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(cpp-300.info)Other Directives

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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.

   This directive is not part of the C standard, but it is not an
official GNU extension either.  We believe it came from System V.

   The `#sccs' directive is recognized on some systems, because it
appears in their header files.  It is a very old, obscure, extension
which we did not invent, and we have been unable to find any
documentation of what it should do, so GCC simply ignores it.

   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.