Error in function argument libusb, works in codeblock but no in class at Qt

[aurquiel]

I have an example for hotplug USB that works in codeblocks, but when i tried to ported the functions to a class in qt i got a invalid parameter in a function.

Every time that i disconnect the device with the vendorid=1256 productid=26720 the count variable set value 2 and the program exit, you can define your own vendorid and productid. If the device is plug in the program waits until unplog.

Codeblocks y link the compiler to -lusb-1.0

#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <libusb-1.0/libusb.h>

static int count = 0;
int hotplug_callback(struct libusb_context *ctx, struct libusb_device *dev,
                     libusb_hotplug_event event, void *user_data) {
  static libusb_device_handle *handle = NULL;
  struct libusb_device_descriptor desc;
  int rc;
  (void)libusb_get_device_descriptor(dev, &desc);
  if (LIBUSB_HOTPLUG_EVENT_DEVICE_ARRIVED == event) {
    rc = libusb_open(dev, &handle);
    if (LIBUSB_SUCCESS != rc) {
      printf("Could not open USB device\n");
    }
  } else if (LIBUSB_HOTPLUG_EVENT_DEVICE_LEFT == event) {
    if (handle) {
      libusb_close(handle);
      handle = NULL;
    }
  } else {
    printf("Unhandled event %d\n", event);
  }
  count++;
  return 0;
}
int main (void) {
  libusb_hotplug_callback_handle handle;
  int rc;
  libusb_init(NULL);
  rc = libusb_hotplug_register_callback(  NULL, (libusb_hotplug_event)(LIBUSB_HOTPLUG_EVENT_DEVICE_ARRIVED | LIBUSB_HOTPLUG_EVENT_DEVICE_LEFT),
 LIBUSB_HOTPLUG_ENUMERATE,
                                           1256,
                                           26720,
                                            LIBUSB_HOTPLUG_MATCH_ANY, 
                                            hotplug_callback,
                                            NULL,
                                            &handle);
  if (LIBUSB_SUCCESS != rc) {
    printf("Error creating a hotplug callback\n");
    libusb_exit(NULL);
    return EXIT_FAILURE;
  }
  while (count < 2) {
    libusb_handle_events_completed(NULL, NULL);
    usleep(10000);
  }
  libusb_hotplug_deregister_callback(NULL, handle);
  libusb_exit(NULL);
  return 0;
}

That works great in a console.

Then moving to a QT class

header

#ifndef BACKEND_H
#define BACKEND_H
#include <QObject>
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <libusb-1.0/libusb.h> //Include libsub

class backend: public QObject
{
    Q_OBJECT
public:
    backend();

    libusb_hotplug_callback_handle handle;

    int hotplug_callback(struct libusb_context *ctx, struct libusb_device *dev,
                         libusb_hotplug_event event, void *user_data);


    static int count;
    void hot();
};


#endif // BACKEND_H

**body **

#include "backend.h"

backend::backend()
{

}

int backend::count=0;

void backend::hot()
{
    int rc;
    libusb_init(NULL);
    rc = libusb_hotplug_register_callback(  NULL, (libusb_hotplug_event) (LIBUSB_HOTPLUG_EVENT_DEVICE_ARRIVED | LIBUSB_HOTPLUG_EVENT_DEVICE_LEFT),
                                            LIBUSB_HOTPLUG_ENUMERATE,
                                            1256,
                                            26720,
                                            LIBUSB_HOTPLUG_MATCH_ANY,
                                            hotplug_callback,
                                            NULL,
                                            &handle);
    if (LIBUSB_SUCCESS != rc) {
        libusb_exit(NULL);
    }
    while (count < 2) {
        libusb_handle_events_completed(NULL, NULL);
        usleep(10000);
    }
    libusb_hotplug_deregister_callback(NULL, handle);
    libusb_exit(NULL);
}

int backend::hotplug_callback(struct libusb_context *ctx, struct libusb_device *dev,
                     libusb_hotplug_event event, void *user_data)
{
  static libusb_device_handle *handle = NULL;
  struct libusb_device_descriptor desc;
  int rc;
  (void)libusb_get_device_descriptor(dev, &desc);
  if (LIBUSB_HOTPLUG_EVENT_DEVICE_ARRIVED == event) {
    rc = libusb_open(dev, &handle);
  } else if (LIBUSB_HOTPLUG_EVENT_DEVICE_LEFT == event) {
    if (handle) {
      libusb_close(handle);
      handle = NULL;
    }
  }
  count++;
  return 0;
}

At this point try to compile and get

/home/zen/QT/hotplug_test/backend.cpp:21: error: invalid use of non-static member function
&handle);
^

I have a problem with handel argument but i don't know why

[SGaist]

Hi,

Because you are trying to pass a member variable as a callback. You can't, your callback should be a free function.