On 21/02/17 21:27, Tobin C. Harding wrote:
Comedi drivers make heavy use of standard types i.e unsigned short. According to Linux Device Drivers standard C data types are not the same size of all architectures. Should we be converting comedi/drivers to use kernel types u8, u16 etc
Linux kernel C data types have stricter requirements than the C standard allows. There is common agreement on the widths of 'char', 'short', 'int', and 'long long', a choice of two widths for 'long'. Signed integers are assumed to use 2's complement representation, and null pointers are assumed to have an "all bits zero" representation. Obviously, the "endianness" of the types will be arch-specific. (I probably missed some stuff. For example, I'm not sure if the Linux kernel requires a plain 'char' to be signed. And I don't know if any of this is officially documented anywhere, so count it as my opinion.)
Follow up question. Some comedi/drivers also use unsigned long for variables that are not memory addresses. If we are to convert standard types to kernel types how does one know the intended size of these variables. I see that the register values come from the manual http://www.ni.com/pdf/manuals/340934b.pdf Is the choice (size) of type in a struct (eg struct ni_private, comedi/drivers/ni_stc.h) a kernel design issue or is it a hardware issue and should I be reading the manual to find it.
It's mostly a design issue. In general, there is no harm in making local variables wider than they need to be up to 'int' size, although the exact-width types such as 'u16' may be useful sometimes. In general, structure members and array elements should not be excessively wide. The use of 'long' or 'unsigned long' should be viewed with suspicion unless there is a specific reason for it.
-- -=( Ian Abbott @ MEV Ltd. E-mail: <abbotti@xxxxxxxxx> )=- -=( Web: http://www.mev.co.uk/ )=- _______________________________________________ devel mailing list devel@xxxxxxxxxxxxxxxxxxxxxx http://driverdev.linuxdriverproject.org/mailman/listinfo/driverdev-devel
