memcpy error: Access violation reading

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.

Code:

---

__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.

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.

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

Code:

---

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.

Quote:

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Originally Posted by gowrishwar

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

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

Code:

---

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().

Code:

---

        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.

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

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

Quote:

---

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.

Quote:

---

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 ?

Quote:

---

Originally Posted by Skizmo

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

Code:

---

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.

Quote:

---

Originally Posted by gowrishwar

Code:

---

        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