The null directive (#)
The null directive performs no action.
It consists of a single # on a line of its own.
The null directive should not be confused with the # operator
or the character that starts a preprocessor directive.
In the following example, if
MINVAL is a defined macro
name, no action is performed. If MINVAL is not a defined
identifier, it is defined 1.
#ifdef MINVAL
#
#else
#define MINVAL 1
#endifRelated information