[Jeremy Higdon]
> Is there a little FAQ of C constructions you should not use if you
> are writing kernel code?  It took a little doing to figure out that
> it was the switch that was causing trouble and not the shifts and
> arithmetic operations, and I'd like to avoid that in the future :-).

Here are some of the big ones:

* 64-bit division.  Most such is by powers of two so you can use shifts
  instead.  Probably the compiler should optimize this case
  automatically, but gcc doesn't, currently.

* floating point.  Use fixed point integers.  Floating point registers
  (at least on i386) are not saved/restored in the right places
  (because doing so is very expensive on some CPUs), so you'll clobber
  someone else's context.

* MMX.  See floating point.

* excess stack usage.  The kernel stack is very very small, so don't
  waste it.  Large automatic arrays are BAD.  Use globals or dynamic
  allocation.

* C++ exceptions, run-time types (RTTI) and other
  features-of-the-month.  Stick to C and you'll be OK.

* global variables initialized to 0.  Not necessary, as uninitialized
  globals are zeroed automatically, and that way they don't take up
  space in the on-disk image.  This is a concern for embedded systems.

Also make sure to read Documentation/CodingStyle for, well, coding style.

Peter
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