The ‘ #ident ’ directive takes one argument, a string constant. On some systems, that string constant is copied into a special segment of the object file. On other systems, the directive is ignored. The ‘ #sccs ’ directive is a synonym for ‘ #ident ’.
These directives are not part of the C standard, but they are not official GNU extensions either. What historical information we have been able to find, suggests they originated with System V.
The null directive consists of a ‘ # ’ followed by a newline, with only whitespace (including comments) in between. A null directive is understood as a preprocessing directive but has no effect on the preprocessor output. The primary significance of the existence of the null directive is that an input line consisting of just a ‘ # ’ will produce no output, rather than a line of output containing just a ‘ # ’. Supposedly some old C programs contain such lines.