString[40];
strcpy_s(otherString,inString);
if(myMethod())
{
memset(otherString, '-', inLen);
}
jstring newString = env->NewStringUTF((const char*)otherString);
return newString;
}
bool myMethod()
{
return true;
}
int main()
{
return 0;
}
Any words of wisdome?
You have to declare your methods before you call them. So in your header type
bool myMethod();
Or you can move the code above your _changeWord function, then the declaration/definition is in one.
Move myMethod() above Java_StringFuncs_changeWord() in the source file.
In C++ you generally have to declare a symbol before you use it. So, somewhere before Java_StringFuncs_changeWord you need to declare myMethod:
bool myMethod();
If this is going to be a shared function (other cpp modules will call it) then you most likely want to put it in a header file which can be #included by other files. If the function only makes sense to be called by this module, you want to put the declaration at the top of the file, after the other #includes.
You can also declare and define the function in one go by moving the whole function above the function that calls it, but this wont always work (if you have two functions which reference eachother you have to have a separate declaration).
Related
I'm trying to use opencv C++ library with another lib in C. So I've defined a structure in an .hpp file which I've added to the .h of the C library
typedef struct {
cv::FileStorage fs;
cv::Mat mat;
} myCPPStruct;
#ifdef __cplusplus
extern "C" {
#endif
typedef struct {
myCPPStruct * mycppstruct;
} myCStruct;
void initialize_myCPPStruct(myCPPStruct * mycppstruct);
#ifdef __cplusplus
}
#endif
In the .c files the initializer for myCStruct calls initialize_myCPPStruct which is defined in a .cpp file somewhat as:
void initialize_myCPPStruct(myCPPStruct * mycppstruct){
mycppstruct = {};
mycppstruct->fs = cv::FileStorage( "file.txt", cv::FileStorage::READ );
mycppstruct->mat = cv::Mat(3,3,CV_8UC3);
}
But once it tries to allocate a value to mycppstruct->fs, it gets a segmentation fault. I assume this is because there is no memory allocation in C, but I've tried
std::memset(stitcher->fs,0,sizeof(cv::FileStorage));
which also doesn't work as it can't parse cv:FileStorage as void *.
Will I have to use the deprecated C OpenCV library to make this work?
EDIT - More details about the compilation.
For the C lib, I recompile (without linking ie with the -c option)all the .c files where I've added C++ functions or structures using g++ while making sure I add the __cplusplus guards in the .h files. All the .c files without C++ code already have .o files compiled with gcc. I then compile the whole program with g++ while making sure to include the relevant library files. There are no compiler errors.
It seems the structure itself has not been created.
You might try this (or something similar):
myCPPStruct* initialize_myCPPStruct()
{
myCPPStruct* result;
result = new myCPPStruct();
result->fs = cv::FileStorage( "file.txt", cv::FileStorage::READ );
result->mat = cv::Mat(3,3,CV_8UC3);
return (result);
}
You need to allocate the memory for your structure:
myCPPStruct = malloc(sizeof(myCPPStruct));
Use the above line instead of:
mycppstruct = {};
Also,since your function is passing in the pointer and allocating the memory internally you need to pass a pointer to the pointer or the allocation will not be passed back:
void initialize_myCPPStruct(myCPPStruct** mycppstruct){
if ( mycppstruct == NULL ) {
//Abort as no address of the pointer to allocate passed
return;
}
*myCPPStruct = malloc(sizeof(myCPPStruct));
(*mycppstruct)->fs = cv::FileStorage( "file.txt", cv::FileStorage::READ );
(*mycppstruct)->mat = cv::Mat(3,3,CV_8UC3);
}
Or you could just change the above to:
myCPPStruct* initialize_myCPPStruct(void) {
myCPPStruct* ptr = malloc(sizeof(myCPPStruct));
ptr->fs = cv::FileStorage( "file.txt", cv::FileStorage::READ );
ptr->mat = cv::Mat(3,3,CV_8UC3);
return ptr;
}
First, let me point out the sentence which confuse me.
there is no memory allocation in C
Yes, there is. Have a look to malloc function.
char *my_allocated_string = malloc(sizeof(char) * 42);
Here you are, you've allocated an array of character of size 42.
Now, have a look to : mycppstruct = {};
That's not how you allocate a structure in C. You have to call... malloc() !
my_struct *s = malloc(sizeof(my_struct));
s->fs = 42;
Ok. Done. Feels better.
Well, first of all, you have to create a wrapper around your C++ code. Why ? Because C++ allows multiple definition of a given function (also call symbol) :
int my_func(int);
int my_func(char);
This is valid in C++. But, think about it, how is the compiler able to let two functions with the same name exist ? Well it's not. It uses a technique named mangling when evaluating the functions to create 2 different names. Mangling is use on everything function and method. Event on single functions.
C is not able (and willing) to create several functions with the same name. Otherwise, you will experience a kind of function-already-implemented error. When you declare :
int my_func(int);
the C compiler will create the symbol : my_func. No mangling.
To make both language interact, you need to reference to a symbol understandable by the C compiler. If you call my_func from a C source file, the C compiler will look for my_func symbol. Bu since C++ will modify its my_func symbol into something like _ZN9myfuncE, the linkage will fail. That's why you have to says to the C++ compiler to not use mangling on the function you expose to C. That's why you need extern "C" { }.
Feeww, so far so good...
Now you have to embed you C API into an extern "C" block :
In my_c_api.h :
void my_func(int);
void my_func(char);
In my_c_api.cpp :
#include "my_c_api.h"
extern "C" void my_func_i(int i) { my_func(i); }
extern "C" void my_func_c(char c) { my_func(c); }
You compile it to create your C++ library.
In your C compilation pipeline, you link against your new C++ library and header files. Then :
#include "<path_to_my_c_api.h>"
void c(int i,char s)
{
my_func_i(i);
my_func_c(c);
}
You cannot compile C++ code with C compiler. You have to compile te C++ code independently.
More on ISO CPP.
I have a significant library of classes written in C++. I'm trying to make use of them through some type of bridge within Swift rather than rewrite them as Swift code. The primary motivation is that the C++ code represents a core library that is used on multiple platforms. Effectively, I'm just creating a Swift based UI to allow the core functionality to work under OS X.
There are other questions asking, "How do I call a C++ function from Swift." This is not my question. To bridge to a C++ function, the following works fine:
Define a bridging header through "C"
#ifndef ImageReader_hpp
#define ImageReader_hpp
#ifdef __cplusplus
extern "C" {
#endif
const char *hexdump(char *filename);
const char *imageType(char *filename);
#ifdef __cplusplus
}
#endif
#endif /* ImageReader_hpp */
Swift code can now call functions directly
let type = String.fromCString(imageType(filename))
let dump = String.fromCString(hexdump(filename))
My question is more specific. How can I instantiate and manipulate a C++ Class from within Swift? I can't seem to find anything published on this.
I've worked out a perfectly manageable answer. How clean you'd like this to be is entirely based upon how much work you're willing to do.
First, take your C++ class and create C "wrapper" functions to interface with it. For example, if we have this C++ class:
class MBR {
std::string filename;
public:
MBR (std::string filename);
const char *hexdump();
const char *imageType();
const char *bootCode();
const char *partitions();
private:
bool readFile(unsigned char *buffer, const unsigned int length);
};
We then implement these C++ functions:
#include "MBR.hpp"
using namespace std;
const void * initialize(char *filename)
{
MBR *mbr = new MBR(filename);
return (void *)mbr;
}
const char *hexdump(const void *object)
{
MBR *mbr;
static char retval[2048];
mbr = (MBR *)object;
strcpy(retval, mbr -> hexdump());
return retval;
}
const char *imageType(const void *object)
{
MBR *mbr;
static char retval[256];
mbr = (MBR *)object;
strcpy(retval, mbr -> imageType());
return retval;
}
The bridge header then contains:
#ifndef ImageReader_hpp
#define ImageReader_hpp
#ifdef __cplusplus
extern "C" {
#endif
const void *initialize(char *filename);
const char *hexdump(const void *object);
const char *imageType(const void *object);
#ifdef __cplusplus
}
#endif
#endif /* ImageReader_hpp */
From Swift, we can now instantiate the object and interact with it like so:
let cppObject = UnsafeMutablePointer<Void>(initialize(filename))
let type = String.fromCString(imageType(cppObject))
let dump = String.fromCString(hexdump(cppObject))
self.imageTypeLabel.stringValue = type!
self.dumpDisplay.stringValue = dump!
So, as you can see, the solution (which is actually rather simple) is to create wrappers that will instantiate an object and return a pointer to that object. This can then be passed back into the wrapper functions which can easily treat it as an object conforming to that class and call the member functions.
Making It Cleaner
While this is a fantastic start and proves that it is completely feasible to use existing C++ classes with a trivial bridge, it can be even cleaner.
Cleaning this up would simply mean that we remove the UnsafeMutablePointer<Void> from the middle of our Swift code and encapsulate it into a Swift class. Essentially, we use the same C/C++ wrapper functions but interface them with a Swift class. The Swift class maintains the object reference and essentially just passes all method and attribute reference calls through the bridge to the C++ object!
Having done this, all of the bridging code is completely encapsulated in the Swift class. Even though we are still using a C bridge, we are effectively using C++ objects transparently without having to resort to recoding them in Objective-C or Objective-C++.
Swift has no C++ interop currently. It's a long-term goal, but is very unlikely to happen in the near future.
In addition to your own solution, there is another way to do it. You can call or directly write C++ code in objective-c++.
So you can create an objective-C++ wrapper on top of your C++ code and create a suitable interface.
Then call objective-C++ code from your swift code. To be able to write objective-C++ code you may have to rename file extension from .m to .mm
Do not forget to release memory allocated by your C++ objects when suitable.
You can use Scapix Language Bridge to automatically bridge C++ to Swift (among other languages). Bridge code automatically generated on the fly directly from C++ header files. Here is an example:
C++:
#include <scapix/bridge/object.h>
class contact : public scapix::bridge::object<contact>
{
public:
std::string name();
void send_message(const std::string& msg, std::shared_ptr<contact> from);
void add_tags(const std::vector<std::string>& tags);
void add_friends(std::vector<std::shared_ptr<contact>> friends);
};
Swift:
class ViewController: UIViewController {
func send(friend: Contact) {
let c = Contact()
contact.sendMessage("Hello", friend)
contact.addTags(["a","b","c"])
contact.addFriends([friend])
}
}
As another answer mentioned, using ObjC++ to interact is much easier. Just name your files .mm instead of .m and xcode/clang, gives you access to c++ in that file.
Note that ObjC++ does not support C++ inheritance. I you want to subclass a c++ class in ObjC++, you can't. You will have to write the subclass in C++ and wrap it around an ObjC++ class.
Then use the bridging header you would normally use to call objc from swift.
This question already has answers here:
Calling C++ dll from Java
(3 answers)
Closed 8 years ago.
I have one c++ dll which is previously used for c# application. now we want to use the same dll for java . i know that we can use JNI technology for this but the problem we have to use the same method signature we don't want to change the method singnature. please advise me.
One option is using JNA instead of JNI. It eliminates the need for the boilerplate native code. An example would look something like this...
import com.sun.jna.Library;
import com.sun.jna.Native;
public class Example {
public interface NativeMath extends Library {
public bool isPrime(int x);
}
public static void main(String[] args) {
int x = 83;
NativeMath nm = (NativeMath) Native.loadLibrary("nm", NativeMath.class);
System.out.println(x + " is prime: " + nm.isPrime(x));
}
}
You don't have to change the method signature, you simply add a native method which then calls the native C++ code. Here is a simple example:
public class Someclass
{
public native void thisCallsCMethod(Someparams);
}
Now create the JNI wrappers:
javac Someclass.java
javah Someclass
This will create a Someclass.h, then you create Someclass.cpp and include the .h in it.
At this point all you have to do is write the C/C++ code for thiCallsCMethod
In the .h you'll see a method signature that you have to implement. Something along the lines of:
#include "clibraryHeader.h"
using namespace clibraryNamespace;
JNIEXPORT void JNICALL thisCallsCMethod(JNIEnv *, someparameters)
{
cout<<"Yeah C code is being called"<<endl;
someCfunction();
}
Obviously you have to massage the parameters in the JNI call, but you can create some temporary variables, then copy back the values you get from the C calls into the incoming parameters (if they need to be returned) etc.
Maybe:
#include "clibraryHeader.h"
using namespace clibraryNamespace;
JNIEXPORT void JNICALL thisCallsCMethod(JNIEnv *, someparameters)
{
cout<<"Yeah C code is being called"<<endl;
Cstruct temp;
temp1.somevar = param1.getSomeVal()
someCfunction(temp);
}
I am exporting a method that can be called from unmanaged code, but the function itself lives in a managed c++ project. The following code results in a compiler error:
error C2526: 'System::Collections::Generic::IEnumerator<T>::Current::get' : C linkage function cannot return C++ class 'System::Collections::Generic::KeyValuePair<TKey,TValue>'
error C2526: 'System::Collections::Generic::Dictionary<TKey,TValue>::KeyCollection::GetEnumerator' : C linkage function cannot return C++ class 'System::Collections::Generic::Dictionary<TKey,TValue>::KeyCollection::Enumerator'
extern "C"
__declspec( dllexport )
bool MyMethod(std::string &name, std::string &path, std::map<std::string, std::string> &mymap)
{
System::Collections::Generic::Dictionary<System::String ^, System::String^> _mgdMap = gcnew System::Collections::Generic::Dictionary<System::String ^, System::String ^>();
// Blah blah processing
}
Looking into this error a little bit, the issue normally has to do with the definition of the method that is marked as 'extern "C"'. So why would it be at all concerned with what goes on inside the method?
If I comment out the Dictionary initialization, or switch it to a HashTable everything compiles beautifully.
The following works as well - if instead of defining a dictionary locally, I avoid the local variable by initializing it in a method.
bool status = CallAnotherMethod(ConvertToDictionary(mymap));
where ConvertToDictionary is declared as
System::Collections::Generic::Dictionary<System::String ^, System::String ^>^ ConvertToDictionary(std::map<std::string, std::string> &map)
{
}
Which tells me that this is a seemingly arbitrary error. I would still like to understand why the compiler thinks this is a problem.
Sorry for necroposting but i need to share my happiness with somebody :)
Seems that you are able to solve this just by creating two wrappers - one for external call marked with "extern C" and one for internal call. In this case everything outside "extern C" will be executed as usual .NET code.
That was answered by the topicstarter here - C++/CLI->C# error C2526: C linkage function cannot return C++ class
void DummyInternalCall(std::string &name, std::string &path, std::map<std::string, std::string> &mymap)
{
System::Collections::Generic::Dictionary<System::String ^, System::String^> _mgdMap = gcnew System::Collections::Generic::Dictionary<System::String ^, System::String ^>();
// Blah blah processing
}
extern "C" __declspec( dllexport )
bool MyMethod(std::string &name, std::string &path, std::map<std::string, std::string> &mymap)
{
DummyInternalCall(name, path, mymap);
}
If you write a function in C++ that is to be called from C, you can't use anything in it's interface (arguments and return type) that isn't plain C. Here all your arguments are C++ objects.
I am working on a plugin system in C++ whereby a C++ executable loads a dll and runs plugin_start(someclass&) via GetProcAddress.
I fully understand how to pass function pointers to the dll, and visa versa, and how the dll may use anything defined in a header file, but I would like the dll to be able to use someclass where someclass is declared in someclass.h BUT DEFINED in someclass.cpp.
The catch is, someclass is compiled into the calling executable which means when the dll tries to call a function it gets a linker error. I even understand why this is, what I don't understand is how to achieve what I want.
I imagine I can pass a pointer to the object, and a pointer to the function ie someclass* somefunction* and then call it as someclass->*somefunction() but this means I would have to pass a pointer to every function in every class.
Is there an easier way to do this, or should I stick to C-style functions and function pointers alone and forget trying to pass entire classes between the two?
Thanks,
Ben
#ifndef EVENTREGISTRAR_H
#define EVENTREGISTRAR_H
#include <vector>
typedef void (__stdcall *error_callback_t)(const char *error);
class EventRegistrar
{
public:
void OnError(error_callback_t fn);
void FireError(const char *error);
private:
std::vector<error_callback_t> errors;
};
#endif
-- Cpp
#include "PluginLoader.h"
void EventRegistrar::OnError(error_callback_t fn)
{
this->errors.push_back(fn);
}
void EventRegistrar::FireError(const char *error)
{
for (std::vector<error_callback_t>::iterator it = this->errors.begin();
it != this->errors.end(); ++it)
{
(*it)(error);
}
}
-- DLL
#include "../plugin.h"
#include <stdio.h>
void __stdcall error(const char *error) { printf("Error: %s\n",error); }
extern "C" int __stdcall plugin_start(plugin_start_data& data)
{
error_callback_t fn = error;
data.events.OnError(fn);
return LOAD_SUCCESS;
}
--Error
Error 1 error LNK2001: unresolved external symbol "public: void __thiscall EventRegistrar::OnError(void (__stdcall*)(char const *))" (?OnError#EventRegistrar##QAEXP6GXPBD#Z#Z) D:\Files\C++ Workspace\BLib\BLib\Example Plugin\main.obj Example Plugin
I did something like this a long time ago. I simply used a straight C interface to keep things simple.
There may be a better way but I think passing a pointer to the object is your best and most straight-forward approach.