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The preprocessing language

   After tokenization, the stream of tokens may simply be passed
straight to the compiler's parser.  However, if it contains any
operations in the "preprocessing language", it will be transformed
first.  This stage corresponds roughly to the standard's "translation
phase 4" and is what most people think of as the preprocessor's job.
   The preprocessing language consists of "directives" to be executed
and "macros" to be expanded.  Its primary capabilities are:
   * Inclusion of header files.  These are files of declarations that
     can be substituted into your program.
   * Macro expansion.  You can define "macros", which are abbreviations
     for arbitrary fragments of C code.  The preprocessor will replace
     the macros with their definitions throughout the program.  Some
     macros are automatically defined for you.
   * Conditional compilation.  You can include or exclude parts of the
     program according to various conditions.
   * Line control.  If you use a program to combine or rearrange source
     files into an intermediate file which is then compiled, you can
     use line control to inform the compiler where each source line
     originally came from.
   * Diagnostics.  You can detect problems at compile time and issue
     errors or warnings.
   There are a few more, less useful, features.
   Except for expansion of predefined macros, all these operations are
triggered with "preprocessing directives".  Preprocessing directives
are lines in your program that start with `#'.  Whitespace is allowed
before and after the `#'.  The `#' is followed by an identifier, the
"directive name".  It specifies the operation to perform.  Directives
are commonly referred to as `#NAME' where NAME is the directive name.
For example, `#define' is the directive that defines a macro.
   The `#' which begins a directive cannot come from a macro expansion.
Also, the directive name is not macro expanded.  Thus, if `foo' is
defined as a macro expanding to `define', that does not make `#foo' a
valid preprocessing directive.
   The set of valid directive names is fixed.  Programs cannot define
new preprocessing directives.
   Some directives require arguments; these make up the rest of the
directive line and must be separated from the directive name by
whitespace.  For example, `#define' must be followed by a macro name
and the intended expansion of the macro.
   A preprocessing directive cannot cover more than one line.  The line
may, however, be continued with backslash-newline, or by a block comment
which extends past the end of the line.  In either case, when the
directive is processed, the continuations have already been merged with
the first line to make one long line.