path: root/usr.bin/unifdef/unifdef.c

/*	$OpenBSD: unifdef.c,v 1.13 2007/06/25 15:57:28 jmc Exp $	*/
/*
 * Copyright (c) 2002, 2003 Tony Finch <dot@dotat.at>
 * Copyright (c) 1985, 1993
 *	The Regents of the University of California.  All rights reserved.
 *
 * This code is derived from software contributed to Berkeley by
 * Dave Yost. Support for #if and #elif was added by Tony Finch.
 *
 * Redistribution and use in source and binary forms, with or without
 * modification, are permitted provided that the following conditions
 * are met:
 * 1. Redistributions of source code must retain the above copyright
 *    notice, this list of conditions and the following disclaimer.
 * 2. Redistributions in binary form must reproduce the above copyright
 *    notice, this list of conditions and the following disclaimer in the
 *    documentation and/or other materials provided with the distribution.
 * 3. Neither the name of the University nor the names of its contributors
 *    may be used to endorse or promote products derived from this software
 *    without specific prior written permission.
 *
 * THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND
 * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
 * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
 * ARE DISCLAIMED.  IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE
 * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
 * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
 * OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
 * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
 * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
 * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
 * SUCH DAMAGE.
 */

#ifndef lint
static const char copyright[] =
"@(#) Copyright (c) 1985, 1993\n\
	The Regents of the University of California.  All rights reserved.\n";

#if 0
static char sccsid[] = "@(#)unifdef.c	8.1 (Berkeley) 6/6/93";
#endif
static const char rcsid[] = "$OpenBSD: unifdef.c,v 1.13 2007/06/25 15:57:28 jmc Exp $";
#endif

/*
 * unifdef - remove ifdef'ed lines
 *
 *  Wishlist:
 *      provide an option which will append the name of the
 *        appropriate symbol after #else's and #endif's
 *      provide an option which will check symbols after
 *        #else's and #endif's to see that they match their
 *        corresponding #ifdef or #ifndef
 *      generate #line directives in place of deleted code
 *
 *   The first two items above require better buffer handling, which would
 *     also make it possible to handle all "dodgy" directives correctly.
 */

#include <ctype.h>
#include <err.h>
#include <stdarg.h>
#include <stdbool.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>

/* types of input lines: */
typedef enum {
	LT_TRUEI,		/* a true #if with ignore flag */
	LT_FALSEI,		/* a false #if with ignore flag */
	LT_IF,			/* an unknown #if */
	LT_TRUE,		/* a true #if */
	LT_FALSE,		/* a false #if */
	LT_ELIF,		/* an unknown #elif */
	LT_ELTRUE,		/* a true #elif */
	LT_ELFALSE,		/* a false #elif */
	LT_ELSE,		/* #else */
	LT_ENDIF,		/* #endif */
	LT_DODGY,		/* flag: directive is not on one line */
	LT_DODGY_LAST = LT_DODGY + LT_ENDIF,
	LT_PLAIN,		/* ordinary line */
	LT_EOF,			/* end of file */
	LT_COUNT
} Linetype;

static char const * const linetype_name[] = {
	"TRUEI", "FALSEI", "IF", "TRUE", "FALSE",
	"ELIF", "ELTRUE", "ELFALSE", "ELSE", "ENDIF",
	"DODGY TRUEI", "DODGY FALSEI",
	"DODGY IF", "DODGY TRUE", "DODGY FALSE",
	"DODGY ELIF", "DODGY ELTRUE", "DODGY ELFALSE",
	"DODGY ELSE", "DODGY ENDIF",
	"PLAIN", "EOF"
};

/* state of #if processing */
typedef enum {
	IS_OUTSIDE,
	IS_FALSE_PREFIX,	/* false #if followed by false #elifs */
	IS_TRUE_PREFIX,		/* first non-false #(el)if is true */
	IS_PASS_MIDDLE,		/* first non-false #(el)if is unknown */
	IS_FALSE_MIDDLE,	/* a false #elif after a pass state */
	IS_TRUE_MIDDLE,		/* a true #elif after a pass state */
	IS_PASS_ELSE,		/* an else after a pass state */
	IS_FALSE_ELSE,		/* an else after a true state */
	IS_TRUE_ELSE,		/* an else after only false states */
	IS_FALSE_TRAILER,	/* #elifs after a true are false */
	IS_COUNT
} Ifstate;

static char const * const ifstate_name[] = {
	"OUTSIDE", "FALSE_PREFIX", "TRUE_PREFIX",
	"PASS_MIDDLE", "FALSE_MIDDLE", "TRUE_MIDDLE",
	"PASS_ELSE", "FALSE_ELSE", "TRUE_ELSE",
	"FALSE_TRAILER"
};

/* state of comment parser */
typedef enum {
	NO_COMMENT = false,	/* outside a comment */
	C_COMMENT,		/* in a comment like this one */
	CXX_COMMENT,		/* between // and end of line */
	STARTING_COMMENT,	/* just after slash-backslash-newline */
	FINISHING_COMMENT	/* star-backslash-newline in a C comment */
} Comment_state;

static char const * const comment_name[] = {
	"NO", "C", "CXX", "STARTING", "FINISHING"
};

/* state of preprocessor line parser */
typedef enum {
	LS_START,		/* only space and comments on this line */
	LS_HASH,		/* only space, comments, and a hash */
	LS_DIRTY		/* this line can't be a preprocessor line */
} Line_state;

static char const * const linestate_name[] = {
	"START", "HASH", "DIRTY"
};

/*
 * Minimum translation limits from ISO/IEC 9899:1999 5.2.4.1
 */
#define	MAXDEPTH        64			/* maximum #if nesting */
#define	MAXLINE         4096			/* maximum length of line */
#define	MAXSYMS         4096			/* maximum number of symbols */

/*
 * Sometimes when editing a keyword the replacement text is longer, so
 * we leave some space at the end of the tline buffer to accommodate this.
 */
#define	EDITSLOP        10

/*
 * Globals.
 */

static bool             complement;		/* -c: do the complement */
static bool             debugging;		/* -d: debugging reports */
static bool             iocccok;		/* -e: fewer IOCCC errors */
static bool             killconsts;		/* -k: eval constant #ifs */
static bool             lnblank;		/* -l: blank deleted lines */
static bool             symlist;		/* -s: output symbol list */
static bool             text;			/* -t: this is a text file */

static const char      *symname[MAXSYMS];	/* symbol name */
static const char      *value[MAXSYMS];		/* -Dsym=value */
static bool             ignore[MAXSYMS];	/* -iDsym or -iUsym */
static int              nsyms;			/* number of symbols */

static FILE            *input;			/* input file pointer */
static const char      *filename;		/* input file name */
static int              linenum;		/* current line number */

static char             tline[MAXLINE+EDITSLOP];/* input buffer plus space */
static char            *keyword;		/* used for editing #elif's */

static Comment_state    incomment;		/* comment parser state */
static Line_state       linestate;		/* #if line parser state */
static Ifstate          ifstate[MAXDEPTH];	/* #if processor state */
static bool             ignoring[MAXDEPTH];	/* ignore comments state */
static int              stifline[MAXDEPTH];	/* start of current #if */
static int              depth;			/* current #if nesting */
static bool             keepthis;		/* don't delete constant #if */

static int              exitstat;		/* program exit status */

static void             addsym(bool, bool, char *);
static void             debug(const char *, ...);
static void             error(const char *);
static int              findsym(const char *);
static void             flushline(bool);
static Linetype         getline(void);
static Linetype         ifeval(const char **);
static void             ignoreoff(void);
static void             ignoreon(void);
static void             keywordedit(const char *);
static void             nest(void);
static void             process(void);
static const char      *skipcomment(const char *);
static const char      *skipsym(const char *);
static void             state(Ifstate);
static int              strlcmp(const char *, const char *, size_t);
static void             usage(void);

#define endsym(c) (!isalpha((unsigned char)c) && !isdigit((unsigned char)c) && c != '_')

/*
 * The main program.
 */
int
main(int argc, char *argv[])
{
	int opt;

	while ((opt = getopt(argc, argv, "i:D:U:I:cdeklst")) != -1)
		switch (opt) {
		case 'i': /* treat stuff controlled by these symbols as text */
			/*
			 * For strict backwards-compatibility the U or D
			 * should be immediately after the -i but it doesn't
			 * matter much if we relax that requirement.
			 */
			opt = *optarg++;
			if (opt == 'D')
				addsym(true, true, optarg);
			else if (opt == 'U')
				addsym(true, false, optarg);
			else
				usage();
			break;
		case 'D': /* define a symbol */
			addsym(false, true, optarg);
			break;
		case 'U': /* undef a symbol */
			addsym(false, false, optarg);
			break;
		case 'I':
			/* no-op for compatibility with cpp */
			break;
		case 'c': /* treat -D as -U and vice versa */
			complement = true;
			break;
		case 'd':
			debugging = true;
			break;
		case 'e': /* fewer errors from dodgy lines */
			iocccok = true;
			break;
		case 'k': /* process constant #ifs */
			killconsts = true;
			break;
		case 'l': /* blank deleted lines instead of omitting them */
			lnblank = true;
			break;
		case 's': /* only output list of symbols that control #ifs */
			symlist = true;
			break;
		case 't': /* don't parse C comments */
			text = true;
			break;
		default:
			usage();
		}
	argc -= optind;
	argv += optind;
	if (nsyms == 0 && !symlist) {
		warnx("must -D or -U at least one symbol");
		usage();
	}
	if (argc > 1) {
		errx(2, "can only do one file");
	} else if (argc == 1 && strcmp(*argv, "-") != 0) {
		filename = *argv;
		if ((input = fopen(filename, "r")) != NULL) {
			process();
			(void) fclose(input);
		} else
			err(2, "can't open %s", *argv);
	} else {
		filename = "[stdin]";
		input = stdin;
		process();
	}

	exit(exitstat);
}

static void
usage(void)
{
	fprintf(stderr,
	    "usage: unifdef [-ceklst] [-Dsym[=val]] [-Ipath] [-iDsym[=val]] "
	    "[-iUsym] [-Usym]\n"
	    "\t[file]\n");
	exit(2);
}

/*
 * A state transition function alters the global #if processing state
 * in a particular way. The table below is indexed by the current
 * processing state and the type of the current line. A NULL entry
 * indicates that processing is complete.
 *
 * Nesting is handled by keeping a stack of states; some transition
 * functions increase or decrease the depth. They also maintain the
 * ignore state on a stack. In some complicated cases they have to
 * alter the preprocessor directive, as follows.
 *
 * When we have processed a group that starts off with a known-false
 * #if/#elif sequence (which has therefore been deleted) followed by a
 * #elif that we don't understand and therefore must keep, we edit the
 * latter into a #if to keep the nesting correct.
 *
 * When we find a true #elif in a group, the following block will
 * always be kept and the rest of the sequence after the next #elif or
 * #else will be discarded. We edit the #elif into a #else and the
 * following directive to #endif since this has the desired behaviour.
 *
 * "Dodgy" directives are split across multiple lines, the most common
 * example being a multi-line comment hanging off the right of the
 * directive. We can handle them correctly only if there is no change
 * from printing to dropping (or vice versa) caused by that directive.
 * If the directive is the first of a group we have a choice between
 * failing with an error, or passing it through unchanged instead of
 * evaluating it. The latter is not the default to avoid questions from
 * users about unifdef unexpectedly leaving behind preprocessor directives.
 */
typedef void state_fn(void);

/* report an error */
static void
Eelif(void)
{
	error("Inappropriate #elif");
}

static void
Eelse(void)
{
	error("Inappropriate #else");
}

static void
Eendif(void)
{
	error("Inappropriate #endif");
}

static void
Eeof(void)
{
	error("Premature EOF");
}

static void
Eioccc(void)
{
	error("Obfuscated preprocessor control line");
}

/* plain line handling */
static void
print(void)
{
	flushline(true);
}

static void
drop(void)
{
	flushline(false);
}

/* output lacks group's start line */
static void
Strue(void)
{
	drop();
	ignoreoff();
	state(IS_TRUE_PREFIX);
}

static void
Sfalse(void)
{
	drop();
	ignoreoff();
	state(IS_FALSE_PREFIX);
}

static void
Selse(void)
{
	drop();
	state(IS_TRUE_ELSE);
}

/* print/pass this block */
static void
Pelif(void)
{
	print();
	ignoreoff();
	state(IS_PASS_MIDDLE);
}

static void
Pelse(void)
{
	print();
	state(IS_PASS_ELSE);
}

static void
Pendif(void)
{
	print();
	--depth;
}

/* discard this block */
static void
Dfalse(void)
{
	drop();
	ignoreoff();
	state(IS_FALSE_TRAILER);
}

static void
Delif(void)
{
	drop();
	ignoreoff();
	state(IS_FALSE_MIDDLE);
}

static void
Delse(void)
{
	drop();
	state(IS_FALSE_ELSE);
}

static void
Dendif(void)
{
	drop();
	--depth;
}

/* first line of group */
static void
Fdrop(void)
{
	nest();
	Dfalse();
}

static void
Fpass(void)
{
	nest();
	Pelif();
}

static void
Ftrue(void)
{
	nest();
	Strue();
}

static void
Ffalse(void)
{
	nest();
	Sfalse();
}

/* variable pedantry for obfuscated lines */
static void
Oiffy(void)
{
	if (iocccok)
		Fpass();
	else
		Eioccc();
	ignoreon();
}

static void
Oif(void)
{
	if (iocccok)
		Fpass();
	else
		Eioccc();
}

static void
Oelif(void)
{
	if (iocccok)
		Pelif();
	else
		Eioccc();
}

/* ignore comments in this block */
static void
Idrop(void)
{
	Fdrop();
	ignoreon();
}

static void
Itrue(void)
{
	Ftrue();
	ignoreon();
}

static void
Ifalse(void)
{
	Ffalse();
	ignoreon();
}

/* edit this line */
static void
Mpass (void)
{
	strncpy(keyword, "if  ", 4);
	Pelif();
}

static void
Mtrue (void)
{
	keywordedit("else\n");
	state(IS_TRUE_MIDDLE);
}

static void
Melif (void)
{
	keywordedit("endif\n");
	state(IS_FALSE_TRAILER);
}

static void
Melse (void)
{
	keywordedit("endif\n");
	state(IS_FALSE_ELSE);
}

static state_fn * const trans_table[IS_COUNT][LT_COUNT] = {
/* IS_OUTSIDE */
{ Itrue, Ifalse,Fpass, Ftrue, Ffalse,Eelif, Eelif, Eelif, Eelse, Eendif,
  Oiffy, Oiffy, Fpass, Oif,   Oif,   Eelif, Eelif, Eelif, Eelse, Eendif,
  print, NULL },
/* IS_FALSE_PREFIX */
{ Idrop, Idrop, Fdrop, Fdrop, Fdrop, Mpass, Strue, Sfalse,Selse, Dendif,
  Idrop, Idrop, Fdrop, Fdrop, Fdrop, Mpass, Eioccc,Eioccc,Eioccc,Eioccc,
  drop,  Eeof },
/* IS_TRUE_PREFIX */
{ Itrue, Ifalse,Fpass, Ftrue, Ffalse,Dfalse,Dfalse,Dfalse,Delse, Dendif,
  Oiffy, Oiffy, Fpass, Oif,   Oif,   Eioccc,Eioccc,Eioccc,Eioccc,Eioccc,
  print, Eeof },
/* IS_PASS_MIDDLE */
{ Itrue, Ifalse,Fpass, Ftrue, Ffalse,Pelif, Mtrue, Delif, Pelse, Pendif,
  Oiffy, Oiffy, Fpass, Oif,   Oif,   Pelif, Oelif, Oelif, Pelse, Pendif,
  print, Eeof },
/* IS_FALSE_MIDDLE */
{ Idrop, Idrop, Fdrop, Fdrop, Fdrop, Pelif, Mtrue, Delif, Pelse, Pendif,
  Idrop, Idrop, Fdrop, Fdrop, Fdrop, Eioccc,Eioccc,Eioccc,Eioccc,Eioccc,
  drop,  Eeof },
/* IS_TRUE_MIDDLE */
{ Itrue, Ifalse,Fpass, Ftrue, Ffalse,Melif, Melif, Melif, Melse, Pendif,
  Oiffy, Oiffy, Fpass, Oif,   Oif,   Eioccc,Eioccc,Eioccc,Eioccc,Pendif,
  print, Eeof },
/* IS_PASS_ELSE */
{ Itrue, Ifalse,Fpass, Ftrue, Ffalse,Eelif, Eelif, Eelif, Eelse, Pendif,
  Oiffy, Oiffy, Fpass, Oif,   Oif,   Eelif, Eelif, Eelif, Eelse, Pendif,
  print, Eeof },
/* IS_FALSE_ELSE */
{ Idrop, Idrop, Fdrop, Fdrop, Fdrop, Eelif, Eelif, Eelif, Eelse, Dendif,
  Idrop, Idrop, Fdrop, Fdrop, Fdrop, Eelif, Eelif, Eelif, Eelse, Eioccc,
  drop,  Eeof },
/* IS_TRUE_ELSE */
{ Itrue, Ifalse,Fpass, Ftrue, Ffalse,Eelif, Eelif, Eelif, Eelse, Dendif,
  Oiffy, Oiffy, Fpass, Oif,   Oif,   Eelif, Eelif, Eelif, Eelse, Eioccc,
  print, Eeof },
/* IS_FALSE_TRAILER */
{ Idrop, Idrop, Fdrop, Fdrop, Fdrop, Dfalse,Dfalse,Dfalse,Delse, Dendif,
  Idrop, Idrop, Fdrop, Fdrop, Fdrop, Dfalse,Dfalse,Dfalse,Delse, Eioccc,
  drop,  Eeof }
/*TRUEI  FALSEI IF     TRUE   FALSE  ELIF   ELTRUE ELFALSE ELSE  ENDIF
  TRUEI  FALSEI IF     TRUE   FALSE  ELIF   ELTRUE ELFALSE ELSE  ENDIF (DODGY)
  PLAIN  EOF */
};

/*
 * State machine utility functions
 */
static void
ignoreoff(void)
{
	ignoring[depth] = ignoring[depth-1];
}

static void
ignoreon(void)
{
	ignoring[depth] = true;
}

static void
keywordedit(const char *replacement)
{
	strlcpy(keyword, replacement, tline + sizeof(tline) - keyword);
	print();
}

static void
nest(void)
{
	depth += 1;
	if (depth >= MAXDEPTH)
		error("Too many levels of nesting");
	stifline[depth] = linenum;
}

static void
state(Ifstate is)
{
	ifstate[depth] = is;
}

/*
 * Write a line to the output or not, according to command line options.
 */
static void
flushline(bool keep)
{
	if (symlist)
		return;
	if (keep ^ complement)
		fputs(tline, stdout);
	else {
		if (lnblank)
			putc('\n', stdout);
		exitstat = 1;
	}
}

/*
 * The driver for the state machine.
 */
static void
process(void)
{
	Linetype lineval;
	state_fn *trans;

	for (;;) {
		linenum++;
		lineval = getline();
		trans = trans_table[ifstate[depth]][lineval];
		if (trans == NULL)
			break;
		trans();
		debug("process %s -> %s depth %d",
		    linetype_name[lineval],
		    ifstate_name[ifstate[depth]], depth);
	}
	if (incomment)
		error("EOF in comment");
}

/*
 * Parse a line and determine its type. We keep the preprocessor line
 * parser state between calls in a global variable.
 */
static Linetype
getline(void)
{
	const char *cp;
	int cursym;
	int kwlen;
	Linetype retval;
	Comment_state wascomment;

	if (fgets(tline, MAXLINE, input) == NULL)
		return (LT_EOF);
	retval = LT_PLAIN;
	wascomment = incomment;
	cp = skipcomment(tline);
	if (linestate == LS_START) {
		if (*cp == '#') {
			linestate = LS_HASH;
			cp = skipcomment(cp + 1);
		} else if (*cp != '\0')
			linestate = LS_DIRTY;
	}
	if (!incomment && linestate == LS_HASH) {
		keyword = tline + (cp - tline);
		cp = skipsym(cp);
		kwlen = cp - keyword;
		/* no way can we deal with a continuation inside a keyword */
		if (strncmp(cp, "\\\n", 2) == 0)
			Eioccc();
		if (strlcmp("ifdef", keyword, kwlen) == 0 ||
		    strlcmp("ifndef", keyword, kwlen) == 0) {
			cp = skipcomment(cp);
			if ((cursym = findsym(cp)) < 0)
				retval = LT_IF;
			else {
				retval = (keyword[2] == 'n')
				    ? LT_FALSE : LT_TRUE;
				if (value[cursym] == NULL)
					retval = (retval == LT_TRUE)
					    ? LT_FALSE : LT_TRUE;
				if (ignore[cursym])
					retval = (retval == LT_TRUE)
					    ? LT_TRUEI : LT_FALSEI;
			}
			cp = skipsym(cp);
		} else if (strlcmp("if", keyword, kwlen) == 0)
			retval = ifeval(&cp);
		else if (strlcmp("elif", keyword, kwlen) == 0)
			retval = ifeval(&cp) - LT_IF + LT_ELIF;
		else if (strlcmp("else", keyword, kwlen) == 0)
			retval = LT_ELSE;
		else if (strlcmp("endif", keyword, kwlen) == 0)
			retval = LT_ENDIF;
		else {
			linestate = LS_DIRTY;
			retval = LT_PLAIN;
		}
		cp = skipcomment(cp);
		if (*cp != '\0') {
			linestate = LS_DIRTY;
			if (retval == LT_TRUE || retval == LT_FALSE ||
			    retval == LT_TRUEI || retval == LT_FALSEI)
				retval = LT_IF;
			if (retval == LT_ELTRUE || retval == LT_ELFALSE)
				retval = LT_ELIF;
		}
		if (retval != LT_PLAIN && (wascomment || incomment)) {
			retval += LT_DODGY;
			if (incomment)
				linestate = LS_DIRTY;
		}
		/* skipcomment should have changed the state */
		if (linestate == LS_HASH)
			abort(); /* bug */
	}
	if (linestate == LS_DIRTY) {
		while (*cp != '\0')
			cp = skipcomment(cp + 1);
	}
	debug("parser %s comment %s line",
	    comment_name[incomment], linestate_name[linestate]);
	return (retval);
}

/*
 * These are the operators that are supported by the expression
 * evaluator. Note that if support for division is added then we also
 * need short-circuiting booleans because of divide-by-zero.
 */
static int
op_lt(int a, int b)
{
	return (a < b);
}

static int
op_gt(int a, int b)
{
	return (a > b);
}

static int
op_le(int a, int b)
{
	return (a <= b);
}

static int
op_ge(int a, int b)
{
	return (a >= b);
}

static int
op_eq(int a, int b)
{
	return (a == b);
}

static int
op_ne(int a, int b)
{
	return (a != b);
}

static int
op_or(int a, int b)
{
	return (a || b);
}

static int
op_and(int a, int b)
{
	return (a && b);
}

/*
 * An evaluation function takes three arguments, as follows: (1) a pointer to
 * an element of the precedence table which lists the operators at the current
 * level of precedence; (2) a pointer to an integer which will receive the
 * value of the expression; and (3) a pointer to a char* that points to the
 * expression to be evaluated and that is updated to the end of the expression
 * when evaluation is complete. The function returns LT_FALSE if the value of
 * the expression is zero, LT_TRUE if it is non-zero, or LT_IF if the
 * expression could not be evaluated.
 */
struct ops;

typedef Linetype eval_fn(const struct ops *, int *, const char **);

static eval_fn eval_table, eval_unary;

/*
 * The precedence table. Expressions involving binary operators are evaluated
 * in a table-driven way by eval_table. When it evaluates a subexpression it
 * calls the inner function with its first argument pointing to the next
 * element of the table. Innermost expressions have special non-table-driven
 * handling.
 */
static const struct ops {
	eval_fn *inner;
	struct op {
		const char *str;
		int (*fn)(int, int);
	} op[5];
} eval_ops[] = {
	{ eval_table, { { "||", op_or } } },
	{ eval_table, { { "&&", op_and } } },
	{ eval_table, { { "==", op_eq },
			{ "!=", op_ne } } },
	{ eval_unary, { { "<=", op_le },
			{ ">=", op_ge },
			{ "<", op_lt },
			{ ">", op_gt } } }
};

/*
 * Function for evaluating the innermost parts of expressions,
 * viz. !expr (expr) defined(symbol) symbol number
 * We reset the keepthis flag when we find a non-constant subexpression.
 */
static Linetype
eval_unary(const struct ops *ops, int *valp, const char **cpp)
{
	const char *cp;
	char *ep;
	int sym;

	cp = skipcomment(*cpp);
	if (*cp == '!') {
		debug("eval%d !", ops - eval_ops);
		cp++;
		if (eval_unary(ops, valp, &cp) == LT_IF)
			return (LT_IF);
		*valp = !*valp;
	} else if (*cp == '(') {
		cp++;
		debug("eval%d (", ops - eval_ops);
		if (eval_table(eval_ops, valp, &cp) == LT_IF)
			return (LT_IF);
		cp = skipcomment(cp);
		if (*cp++ != ')')
			return (LT_IF);
	} else if (isdigit((unsigned char)*cp)) {
		debug("eval%d number", ops - eval_ops);
		*valp = strtol(cp, &ep, 0);
		cp = skipsym(cp);
	} else if (strncmp(cp, "defined", 7) == 0 && endsym(cp[7])) {
		cp = skipcomment(cp+7);
		debug("eval%d defined", ops - eval_ops);
		if (*cp++ != '(')
			return (LT_IF);
		cp = skipcomment(cp);
		sym = findsym(cp);
		if (sym < 0 && !symlist)
			return (LT_IF);
		*valp = (value[sym] != NULL);
		cp = skipsym(cp);
		cp = skipcomment(cp);
		if (*cp++ != ')')
			return (LT_IF);
		keepthis = false;
	} else if (!endsym(*cp)) {
		debug("eval%d symbol", ops - eval_ops);
		sym = findsym(cp);
		if (sym < 0 && !symlist)
			return (LT_IF);
		if (value[sym] == NULL)
			*valp = 0;
		else {
			*valp = strtol(value[sym], &ep, 0);
			if (*ep != '\0' || ep == value[sym])
				return (LT_IF);
		}
		cp = skipsym(cp);
		keepthis = false;
	} else
		return (LT_IF);

	*cpp = cp;
	debug("eval%d = %d", ops - eval_ops, *valp);
	return (*valp ? LT_TRUE : LT_FALSE);
}

/*
 * Table-driven evaluation of binary operators.
 */
static Linetype
eval_table(const struct ops *ops, int *valp, const char **cpp)
{
	const struct op *op;
	const char *cp;
	int val;

	debug("eval%d", ops - eval_ops);
	cp = *cpp;
	if (ops->inner(ops+1, valp, &cp) == LT_IF)
		return (LT_IF);
	for (;;) {
		cp = skipcomment(cp);
		for (op = ops->op; op->str != NULL; op++)
			if (strncmp(cp, op->str, strlen(op->str)) == 0)
				break;
		if (op->str == NULL)
			break;
		cp += strlen(op->str);
		debug("eval%d %s", ops - eval_ops, op->str);
		if (ops->inner(ops+1, &val, &cp) == LT_IF)
			return (LT_IF);
		*valp = op->fn(*valp, val);
	}

	*cpp = cp;
	debug("eval%d = %d", ops - eval_ops, *valp);
	return (*valp ? LT_TRUE : LT_FALSE);
}

/*
 * Evaluate the expression on a #if or #elif line. If we can work out
 * the result we return LT_TRUE or LT_FALSE accordingly, otherwise we
 * return just a generic LT_IF.
 */
static Linetype
ifeval(const char **cpp)
{
	int ret;
	int val;

	debug("eval %s", *cpp);
	keepthis = killconsts ? false : true;
	ret = eval_table(eval_ops, &val, cpp);
	return (keepthis ? LT_IF : ret);
}

/*
 * Skip over comments and stop at the next character position that is
 * not whitespace. Between calls we keep the comment state in the
 * global variable incomment, and we also adjust the global variable
 * linestate when we see a newline.
 * XXX: doesn't cope with the buffer splitting inside a state transition.
 */
static const char *
skipcomment(const char *cp)
{
	if (text || ignoring[depth]) {
		for (; isspace((unsigned char)*cp); cp++)
			if (*cp == '\n')
				linestate = LS_START;
		return (cp);
	}
	while (*cp != '\0')
		if (strncmp(cp, "\\\n", 2) == 0)
			cp += 2;
		else switch (incomment) {
		case NO_COMMENT:
			if (strncmp(cp, "/\\\n", 3) == 0) {
				incomment = STARTING_COMMENT;
				cp += 3;
			} else if (strncmp(cp, "/*", 2) == 0) {
				incomment = C_COMMENT;
				cp += 2;
			} else if (strncmp(cp, "//", 2) == 0) {
				incomment = CXX_COMMENT;
				cp += 2;
			} else if (strncmp(cp, "\n", 1) == 0) {
				linestate = LS_START;
				cp += 1;
			} else if (strchr(" \t", *cp) != NULL) {
				cp += 1;
			} else
				return (cp);
			continue;
		case CXX_COMMENT:
			if (strncmp(cp, "\n", 1) == 0) {
				incomment = NO_COMMENT;
				linestate = LS_START;
			}
			cp += 1;
			continue;
		case C_COMMENT:
			if (strncmp(cp, "*\\\n", 3) == 0) {
				incomment = FINISHING_COMMENT;
				cp += 3;
			} else if (strncmp(cp, "*/", 2) == 0) {
				incomment = NO_COMMENT;
				cp += 2;
			} else
				cp += 1;
			continue;
		case STARTING_COMMENT:
			if (*cp == '*') {
				incomment = C_COMMENT;
				cp += 1;
			} else if (*cp == '/') {
				incomment = CXX_COMMENT;
				cp += 1;
			} else {
				incomment = NO_COMMENT;
				linestate = LS_DIRTY;
			}
			continue;
		case FINISHING_COMMENT:
			if (*cp == '/') {
				incomment = NO_COMMENT;
				cp += 1;
			} else
				incomment = C_COMMENT;
			continue;
		default:
			/* bug */
			abort();
		}
	return (cp);
}

/*
 * Skip over an identifier.
 */
static const char *
skipsym(const char *cp)
{
	while (!endsym(*cp))
		++cp;
	return (cp);
}

/*
 * Look for the symbol in the symbol table. If it is found, we return
 * the symbol table index, else we return -1.
 */
static int
findsym(const char *str)
{
	const char *cp;
	int symind;

	cp = skipsym(str);
	if (cp == str)
		return (-1);
	if (symlist)
		printf("%.*s\n", (int)(cp-str), str);
	for (symind = 0; symind < nsyms; ++symind) {
		if (strlcmp(symname[symind], str, cp-str) == 0) {
			debug("findsym %s %s", symname[symind],
			    value[symind] ? value[symind] : "");
			return (symind);
		}
	}
	return (-1);
}

/*
 * Add a symbol to the symbol table.
 */
static void
addsym(bool ignorethis, bool definethis, char *sym)
{
	int symind;
	char *val;

	symind = findsym(sym);
	if (symind < 0) {
		if (nsyms >= MAXSYMS)
			errx(2, "too many symbols");
		symind = nsyms++;
	}
	symname[symind] = sym;
	ignore[symind] = ignorethis;
	val = sym + (skipsym(sym) - sym);
	if (definethis) {
		if (*val == '=') {
			value[symind] = val+1;
			*val = '\0';
		} else if (*val == '\0')
			value[symind] = "";
		else
			usage();
	} else {
		if (*val != '\0')
			usage();
		value[symind] = NULL;
	}
}

/*
 * Compare s with n characters of t.
 * The same as strncmp() except that it checks that s[n] == '\0'.
 */
static int
strlcmp(const char *s, const char *t, size_t n)
{
	while (n-- && *t != '\0')
		if (*s != *t)
			return ((unsigned char)*s - (unsigned char)*t);
		else
			++s, ++t;
	return ((unsigned char)*s);
}

/*
 * Diagnostics.
 */
static void
debug(const char *msg, ...)
{
	va_list ap;

	if (debugging) {
		va_start(ap, msg);
		vwarnx(msg, ap);
		va_end(ap);
	}
}

static void
error(const char *msg)
{
	if (depth == 0)
		warnx("%s: %d: %s", filename, linenum, msg);
	else
		warnx("%s: %d: %s (#if line %d depth %d)",
		    filename, linenum, msg, stifline[depth], depth);
	errx(2, "output may be truncated");
}