Do you know about %#x, in C language format string.

Question

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

It's weird. What does sharp mean?

Answer 1

# indcates an alternative format. For x this means that 0x is prepended to the output.

Answer 2

printf when used with specifier x, # causes the output to be prefixed with 0x provided the value being printed is other than 0.

Answer 3

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.conversions, the result is undefined.

MSDN docs on the flags are here

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