(cpp) Other Directives

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 8 Other Directives
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 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.
 

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