Question

KdPrint((
         "Unknown IoControlCode %#x\n",
                io_stack->Parameters.DeviceIoControl.IoControlCode
        ));

It’s weird. What does sharp mean?

Answer 1

The printf documentation says:

The character % is followed by zero or more of the following flags:

# The value should be converted to an ‘‘alternate form’’.
For o conversions, the first character
of the output string is made zero (by
prefixing a 0 if it was not zero
already). For x and X conversions, a
non-zero result
has the string ‘0x’ (or ‘0X’ for X conversions) prepended to
it. For a, A, e, E, f, F, g, and G
conversions, the result will always
contain a decimal point, even if no
digits follow it (normally, a decimal
point appears
in the results of those conversions only if a digit follows).
For g and G conversions, trailing
zeros are not removed from the result
as they would otherwise be. For other
conversions, the result is undefined.

MSDN docs on the flags are here.

so for %#x the value is simply prefixed with 0x. Where %x would yield 34ab, %#x would yield 0x34ab.