I'm wrapping a C API with a C++/CLI wrapper in order to consume it from a C# project.
The C API requires a callback for one of its functions. The callback signature takes a void* user data parameter.
I would like to pass in this as the void* user data parameter to the C API but I have been unsuccessful (failing to compile). Furthermore it's not as easy to pass in 'this' as this is referring to a managed class, so of course I'd have to pin the reference first prior to passing it in.
I'm also unsure how to cast the void* parameter back to a ManagedClass^ in the callback function.
Code:
public ref class ManagedClass
{
public:
static void __stdcall CallbackFunc( int, void* userData )
{
// How can I cast the void* parameter back to
// ManagedClass^ ??
}
void Open( )
{
// I know I need to pin the reference to myself
// but I'm unsure how to go about it. How can I store
// this in a pin_ptr??
ptr = this;
// Open the third party C-API passing in
// my callback function and a reference to
// myself.
thirdPartyApiOpen( &CallbackFunc, ptr );
}
private:
pin_ptr<ManagedClass^> ptr;
};
Related
Condition
I use a framework that has an custom type as bellow:
typedef log (*CustomType) (
int timeStamp,
const char* data,
int dataSize,
void* userData,
int dataType,
int viewId
)
and MyClass init method as bellow:
MyClass_Init (void **output, CustomType video, CustomType audio, void* userData)
Question
I used init method like bellow but always receive error (error content is not displayed because i use a framework). pls point me what is missed.
CustomType videoInput;
CustomType audioInput;
void *output = malloc(sizeof(void*);
void *userData = malloc(sizeof(void*));
long result = MyClass_Init(&output, videoInput, audioInput, userData);
A number of things wrong with this code:
You can't intermix function pointers and method pointers. What it boils down to is that the this for a method has to be included in the method call signature. Since the function pointer doesn't include the this pointer (it is a function, not a method pointer), the two can not match.
Most C-based API includes some sort of reference value (most frameworks call those refCon, context or userData), so what you can do is create an adapter function that calls your method. The userData parameter in your CustomType parameter list looks like it is one of those (consult the docs to be sure).
You can probably provide a userData wherever you set MyClass_Init as your callback now. So, if that function to provide a callback to the library was called set_callback( MyCustomType callback, void* userData ), do something like
MyClass *obj = new MyClass; // Or however you create your object
set_callback( MyClassCallbackAdapterFunction, obj );
with an adapter function like:
log MyClassCallbackAdapterFunction( int timeStamp, const char* data, int dataSize, void* userData, int dataType, int viewId )
{
MyClass *myThis = (MyClass*) userData;
// Here you can now call myThis->MyClass_Init( ... ) however you want to.
}
The malloc( sizeof(void*) ) statements look like you're misunderstanding return parameters (also called "side effects" by some teachers). I don't have the docs to whatever API/library you're using, but I'm pretty certain you're supposed to not just pass in buffers the size of a pointer. Either you'd just provide a pointer on the stack in which a buffer will be returned, or you provide a whole buffer (e.g. an array) and its size, and that is where the callback will write to or so.
I have been struggling for days to figure out the probably obvious reason why i cant get my code to compile.
I have a class (based on wxThread) where the callback is defined:
-- Header file --
class TestClass : public wxThread
{
private:
static void WlanNotification(WLAN_NOTIFICATION_DATA *wlanNotifData, VOID *p);
};
-- Code file --
I call the WlanRegisterNotification function, that needs the above callback function as a parameter:
dwResult = WlanRegisterNotification(hClient, WLAN_NOTIFICATION_SOURCE_ALL, true, (WLAN_NOTIFICATION_CALLBACK) WlanNotification, this, 0, &dwPrevNotif);
This compiles and works fine, but the problem is the function is marked as static, so i cant access my non static stuff from the callback (which i need for other reasons).
I have tried every single combination i can think of to pass in the callback as non static:
-- Header file --
void WINAPI WlanNotification(PWLAN_NOTIFICATION_DATA data, PVOID context);
-- Code file --
dwResult = WlanRegisterNotification(hClient, WLAN_NOTIFICATION_SOURCE_ALL, true, (WLAN_NOTIFICATION_CALLBACK)WlanNotification, this, 0, &dwPrevNotif);
i just get:
error C2660: 'WlanRegisterNotification' : function does not take 6
arguments
error C2440: 'type cast' : cannot convert from 'overloaded-function'
to 'WLAN_NOTIFICATION_CALLBACK'
I'm thinking its related to the typedef somehow:
typedef VOID (WINAPI *WLAN_NOTIFICATION_CALLBACK) (PWLAN_NOTIFICATION_DATA, PVOID);
I have tried googling for examples of using the WlanRegisterNotification function, but none of the examples i could find is calling it from a class, which is what seems to be an issue here, so i'm really lost.
A non-static class method has a hidden this parameter that the callback is not expecting let alone know how to fill in. That is why you cannot use it as a callback unless you either 1) use static to remove that parameter, or 2) create a thunk to use as the actual callback and then have it internally delegate to a non-static class method. Remember that the Windows API is designed for C, not C++. There are no classes or implicit this pointers in C.
In this case, a static callback can access non-static members of your class because you are explicitly passing the object's this pointer as the pCallbackContext of WlanRegisterNotification(), which is then passed as-is to the context of the callback:
class TestClass : public wxThread
{
private:
static VOID WINAPI WlanNotification(PWLAN_NOTIFICATION_DATA wlanNotifData, PVOID context);
};
VOID WINAPI TestClass::WlanNotification(PWLAN_NOTIFICATION_DATA wlanNotifData, PVOID context)
{
TestClass *pThis = (TestClass*) context;
// use pThis-> to access non-static members as needed..
}
// get rid of the typecast when passing the callback. If it does
// not compile, then it is not declared in a compatible manner...
dwResult = WlanRegisterNotification(hClient, WLAN_NOTIFICATION_SOURCE_ALL, TRUE, &WlanNotification, this, 0, &dwPrevNotif);
is there a possibility or a workaround to pass a member function to the Windows API function QueueUserAPC()?
Okay, I could pass a static member function.
But then I won't have full access to local member variables...
So is there a possibility to combine both, passing as member function and full access to non-static member variables?
I tried to work out a solution related to this but without any success yet.
Hopefully someone got an idea to solve this.
This is a kind of standard pattern to use when having C-style callbacks call your C++ functions.
You create a free function (or static member) that forwards the call ...
VOID CALLBACK ForwardTo_MyClass_func( _In_ ULONG_PTR dwParam )
{
auto* p = (MyClass*)dwParam;
p->func();
}
... and you then set it up by passing the instance pointer as the third parameter to QueueUserAPC:
QueueUserAPC( ForwardToMyClass_func, hThread, (ULONG_PTR)pMyClass );
If you need further arguments, you will have to create some kind of structure to hold both the instance pointer and the arguments.
The answer is no.
Windows API has a C interface, and therefor cannot handle name mangled signatures, such as C++ member functions. The function you pass must be a C style free function.
By the way, nesting it in a namespace is acceptable, if less scalable:
namespace apc_stuff
{
static MyStruct some_static_data;
static void __stdcall MyApcFunc(ULONG_PTR data); // PAPCFUNC
};
using namespace apc_stuff;
MyClass::DoSomething(...)
{
auto my_data = new MyData(...);
auto data = reinterpret_cast<ULONG_PTR>(my_data);
QueueUserAPC(MyApcFunc, hThread, data)
}
/*static*/ void __stdcall apc_stuff::MyApcFunc(ULONG_PTR data)
{
auto my_data = reinterpret_cast<MyData *>(data);
//
// use my_data
// use some_static_data
//
}
I am using libusb to interact with a usb device. I have created a Class and a member function will receive the input from the device and process it. I am using Asynchronous api. Now the libusb_fill_bulk_transfer() function call throws a compilation error:
void MyDeviceClass::on_connectButton_clicked()
{
int r; //for return values
ssize_t cnt; //holding number of devices in list
r = libusb_init(&ctx); //initialize the library for the session we just declared
if(r < 0) {
qDebug()<<"Init Error "<<r<<endl; //there was an error
return;
}
...
...
...
libusb_fill_bulk_transfer( transfer_in, dev_handle, USB_ENDPOINT_IN,
inBuffer, LEN_IN_BUFFER,readDataFromBuffer,NULL,0);
libusb_submit_transfer(transfer_in);
QtConcurrent::run (this,&MyDeviceClass::eventThread);
}
The compiler suggests using &MyDeviceClass::readDataFromBuffer as a function pointer but that still throws an error. I tried using static members, and even static non-member functions but all in vain. Please help me in passing the callback to this function. I am using Qt, and probably won't like to use boost libraries.
Please note that libusb_fill_bulk_transfer(...) is invoked inside a member function of MyDeviceClass.
I made the member function static and prototype needed to be modified:
static void LIBUSB_CALL readDataFromBuffer(struct libusb_transfer *);
what is the compile error when you use a non member function?
Edit: this should be LIBUSB_CALL:
void LIBUSB_CALL BulkTransferCallback(struct libusb_transfer *transfer) {
// This assumes that you set transfer::user_data to be a pointer to
// an instance of the MyDeviceClass class.
MyDeviceClass* mdc = reinterpret_cast<MyDeviceClass*>(transfer->user_data);
mdc->onBulkTransferCallback(transfer);
}
Then when you set the pointer, use:
transfer.callback = &BulkTransferCallback;
As you answered yourself, actually it should be __stdcall which is defined by LIBUSB_CALL. Their documentation should include this in the function pointer typedef to remove the ambiguity.
The lib_usb api defines the function libusb_fill_bulk_transfer with:
static void libusb_fill_bulk_transfer (struct libusb_transfer *transfer, libusb_device_handle *dev_handle, unsigned char endpoint, unsigned char *buffer, int length, libusb_transfer_cb_fn callback, void *user_data, unsigned int timeout)
It is possible to add a static class function to your class and call the non-static member-function
of the class within this static function.
You this Object would be at the place of void *user_data.
In the static class function you would have to cast the member of
transfer->user_data correctly to your class.
Using C++.
pthread_t threads[STORAGE]; // 0-99
...
void run()
Error>>> int status = pthread_create(&threads[0], NULL, updateMessages, (void *) NULL);
if (status != 0)
{
printf("pthread_create returned error code %d\n", status);
exit(-1);
}
...
void ClientHandler::updateMessages(void *)
{
string reqUpdate = "91"; // Request for update
string recvMSG;
while (true)
{
sleep(5);
sending(sock,reqUpdate); // send
recvMSG = receiving(sock); // receive
QString output(recvMSG);
emit signal_chat(output, 0); // Print message to text box
}
}
...
Compile Error:
TCPClient.cpp:109: error: argument of type ‘void (ClientHandler::)(void*)’ does not match ‘void* (*)(void*)’
I can't figure out whats wrong.
Thanks in advance.
A pointer to a member function is different from a global function with the same signature since the member function needs an additional object on which it operates. Therefore pointers to these two types of functions are not compatible.
In this case this means that you cannot pass a member function pointer to pthread_create but only a pointer to a non-member (or static) function. A work around for this problem is to use the forth parameter of pthread_create to pass a pointer to a object to a global function which then calls the method of the passed object:
class ClientHandler {
public:
void updateMessages();
void run();
};
// Global function that will be the threads main function.
// It expects a pointer to a ClientHandler object.
extern "C"
void *CH_updateMessages(void *ch) {
// Call "real" main function
reinterpret_cast<ClientHandler*>(ch)->updateMessages();
return 0;
}
void ClientHandler::run() {
// Start thread and pass pointer to the current object
int status = pthread_create(&threads[0], NULL, CH_updateMessages, (void*)this);
...
}
It's nothing to do with threads, it's a normal C++ error, you're just passing an incompatible type of function pointer.
A function pointer is not the same as a member instance function pointer, even if their signature is the same; this is because there is an implicit reference to *this passed. You can't avoid this.
As pthread_create takes a free function, create a static function(is a free function) inside ClientHandler
static void Callback(void * this_pointer,int other_arg) {
ClientHandler* self = static_cast< ClientHandler*>(this_pointer);
self-> updateMessages(other_arg);
}
and call pthread_create as follows
pthread_create(&threads[0], NULL, &ClientHandler::Callback, (void *) pointer_to_ClientHandler,int other_arg);
That works because Callback is free function
YoLinux has a nice pthread tutorial that my help you in learning about threads.
As others have already said, the problem is that the signatures between the functions are different. Class member functions always have a "secret" extra parameter, the this pointer. So you can never pass a member function where a global function is expected. You can hack around this either with libraries such as Boost.Bind, or by making the function a static member of the class.
But the simplest, and most elegant solution is to use a different threading API.
Boost.Thread is a very nice threading library for C++ (pthreads is designed for C, and that's why it doesnt play well with C++ features such as class methods).
I'd recommend using that.
Your code could be rewritten as something like this:
class ClientHandler {
public:
ClientHandler(/* All the parameters you want to pass to the thread. Unlike pthreads you have complete type safety and can pass as many parameters to this constructor as you like */){...}
void operator()() // boost.thread calls operator() to run the thread, with no parameters. (Since all parameters were passed in the constructor and saved as member variables
{
string reqUpdate = "91"; // Request for update
string recvMSG;
while (true)
{
sleep(5);
sending(sock,reqUpdate); // send
recvMSG = receiving(sock); // receive
QString output(recvMSG);
emit signal_chat(output, 0); // Print message to text box
}
}
// whatever arguments you want to pass to the thread can be stored here as member variables
};
boost::threead_group gr; // can store all your threads here, rather than being limited to your fixed-size array
gr.create_thread(ClientHandler(/* construct a ClientHandler object with the parameters you like*/));
You're passing a member function instead of a global, normal, one.
Just define:
void updateMessages(void *) {
static ClientHandler c;
// use c..
}