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Pointers Problem

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LOSTISLAND

Posted: Jun 25, 2012 - 11:16 AM

Joined: May 05, 2010
Posts: 200

Hi all,
Could anyone say what’s happening here ?
Is there any tutorial explaining how to make use of pointers in this way ?

Code:

#define P_EP(n) ((USB_EP_EVENT & (1 << (n))) ? USB_EndPoint##n : NULL)

/* USB Endpoint Events Callback Pointers */

void (* const USB_P_EP[16]) (uint32_t event) = {

  P_EP(0),

  P_EP(1),

  P_EP(2),

  P_EP(3),

  P_EP(4),

  P_EP(5),

  P_EP(6),

  P_EP(7),

  P_EP(8),

  P_EP(9),

  P_EP(10),

  P_EP(11),

  P_EP(12),

  P_EP(13),

  P_EP(14),

  P_EP(15),

};

Thanks in advanced

jayjay1974

Posted: Jun 25, 2012 - 12:22 PM

Joined: Oct 30, 2002
Posts: 5720
Location: The Netherlands

Those are function pointers.

http://en.wikipedia.org/wiki/Function_pointer

clawson

Posted: Jun 25, 2012 - 01:18 PM

Joined: Jul 18, 2005
Posts: 62324
Location: (using avr-gcc in) Finchingfield, Essex, England

The form of a function pointer definition is:

Code:

return-type (*name_of_fn_ptr)(input-type [varname], input-type [varname]...)

The simplest would be for a function of the form:

Code:

void a_function(void);

in which case the definition of a pointer to this kind of function would be:

Code:

void (* ptr)(void);

and now you could say:

Code:

ptr = a_function;

and assign the address of a_function() to that variable. If a_function were more complex:

Code:

int a_function(long l, char * array);

then the function pointer definition would need to be more complex:

Code:

int (* ptr)(long, char 8);

and once again you could use:

Code:

ptr = a_function;

Now if you wanted to create a load of function pointers in an array you just add an index to it:

Code:

int (* ptr[8])(long, char 8);

So ptr[] is an array of 8 pointers to functions that return an int and take a long and a char pointer. You could then say:

Code:

ptr[3] = a_function;

ptr[7] = another_function; // of that same type

and so on

BTW is the above code you quote from LUFA or ASF by any chance? End point call backs are a bit like "soft interrupts". It gives the USB stack the chance to call back a function you can provide if you need to know when a certain even has occurred (such as data arriving on an endpoint).

_________________

LOSTISLAND

Posted: Jun 25, 2012 - 06:07 PM

Joined: May 05, 2010
Posts: 200

Thanks alot ,

Quote:

It's from USBHID sample of CMSIS for LPC17xx on uVision4 IDE.

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