xref: /csrg-svn/old/cpp/README (revision 16236)

#
# @(#)README 1.3 03/24/84
#
August 30, 1982
Fixed by Kurt Shoens, UCB
If the "#line n name" occurs, then all future references
to the current file are generated in terms of "name", instead
of the name of file given to cpp in its command argument

August 25, 1978

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 (on UNIX systems) 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:
	: safety first: backup the existing version
	cp /lib/cpp /lib/ocpp
	: install the new version
	cp cpp /lib/cpp

Invocation
	cpp [-CEPRM] [-Dname] ... [-Dname=def] ... [-Idirectory] ...
		[-Uname] ... [<infile>] [<outfile>]

	If there are two non-flag arguments then the first is the name of the
	input file and the second is the name of the output file.  If there is
	one non-flag argument then it is the name of the input file and the
	output is written on the standard output.  If there are no non-flag
	arguments then the input is taken from the standard input and the output
	is written on the standard output.  Flag arguments are:

		-C	retain comments in output
		-Dname	define name as "1"
		-Dname=def	define name as def
		-E	ignored
		-Idirectory	add directory to search list for #include files
		-M	generate Makefile dependencies (-C and -M ignored)
		-P	don't insert lines "# 12 \"foo.c\"" into output
		-R	allow recursive macros
		-Uname	undefine name

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".
	The directory search order for #include files is
		1) the directory of the file which contains the #include request
		   (e.g. #include is relative to the file being scanned when
		   the request is made)
		2) the directories specified by -I, in left-to-right order
		3) the standard directory(s) (which for UNIX is /usr/include)
	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 (except if the argument -C is specified).
		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 formal parameters are recognized in '#define' bodies even inside
		character constants and quoted strings.  The output from
			#define foo(a) '\a'
			foo(bar)
		is the seven characters " '\\bar'".  Macro names are not recognized
		inside character constants or quoted strings during the regular scan.
		Thus
			#define foo bar
			printf("foo");
		does not expand 'foo' in the second line, because it is inside
		a quoted string which is not part of a '#define' macro definition.
	Macros are not expanded while processing a '#define' or '#undef'.
		Thus
			#define foo bletch
			#define bar foo
			#undef foo
			bar
		produces "foo".  The token appearing immediately after a
		'#ifdef' or '#ifndef' is not expanded (of course!).
	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
	If the -R flag is not specified then the invocation of some recursive
		macros is trapped and the recursion forcibly terminated with an
		error message.  The recursions that are trapped are the ones
		in which the nesting level is non-decreasing from some point on.
		In particular,
			#define a a
			a
		will be detected.  (Use "#undef a" if that is what you want.)
		The recursion
			#define a c b
			#define b c a
			#define c foo
			a
		will not be detected because the nesting level decreases after
		each expansion of "c".
	The -R flag specifically allows recursive macros and recursion will
		be strictly obeyed (to the extent that space is available).
		Assuming that -R is specified:
			#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 (unless -P).
	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.