usr/src/cmd/cpp/README

Files in this directory form the C preprocessor, which handles '#include'
files and macro definition and expansion for the C compiler.
This new version was written by John F. Reiser and is from 5 to 12
times faster than the old.

To create the executable file 'cpp' in the current directory:
        make

To install the preprocessor 'cpp' so it will be used by the C compiler:
        : backup the existing version
        cp /lib/cpp /lib/ocpp
        : install the new version
        cp cpp /lib/cpp

Documentation clarifications:
        Symbols defined on the command line by "-Dfoo" are defined as "1",
                i.e., as if they had been defined by "#define foo 1" or "-Dfoo=1".
        An unescaped linefeed (the single character "\n") terminates a
                character constant or quoted string.
        An escaped linefeed (the two-character sequence "\\\n") may be
                used in the body of a '#define' statement to continue
                the definition onto the next line.  The escaped linefeed is
                not included in the macro body.
        Comments are uniformly removed.  They are also ignored, except
                that a comment terminates a token.
                Thus "foo/* la di da */bar" may expand 'foo' and 'bar' but
                will never expand 'foobar'.  If neither 'foo' nor 'bar' is a
                macro then the output is "foobar", even if 'foobar'
                is defined as something else.  The file
                        #define foo(a,b)b/**/a
                        foo(1,2)
                produces "21" because the comment causes a break which enables
                the recognition of 'b' and 'a' as formals in the string "b/**/a".
        Macro formals are recognized even inside character constants
                and quoted strings.  The output from
                        #define foo(a) '\a'
                        foo(bar)
                is the seven characters " '\\bar'".
        Macros are not expanded while processing a '#define' or '#undef'.
                Thus
                        #define foo bletch
                        #define bar foo
                        #undef foo
                        bar
                produces "foo".
        Macros are not expanded during the scan which determines the actual
                parameters to another macro call.  Thus
                        #define foo(a,b)b a
                        #define bar hi
                        foo(bar,
                        #define bar bye
                        )
                produces " bye" (and warns about the redefinition of 'bar').

There are some differences between the new and the old preprocessor.
Bugs fixed:
        "1.e4" is recognized as a floating-point number, rather than as an
                opportunity to expand the possible macro name "e4".
        Any kind and amount of white space (space, tab, linefeed, vertical tab,
                formfeed, carriage return) is allowed between a macro name and
                the left parenthesis which introduces its actual parameters.
        The comma operator is legal in preprocessor '#if' statements.
        Macros with parameters are legal in preprocessor '#if' statements.
        Single-character character constants are legal in preprocessor '#if' statements.
        Linefeeds are put out in the proper place when a multiline comment
                is not passed through to the output.
        The following example expands to "# # #" :
                #define foo #
                foo foo foo
        Recursion in macro definitions is strictly obeyed (to the extent that
                space is available).  In particular,
                        #define a a
                        a
                causes an infinite loop with very little output.  The tail recursion
                        #define a <b
                        #define b >a
                        a
                causes the string "<>" to be output infinitely many times.  The
                non-tail recursion
                        #define a b>
                        #define b a<
                        a
                complains "too much pushback", dumps the pushback, and continues
                (again, infinitely).

Stylistic choice:
        Nothing (not even linefeeds) is output while a false '#if', '#ifdef',
                or '#ifndef' is in effect.  Thus when all conditions become true
                a line of the form "# 12345 foo.c" is output.
        Error and warning messages always appear on standard error (file
                descriptor 2).
        Mismatch between the number of formals and actuals in a macro call
                produces only a warning, and not an error.  Excess actuals
                are ignored; missing actuals are turned into null strings.

Incompatibility:
        The virgule '/' in "a=/*b" is interpreted as the first character of
                the pair "/*" which introduces a comment, rather than as the
                second character of the divide-and-replace operator "=/".
                This incompatibility reflects the recent change in the C language
                which made "a/=*b" the legal way to write such a statement
                if the meaning "a=a/ *b" is intended.