memcpy error: Access violation reading

[gowrishwar]

Hi all,

I am working on a camera to collect data. In the process one of the functions is throwing errors while copying the data from the buffer on board to local system buffer.

The message which I am getting is


Unhandled exception at 0x1026ef58 (msvcr90d.dll) in Camera.exe: 0xC0000005: Access violation reading location 0xd41dddcc.

__declspec(dllexport) int intf_daq_get_frame (void * intf_p, unsigned char * frame_data, unsigned long frame_size)
{	
	BFDev *bf_p;
	bf_p = (BFDev *) intf_p;
	if (bf_p == NULL)
	{
		return FAILURE;
	}	
	

	// Wait for frame to be acquired.
	bf_p->rv = BiCirWaitDoneFrame(bf_p->Board, &bf_p->BufArray, INFINITE,  &bf_p->CirHandle);
	
	// Process newly acquired frame 		
	memcpy(frame_data, (unsigned char *) (bf_p->CirHandle.pBufData), frame_size);

	// Mark the processed buffer available for acquisition
	bf_p->rv = BiCirStatusSet(bf_p->Board, &bf_p->BufArray,  bf_p->CirHandle,  BIAVAILABLE);
	if(bf_p->rv != BI_OK)
	{		
		BiErrorShow(bf_p->Board, bf_p->rv);

		return FAILURE;
	}


	return SUCCESS;
}

Can someone tell me what would have went wrong and any suggestions.

Thanks in advance.

[Skizmo]

I know nothing about cameras and stuff, but if you are sure that the memcpy is the failure, then I have to assume that 'bf_p->CirHandle.pBufData' is not valid. So, or 'bf_p' is not valid or 'pbBufData' is not valid. You can use the debugger to inspect bot pointers and see what they contain.

[gowrishwar]

Thanks for the reply Skizmo

Both pointers are valid, the data in bf_p->CirHandle.pBufData is 3677536032, which is unsigned int and I am Type Casting it to unsigned char.

When I debug, the output shows

First-chance exception at 0x1026ef58 (msvcr90d.dll) in Camera.exe: 0xC0000005: Access violation reading location 0xd41dddcc.
Unhandled exception at 0x1026ef58 (msvcr90d.dll) in Camera.exe: 0xC0000005: Access violation reading location 0xd41dddcc.

Is this a problem with Overlapping Buffers? or something else.

[ZuK]

Is this a problem with Overlapping Buffers? or something else.

If you think that the problem might be overlapping buffers use memmove() instead of memcpy().
Kurt

[gowrishwar]

I have tried memmove(). Even then it is throwing the same exception.

Output:
First-chance exception at 0x1026f448 (msvcr90d.dll) in Camera.exe: 0xC0000005: Access violation reading location 0xd41dddcc.
Unhandled exception at 0x1026f448 (msvcr90d.dll) in Camera.exe: 0xC0000005: Access violation reading location 0xd41dddcc.

Then I try to debug more by adding the following piece of code in the place of memcpy().

	unsigned char *FData;
	if ((FData = (unsigned char *) calloc(1 , sizeof(unsigned char))) == NULL)
	{
		return NULL;
	}
	FData = (unsigned char *) frame_data;


 if (FData <= (unsigned char *) (bf_p->CirHandle.pBufData) || FData >= ((unsigned char *) (bf_p->CirHandle.pBufData) + frame_size))
	 {
		 /*
          * Non-Overlapping Buffers
          * copy from lower addresses to higher addresses
          */
         while (frame_size--)
              *FData++ = *(unsigned char *) (bf_p->CirHandle.pBufData)++;
	 }
	else
	 {
		 /*
         * Overlapping Buffers
         * copy from higher addresses to lower addresses
         */
         FData += frame_size - 1;
         *(unsigned char *) (bf_p->CirHandle.pBufData) = *(unsigned char *) (bf_p->CirHandle.pBufData) + (frame_size - 1);
		 while (frame_size--)
             *FData-- = *(unsigned char *) (bf_p->CirHandle.pBufData)--;
	   }

Then the exception I am getting is
Unhandled exception at 0x0077193b in Camera.exe: 0xC0000005: Access violation writing location 0x9b9b9b99.

[Lindley]

Clearly, one or more of those pointers aren't valid......

[S_M_A]

Casts are pure evil when used (without a deep understanding) just to solve a compiler error and I think this shows that you are in deep water at the moment

unsigned char *FData;
if ((FData = (unsigned char *) calloc(1 , sizeof(unsigned char))) == NULL)
{
return NULL;
}
FData = (unsigned char *) frame_data;

I would advise you to get rid of all casts and solve the compilation issues by using the correct types instead.

[Skizmo]

I would advise you to get rid of all casts and solve the compilation issues by using the correct types instead.

And how would you like to do that if 'bf_p->CirHandle.pBufData' is a void* that actually points to a byte buffer ?

[S_M_A]

And how would you like to do that if 'bf_p->CirHandle.pBufData' is a void* that actually points to a byte buffer ?

memcpy is declared like

void* memcpy(void* dst, void* src, size_t size);

which is to say it accepts any pointer for the params so why should it be any problem to use a pointer to the correct type? The cast in the call to memcpy just tells the compiler: "ignore that this looks strange, I know what I'm doing"

So, if pBufData actually points at a byte buffer I prefer to declare it as an unsigned char* to help the compiler help me in finding my mistakes.

[ZuK]

	unsigned char *FData;
	if ((FData = (unsigned char *) calloc(1 , sizeof(unsigned char))) == NULL)
	{
		return NULL;
	}
	FData = (unsigned char *) frame_data;

This really looks strange.
You calloc a singe byte just to immediately overwrie the returned pointer creating a memory leak.
Kurt