The RenderMan Interface - Paul Bourke

Logical expressions have the value 1 for true, 0 for false. As in C, a non-zero logical expression
is deemed to be true. In general, logical expressions are only defined for scalar types.
The exception is == and != which are defined for every type.
B.4 Preprocessor
Shading Language compilers will respect ANSI C preprocessor directives, including:
#define identifier token-string
#define identifier ( identifier , ... , identifier ) token string
#undef identifier
#include ”filename”
#include
#if constant-expression
#ifdef identifier
#ifndef identifier
#else
#endif
#line constant identifier
#pragma token-string
The behaviors of these preprocessor directives should be identical to that expected from an
ANSI C compiler, and it is reasonable to assume that a Shading Language compiler may
use whatever compliant ANSI C preprocessor is available on a system. It should, therefore,
not be assumed that the preprocessor truly understands the semantics of the Shading
Language. For example, simple constant floating-point or boolean expressions that can be
evaluated at compile time may be used for constant-expression, but not expression involving
vectors or other types specific to the Shading Language.
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