topic really just another question on pointer scope in Kinetis Microcontrollers

really just another question on pointer scope

ryanlush — Mon, 08 Jan 2018 22:15:10 GMT

I wrote this code to demonstrate something I found in the KSDK USB library. I took (interpreted?) this code from usb_device_ch9.c starting around line 862.

typedef struct _usb_device_control_request_struct
{
    usb_setup_struct_t *setup; /*!< The pointer of the setup packet data. */
    uint8_t *buffer;           /*!< Pass the buffer address. */
    uint32_t length;           /*!< Pass the buffer length or requested length. */
    uint8_t isSetup;           /*!< Indicates whether a setup packet is received. */
} usb_device_control_request_struct_t;

void main()
{
function1();
}

void function1()
{
   uint8_t *buffer = (uint8_t *)NULL;
    uint32_t length = 0U;
   usb_device_control_request_struct_t controlRequest;

   controlRequest.buffer = (uint8_t *)NULL

   function2(&controlRequest);

   length = controlRequest.length;
   buffer = controlRequest.buffer;

}

void function2(void* param)
{
usb_device_control_request_struct_t* controlRequest = (usb_device_control_request_struct_t*)param;

function3(controlRequest)
}

uint8_t buffer[8]
void function3(usb_device_control_request_struct_t* controlRequest)
{
controlRequest.buffer = buffer;
controlRequest.length = 8;
}

The expectation is function2 will give controlRequest.buffer something to point to but controlRequest.buffer is still null when in the context of function1() just after calling function2(). I would never do it this way but this is the framework of the KSDK USB driver. I'm probably forgetting to explain something here but hopefully I can at least get a conversation going?

Re: really just another question on pointer scope

danielchen — Wed, 10 Jan 2018 06:08:28 GMT

Hi Ryan Lush

I am not sure I fully understand you, I copied your code to hello world project, and found the controlRequest.buffer point to 0x2002FFE0. Please see below picture.

Have a great day,
Daniel

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Re: really just another question on pointer scope

bobpaddock — Wed, 10 Jan 2018 13:22:34 GMT

Isn't function3 wrong, among other issues?

void function3(usb_device_control_request_struct_t* controlRequest)
{
controlRequest.buffer = buffer;
controlRequest.length = 8;
}

should be:

{

controlRequest->buffer = buffer;

...

}

Re: really just another question on pointer scope

danielchen — Wed, 10 Jan 2018 14:08:05 GMT

Yes, controlRequest is a pointer in function3, should be controlRequest->buffer = buffer; or else there will be a compile error.

Regards

Daniel