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<h2 class="chapter">Other Directives</h2>
<p>The <code>#ident</code> 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.
<p>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.
<p>The <code>#sccs</code> directive is recognized, because it appears in the
header files of some systems. 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.
<p>The <dfn>null directive</dfn> consists of a <code>#</code> 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 <code>#</code> will
produce no output, rather than a line of output containing just a
<code>#</code>. Supposedly some old C programs contain such lines.
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