accessing Double pointer

Question

typedef struct _WDF_USB_DEVICE_SELECT_CONFIG_PARAMS {

ULONG Size;

WdfUsbTargetDeviceSelectConfigType Type;

union {

struct {
  PUSB_CONFIGURATION_DESCRIPTOR  ConfigurationDescriptor;
  PUSB_INTERFACE_DESCRIPTOR*  InterfaceDescriptors;
  ULONG NumInterfaceDescriptors;
} Descriptor;
struct {
  PURB  Urb;
} Urb;
struct {
  UCHAR  NumberConfiguredPipes;
  WDFUSBINTERFACE  ConfiguredUsbInterface;
} SingleInterface;
struct {
  UCHAR  NumberInterfaces;
  PWDF_USB_INTERFACE_SETTING_PAIR  Pairs;
  UCHAR  NumberOfConfiguredInterfaces;
} MultiInterface;

} Types;

} WDF_USB_DEVICE_SELECT_CONFIG_PARAMS, *PWDF_USB_DEVICE_SELECT_CONFIG_PARAMS;

WDF_USB_DEVICE_SELECT_CONFIG_PARAMS params;

typedef struct _USB_INTERFACE_DESCRIPTOR {

UCHAR bLength ;

UCHAR bInterfaceClass ;

UCHAR bInterfaceSubClass ;

} USB_INTERFACE_DESCRIPTOR, *PUSB_INTERFACE_DESCRIPTOR ;

Able to acess NumInterfaceDescriptors via -> params.Types.Descriptor.NumInterfaceDescriptors

I want to acess bInterfaceClass via WDF_USB_DEVICE_SELECT_CONFIG_PARAMS . Please note that this structure is filled by the library I have to just access it

Why do you want the type IntDesc to be a double pointer? typedef PUSB_INTERFACE_DESCRIPTOR* IntDesc; Wouldn't it be easier just to use a single pointer? What's the benefit of another layer of indirection here? — Tim Lovell-Smith
– Tim Lovell-Smith, Commented Jan 18, 2010 at 7:07
Hey, dude... you've changed the question significantly two times now, each time breaking previously valid answers, and each time breaking the layout others had fixed for you previously. I have very little patience with people who are that careless with other people's time. -1 from me, and I won't bother to edit / answer your question again. — DevSolar
– DevSolar, Commented Jan 18, 2010 at 13:49
Oh, and by the way, this is now a duplicate to stackoverflow.com/questions/2085761/… — DevSolar
– DevSolar, Commented Jan 18, 2010 at 13:54

Ignacio Vazquez-Abrams · Accepted Answer · 2010-01-18 05:41:20Z

6

(*someIntDesc)->iInterface

answered Jan 18, 2010 at 5:41

Ignacio Vazquez-Abrams

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Comments

DevSolar · Accepted Answer · 2010-01-18 08:20:56Z

2

IntDesc foo;
// somehow initialize foo to actually point to (a pointer to) a valid structure
(*foo)->iInterface = 10;

edited Jan 18, 2010 at 8:20

answered Jan 18, 2010 at 5:43

DevSolar

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1 Comment

DevSolar Over a year ago

You're right, of course. I shouldn't take linguistic shortcuts.

akif · Accepted Answer · 2010-01-18 06:08:58Z

1

Deference it like this

(*intDesc)->iInterface

edited Jan 18, 2010 at 6:08

answered Jan 18, 2010 at 5:44

akif

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Comments

R Samuel Klatchko · Accepted Answer · 2010-01-18 06:11:22Z

IntDesc is a type, not a variable. So the first thing you need to do is create a variable of the correct type:

IntDesc id;

Next, you'll need to have it point to allocated memory. I'm going to put everything on the stack, you may have other needs:

USB_INTERFACE_DESCRIPTOR usb;
PUSB_INTERFACE_DESCRIPTOR pusb = &usb;
id = &pusb;

Now that you have a valid pointer, you can go ahead an dereference it. Since this is a double pointer, you will need to dereference it twice:

(*(*id)).iInterface = 10;

Because C defines -> as a combination of * and ., you can express that more succinctly with:

(*id)->iInterface = 10;

caf · Accepted Answer · 2010-01-18 07:26:38Z

1

From the name InterfaceDescriptors, it would appear to point to an array of pointers to the structure. So the more idiomatic way would be:

InterfaceDescriptors[0]->iInterface = 10;

answered Jan 18, 2010 at 7:26

caf

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Comments

piotr · Accepted Answer · 2010-01-18 10:57:56Z

0

Your code is quite wrong

NODE* ptr;
k.N.iInterface = 100;
ptr = (NODE*)malloc(sizeof(NODE));

And before accesing:

ptr->N1->iInterface

N1 should be initialized to something.

answered Jan 18, 2010 at 10:57

piotr

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