gcc/doc/cpp/other-directives.rst

   1 ..
   2   Copyright 1988-2022 Free Software Foundation, Inc.
   3   This is part of the GCC manual.
   4   For copying conditions, see the copyright.rst file.
   5
   6 .. index:: #ident, #sccs
   7
   8 .. _other-directives:
   9
  10 Other Directives
  11 ----------------
  12
  13 The :samp:`#ident` directive takes one argument, a string constant.  On
  14 some systems, that string constant is copied into a special segment of
  15 the object file.  On other systems, the directive is ignored.  The
  16 :samp:`#sccs` directive is a synonym for :samp:`#ident`.
  17
  18 These directives are not part of the C standard, but they are not
  19 official GNU extensions either.  What historical information we have
  20 been able to find, suggests they originated with System V.
  21
  22 .. index:: null directive
  23
  24 The :dfn:`null directive` consists of a :samp:`#` followed by a newline,
  25 with only whitespace (including comments) in between.  A null directive
  26 is understood as a preprocessing directive but has no effect on the
  27 preprocessor output.  The primary significance of the existence of the
  28 null directive is that an input line consisting of just a :samp:`#` will
  29 produce no output, rather than a line of output containing just a
  30 :samp:`#`.  Supposedly some old C programs contain such lines.