[PATCH] introduction: document #define syntax

We use the C #define syntax to refer to numeric values. Let's document that.  Signed-off-by: Michael S. Tsirkin <> ---  introduction.tex
21 +++++++++++++++++++++  1 file changed, 21 insertions(+)  diff --git a/introduction.tex b/introduction.tex index cc38e29..4febed2 100644 --- a/introduction.tex +++ b/introduction.tex @@ -210,6 +210,27 @@ section{Structure Specifications}  egin{lstlisting}  CPU_TO_BE16(B << 15
A)  end{lstlisting} +section{Constant Specifications} + +In many cases, numberic values used in the interface between the device +and the driver are documented using the C #define and /* */ +comment syntax. Multiple related values are grouped together with +a common name as a prefix, using _ as a separator. +Using _XXX as a suffix refers to all values in a group. +For example: + +egin{lstlisting} +/* Value A description */ +#define VIRTIO_VALUE_A
(1 << 0) +/* Value B description */ +#define VIRTIO_VALUE_B
(1 << 1) +end{lstlisting} +documents two numeric values: 1 meaning Value A and 2 meaning +Value B.  Note that $<<$ refers to the shift-left operation. + +Further, in this case VIRTIO_VALUE_A and VIRTIO_VALUE_B +refer to 1 and 2 respectively. Further, VIRTIO_VALUE_XXX refers to +either VIRTIO_VALUE_A or VIRTIO_VALUE_B.
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