Segmentation fault when using va_list

[John Palmieri]

When using dbus_message_append_* api all arguments must be pointers:

You have this code:

    dbus_send_signal("/IO/Processor","Recorder.Set", "dviRecordingPolicy",
               DBUS_TYPE_UINT32,
               32,
               DBUS_TYPE_UINT32
               42);

You should have this code:

    int first_arg = 32;
    int second_arg = 42;
    dbus_send_signal("/IO/Processor","Recorder.Set", "dviRecordingPolicy",
               DBUS_TYPE_UINT32,
               &first_arg,
               DBUS_TYPE_UINT32
               &second_arg);

You need to be careful though because this code will also crash due to the way C handles asking for the address of an  array (it returns the address of the first element - in essences the same address as the array):

    char str_arg[] = "hello";
    dbus_send_signal("/IO/Processor","Recorder.Set", "dviRecordingPolicy",
               DBUS_TYPE_STR,
               &str_arg);

You would want to do one of two things here:

     char *str_arg = "hello"

or

     char str_arg[] = "hello";
     char *str_argp = str_arg;

Strange, and the cause of many frustrating errors, but that is the way C has defined the difference between arrays and pointer.

Check out the dbus_message_append_args documentation at http://dbus.freedesktop.org/doc/dbus/api/html/group__DBusMessage.html#gdf644b7816754d01040cce6dc232469d

It goes over the different cases.  There is also a link there to the code which uses append_valist in the background which is basically how you are using the API.

--
John (J5) Palmieri

----- "Priya Raghavendra" <Priya_Raghavendra at mindtree.com> wrote:

> Hi,
> 
> 
> 
> I am a newbie to DBUS. I am trying to create a signal which takes
> va_list as its arguments.
> 
> 
> 
> But the program crashes with “Segmentation Fault” when I execute this
> code.
> 
> 
> 
> The program crashes on this line of code
> 
> dbus_message_append_args_valist(message_ptr, first_arg_type, ap)
> 
> 
> 
> Find attached the complete listing of the test code.
> 
> 
> 
> Thanks,
> 
> Priya
> 
> 
> 
> 
> 
> http://www.mindtree.com/email/disclaimer.html
> 
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-- 
--
John (J5) Palmieri
Software Engineer
Red Hat, Inc.