Handling exception while writing to a file in usb and usb removed while writing in progress

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Hi,
i am writing to a file in USB and while in progress i removed the USB. In the code i am using fwrite to write to the file. i put a try{} catch(...){} block to handle the exception. Exception wasn't getting caught.

i have a requirement to upgrade the firware from USB. so i am using fread to read a chunk of data from the file and sending the command to the module. While the reading is in progress, i removed the USB. Same try{} catch(...){} block to handle the exception. but the exception is not gettign caught

Thanks
Ajai

Posted 19-Mar-13 3:55am

ajaiisnow

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Sergey Alexandrovich Kryukov 19-Mar-13 9:59am

If exception is not caught, does it mean the file was read successfully? It is actually possible...
Did you run it under the debugger? Could you see the result of reading?
Also, it's usually important not to catch exception locally, but catch it up the stack (this won't solve your problem, just for general design).
—SA

ajaiisnow 19-Mar-13 10:06am

yes the file read got completed successfully with out any exception.
i ran in debugger and i am suppose to read 64 bytes in each iteration and it is reading successfully 64 bytes. the return code for fread is returning 64.

in case of fwrite, as the file is invalid the system hangs if i remove the usb in between.

Jochen Arndt 19-Mar-13 10:09am

Which exception did you expect? fwrite() does not throw exceptions. Use the CFile class if you want exceptions.

ajaiisnow 19-Mar-13 10:13am

i am new to WINCE. Is it specific to WINCE CRT version that exception trowing is avoided.

thanks
Ajai

Matthew Faithfull 19-Mar-13 11:08am

It's not specific to WinCE CRT but in general WinCE doesn't code doesn't use exceptions, at least it didn't with WinCE 3 through 5 on an ARMV4I device which is what I used to work on. Generally the removal of USB storage in the middle of reading and writing is not safe on
WinCE devices and I don't believe you can make it so. Even if you can at the WinCE end you'll need thouroughly tested industrial grade flash storage on the other end or 1 in every few hundred times your storage device will be effectively destroyed anyway.

Jochen Arndt 19-Mar-13 13:03pm

fwrite() is a standard C library function. It will never throw an exception regardless of the OS (Win, Win CE, Linux, ...) because it is C which does not know exceptions like C++.

All you can do is checking the return value of fwrite() and throw your own exception.

If a device is removed during any IO operation, you will have corrupted data. Such an event can't be handled properly. All you can (and should) do, is checking the return value of each IO operation and stop processing (don't call other IO functions) to decrease the chance of a locked system.

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Philippe Mori · Accepted Answer · 2013-03-19T07:42:00

If possible you should first copy the firmware from the USB drive to a safe memory location and when complete you should validate the data on the copy and then upgrade the firmware.

Directly upgrading from the USB key is not safe as the user can remove the key at any point.

You should at least validate that all bytes were copied to the temporary location and if possible you might want to do a checksum or other validation to ensure that the firmware is valid before using it.

And in any case, you should tell the user not to remove the USB key while upgrading as you want to be on the save side and if somethying goes wrong then you can ask the user if he has followed the instructions to help ensure that users won't abuse your technical support.

Finally, the hardware should be designed in a way to be safe enough. In a case like yours, I would recommand you to have memory specifically reserved for upgrading the firmware.