Re: unsigned integer wrap 'trap'.

[Ian Lance Taylor <iant at google dot com>]

samboy <sam.liapis@global360.com.au> writes:

> I have a storage application to which I can record or retrieve files. Its
> multi-process C/C++ compiled 
> to run on the SPARC platform. The problem is the maximum file size which can
> be stored is 4GB due to 
> the size being internally conveyed using an unsigned integer. I need to
> extend the range so it accepts
> much larger files i.e. convert all uints to long longs. The approach at
> first was to work through the code 
> line-by-line to locate and upsize all variables as required. The problem is
> there's almost a million lines of
> code and no guarantee every instance of filesize usage will be located &
> upsized. Another more elegant
> approach which came to mind was to somehow 'trap' the occurrence of
> *unsigned* integer overflow...or
> should that be wrapping - yes I know its completely acceptable behaviour. If
> I could get gcc to do this 
> for me I could simply run a full suite of end-to-end functional testing then
> wait for the trap to fire and 
> abort. After each abort I can identify the undersized variable (via the
> backtrace) then apply a fix and
> repeat the process. This approach guarantees I'll target all variables
> restricting >4GB file records.

That seems doable but somewhat difficult.  You may get some traction
with the -Wconversion option in mainline gcc.  It will emit a warning
every time you convert from, e.g, 'unsigned long long' to 'unsigned
int'.  If the file size is coming in as 'unsigned long long' somewhere,
then you can use -Wconversion to trace it as it goes through the code.
Assuming there are no explicit casts: -Wconversion will only warn about
implicit casts.

Ian