[libusb] How do I add a log callback function via init options?

[Andreas J. <and...@gm...>]

Hi,

I'm Andy and new to the USB world. So, I'm trying my first baby steps
and just stumbled over a problem that I cannot solve:

I want to create a context like this:

    int res = libusb_init_context ( &ctx, opts, optionsLength );

Obviously I need options:

    int optionsLength = 3;
    struct libusb_init_option opts[optionsLength];

I can set most options successfully but not the one that takes a
callback function:

    void my_log_callback ( libusb_context *ctx, enum libusb_log_level
level, const char *str )

    opts[2] = ( struct libusb_init_option ) {
          LIBUSB_OPTION_LOG_CB,   { my_log_callback }
    };

    This fails with:
    error: invalid conversion from ‘void (*)(libusb_context*,
libusb_log_level, const char*)’ to ‘int’ [-fpermissive]

    I tried a function pointer:
    void ( *cbPtr ) ( libusb_context*, enum libusb_log_level, const
char* ) = my_log_callback;

    And:
    libusb_log_cb cb = cbPtr;


All with similar results. I'm using libusb 1.0.30 with C++.

What am I doing wrong? How do I properly pass on that function?