#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 ‘
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 ‘
produce no output, rather than a line of output containing just a
#’. Supposedly some old C programs contain such lines.