Re: [Libusb-devel] Segmentation fault - not sure what is happening

[Raghav P. <rag...@gm...>]

Tim Roberts wrote:
> Raghav Puranmalka wrote:
>> Hi,
>>
>> I am trying to communicate with a FX2 USB device that I have. I am
>> able to submit control transfers just fine. However, when I try to do
>> a bulk transfer, I get a segmentation fault. I'm not sure what is
>> happening. Any help would be appreciated.
>>
>> The code segment that is segfaulting is below:
>>
>>
>> #define FLEX_2_EP 0x02
>>
>> //m_hDeviceHandle is a pointer libusb_device_handle
>>
>> int *nBytes;
>> int result;
>>
>> char c = 'R';
>> unsigned char endpoint = FLEX_2_EP;
>> result = libusb_bulk_transfer(m_hDeviceHandle, endpoint, (unsigned
>> char*) &c, sizeof(c), nBytes, 0); //segfault occurs here
>>   
> 
> Yes.  You have a C misunderstanding here.  The next to the last
> parameter is an OUTPUT parameter.  It doesn't want you to hand it an
> "int *", it wants you to hand it the address of an "int" so that it can
> fill in the number of bytes it actually sent.  As it is, nBytes contains
> garbage, and libusb tries to store the number of bytes at that garbage
> address.
> 
> Change it to this:
>     int nBytes;
>     result = libusb_bulk_transfer( ..., sizeof(c), &nBytes, 0 );
> 

Ah. I see. I'm pretty new at C, so I wasn't sure what to do there. Thank 
you so much!

Raghav