It has been mentioned here that (all) identifiers (or only in (all) pre-processor directives?) with leading underscores are reserved for the implementation.
So something like:
#ifndef _HEADER_H
#define _HEADER_H
to implement header guards would be bad practice.
However, I cant seem to find where this is stated in ISO/IEC 14882:2003
Does anyone know where I can find the relevant information in the standard?