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Full .nr nS support, unbreaking the kernel manuals.

Kristaps coded this from scratch after reading my .nr patch;
it is simpler and more powerful.

Registers live in struct regset in regs.h, struct man and struct mdoc
contain pointers to it.  The nS register is cleared when parsing .Sh.
Frontends respect the MDOC_SYNPRETTY flag set in mdoc node_alloc.

/*	$Id: roff.c,v 1.6 2010/06/27 21:54:42 schwarze Exp $ */
/*
 * Copyright (c) 2010 Kristaps Dzonsons <kristaps@bsd.lv>
 *
 * Permission to use, copy, modify, and distribute this software for any
 * purpose with or without fee is hereby granted, provided that the above
 * copyright notice and this permission notice appear in all copies.
 *
 * THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES
 * WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF
 * MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR
 * ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
 * WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN
 * ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF
 * OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
 */
#ifdef HAVE_CONFIG_H
#include "config.h"
#endif

#include <assert.h>
#include <errno.h>
#include <ctype.h>
#include <limits.h>
#include <stdlib.h>
#include <string.h>
#include <stdio.h>

#include "mandoc.h"
#include "regs.h"
#include "roff.h"

#define	RSTACK_MAX	128

#define	ROFF_CTL(c) \
	('.' == (c) || '\'' == (c))

enum	rofft {
	ROFF_am,
	ROFF_ami,
	ROFF_am1,
	ROFF_de,
	ROFF_dei,
	ROFF_de1,
	ROFF_ds,
	ROFF_el,
	ROFF_ie,
	ROFF_if,
	ROFF_ig,
	ROFF_rm,
	ROFF_tr,
	ROFF_cblock,
	ROFF_ccond,
	ROFF_nr,
	ROFF_MAX
};

enum	roffrule {
	ROFFRULE_ALLOW,
	ROFFRULE_DENY
};

struct	roff {
	struct roffnode	*last; /* leaf of stack */
	mandocmsg	 msg; /* err/warn/fatal messages */
	void		*data; /* privdata for messages */
	enum roffrule	 rstack[RSTACK_MAX]; /* stack of !`ie' rules */
	int		 rstackpos; /* position in rstack */
	struct regset	*regs; /* read/writable registers */
};

struct	roffnode {
	enum rofft	 tok; /* type of node */
	struct roffnode	*parent; /* up one in stack */
	int		 line; /* parse line */
	int		 col; /* parse col */
	char		*end; /* end-rules: custom token */
	int		 endspan; /* end-rules: next-line or infty */
	enum roffrule	 rule; /* current evaluation rule */
};

#define	ROFF_ARGS	 struct roff *r, /* parse ctx */ \
			 enum rofft tok, /* tok of macro */ \
		 	 char **bufp, /* input buffer */ \
			 size_t *szp, /* size of input buffer */ \
			 int ln, /* parse line */ \
			 int ppos, /* original pos in buffer */ \
			 int pos, /* current pos in buffer */ \
			 int *offs /* reset offset of buffer data */

typedef	enum rofferr (*roffproc)(ROFF_ARGS);

struct	roffmac {
	const char	*name; /* macro name */
	roffproc	 proc; /* process new macro */
	roffproc	 text; /* process as child text of macro */
	roffproc	 sub; /* process as child of macro */
	int		 flags;
#define	ROFFMAC_STRUCT	(1 << 0) /* always interpret */
	struct roffmac	*next;
};

static	enum rofferr	 roff_block(ROFF_ARGS);
static	enum rofferr	 roff_block_text(ROFF_ARGS);
static	enum rofferr	 roff_block_sub(ROFF_ARGS);
static	enum rofferr	 roff_cblock(ROFF_ARGS);
static	enum rofferr	 roff_ccond(ROFF_ARGS);
static	enum rofferr	 roff_cond(ROFF_ARGS);
static	enum rofferr	 roff_cond_text(ROFF_ARGS);
static	enum rofferr	 roff_cond_sub(ROFF_ARGS);
static	enum rofferr	 roff_line(ROFF_ARGS);
static	enum rofferr	 roff_nr(ROFF_ARGS);
static	enum roffrule	 roff_evalcond(const char *, int *);

/* See roff_hash_find() */

#define	ASCII_HI	 126
#define	ASCII_LO	 33
#define	HASHWIDTH	(ASCII_HI - ASCII_LO + 1)

static	struct roffmac	*hash[HASHWIDTH];

static	struct roffmac	 roffs[ROFF_MAX] = {
	{ "am", roff_block, roff_block_text, roff_block_sub, 0, NULL },
	{ "ami", roff_block, roff_block_text, roff_block_sub, 0, NULL },
	{ "am1", roff_block, roff_block_text, roff_block_sub, 0, NULL },
	{ "de", roff_block, roff_block_text, roff_block_sub, 0, NULL },
	{ "dei", roff_block, roff_block_text, roff_block_sub, 0, NULL },
	{ "de1", roff_block, roff_block_text, roff_block_sub, 0, NULL },
	{ "ds", roff_line, NULL, NULL, 0, NULL },
	{ "el", roff_cond, roff_cond_text, roff_cond_sub, ROFFMAC_STRUCT, NULL },
	{ "ie", roff_cond, roff_cond_text, roff_cond_sub, ROFFMAC_STRUCT, NULL },
	{ "if", roff_cond, roff_cond_text, roff_cond_sub, ROFFMAC_STRUCT, NULL },
	{ "ig", roff_block, roff_block_text, roff_block_sub, 0, NULL },
	{ "rm", roff_line, NULL, NULL, 0, NULL },
	{ "tr", roff_line, NULL, NULL, 0, NULL },
	{ ".", roff_cblock, NULL, NULL, 0, NULL },
	{ "\\}", roff_ccond, NULL, NULL, 0, NULL },
	{ "nr", roff_nr, NULL, NULL, 0, NULL },
};

static	void		 roff_free1(struct roff *);
static	enum rofft	 roff_hash_find(const char *);
static	void		 roff_hash_init(void);
static	void		 roffnode_cleanscope(struct roff *);
static	int		 roffnode_push(struct roff *, 
				enum rofft, int, int);
static	void		 roffnode_pop(struct roff *);
static	enum rofft	 roff_parse(const char *, int *);
static	int		 roff_parse_nat(const char *, unsigned int *);

/* See roff_hash_find() */
#define	ROFF_HASH(p)	(p[0] - ASCII_LO)

static void
roff_hash_init(void)
{
	struct roffmac	 *n;
	int		  buc, i;

	for (i = 0; i < (int)ROFF_MAX; i++) {
		assert(roffs[i].name[0] >= ASCII_LO);
		assert(roffs[i].name[0] <= ASCII_HI);

		buc = ROFF_HASH(roffs[i].name);

		if (NULL != (n = hash[buc])) {
			for ( ; n->next; n = n->next)
				/* Do nothing. */ ;
			n->next = &roffs[i];
		} else
			hash[buc] = &roffs[i];
	}
}


/*
 * Look up a roff token by its name.  Returns ROFF_MAX if no macro by
 * the nil-terminated string name could be found.
 */
static enum rofft
roff_hash_find(const char *p)
{
	int		 buc;
	struct roffmac	*n;

	/*
	 * libroff has an extremely simple hashtable, for the time
	 * being, which simply keys on the first character, which must
	 * be printable, then walks a chain.  It works well enough until
	 * optimised.
	 */

	if (p[0] < ASCII_LO || p[0] > ASCII_HI)
		return(ROFF_MAX);

	buc = ROFF_HASH(p);

	if (NULL == (n = hash[buc]))
		return(ROFF_MAX);
	for ( ; n; n = n->next)
		if (0 == strcmp(n->name, p))
			return((enum rofft)(n - roffs));

	return(ROFF_MAX);
}


/*
 * Pop the current node off of the stack of roff instructions currently
 * pending.
 */
static void
roffnode_pop(struct roff *r)
{
	struct roffnode	*p;

	assert(r->last);
	p = r->last; 

	if (ROFF_el == p->tok)
		if (r->rstackpos > -1)
			r->rstackpos--;

	r->last = r->last->parent;
	if (p->end)
		free(p->end);
	free(p);
}


/*
 * Push a roff node onto the instruction stack.  This must later be
 * removed with roffnode_pop().
 */
static int
roffnode_push(struct roff *r, enum rofft tok, int line, int col)
{
	struct roffnode	*p;

	if (NULL == (p = calloc(1, sizeof(struct roffnode)))) {
		(*r->msg)(MANDOCERR_MEM, r->data, line, col, NULL);
		return(0);
	}

	p->tok = tok;
	p->parent = r->last;
	p->line = line;
	p->col = col;
	p->rule = p->parent ? p->parent->rule : ROFFRULE_DENY;

	r->last = p;
	return(1);
}


static void
roff_free1(struct roff *r)
{

	while (r->last)
		roffnode_pop(r);
}


void
roff_reset(struct roff *r)
{

	roff_free1(r);
}


void
roff_free(struct roff *r)
{

	roff_free1(r);
	free(r);
}


struct roff *
roff_alloc(struct regset *regs, const mandocmsg msg, void *data)
{
	struct roff	*r;

	if (NULL == (r = calloc(1, sizeof(struct roff)))) {
		(*msg)(MANDOCERR_MEM, data, 0, 0, NULL);
		return(0);
	}

	r->regs = regs;
	r->msg = msg;
	r->data = data;
	r->rstackpos = -1;
	
	roff_hash_init();
	return(r);
}


enum rofferr
roff_parseln(struct roff *r, int ln, char **bufp, 
		size_t *szp, int pos, int *offs)
{
	enum rofft	 t;
	int		 ppos;

	/*
	 * First, if a scope is open and we're not a macro, pass the
	 * text through the macro's filter.  If a scope isn't open and
	 * we're not a macro, just let it through.
	 */

	if (r->last && ! ROFF_CTL((*bufp)[pos])) {
		t = r->last->tok;
		assert(roffs[t].text);
		return((*roffs[t].text)
				(r, t, bufp, szp, ln, pos, pos, offs));
	} else if ( ! ROFF_CTL((*bufp)[pos]))
		return(ROFF_CONT);

	/*
	 * If a scope is open, go to the child handler for that macro,
	 * as it may want to preprocess before doing anything with it.
	 */

	if (r->last) {
		t = r->last->tok;
		assert(roffs[t].sub);
		return((*roffs[t].sub)
				(r, t, bufp, szp, ln, pos, pos, offs));
	}

	/*
	 * Lastly, as we've no scope open, try to look up and execute
	 * the new macro.  If no macro is found, simply return and let
	 * the compilers handle it.
	 */

	ppos = pos;
	if (ROFF_MAX == (t = roff_parse(*bufp, &pos)))
		return(ROFF_CONT);

	assert(roffs[t].proc);
	return((*roffs[t].proc)
			(r, t, bufp, szp, ln, ppos, pos, offs));
}


int
roff_endparse(struct roff *r)
{

	if (NULL == r->last)
		return(1);
	return((*r->msg)(MANDOCERR_SCOPEEXIT, r->data, r->last->line, 
				r->last->col, NULL));
}


/*
 * Parse a roff node's type from the input buffer.  This must be in the
 * form of ".foo xxx" in the usual way.
 */
static enum rofft
roff_parse(const char *buf, int *pos)
{
	int		 j;
	char		 mac[5];
	enum rofft	 t;

	assert(ROFF_CTL(buf[*pos]));
	(*pos)++;

	while (buf[*pos] && (' ' == buf[*pos] || '\t' == buf[*pos]))
		(*pos)++;

	if ('\0' == buf[*pos])
		return(ROFF_MAX);

	for (j = 0; j < 4; j++, (*pos)++)
		if ('\0' == (mac[j] = buf[*pos]))
			break;
		else if (' ' == buf[*pos] || (j && '\\' == buf[*pos]))
			break;

	if (j == 4 || j < 1)
		return(ROFF_MAX);

	mac[j] = '\0';

	if (ROFF_MAX == (t = roff_hash_find(mac)))
		return(t);

	while (buf[*pos] && ' ' == buf[*pos])
		(*pos)++;

	return(t);
}


static int
roff_parse_nat(const char *buf, unsigned int *res)
{
	char		*ep;
	long		 lval;

	errno = 0;
	lval = strtol(buf, &ep, 10);
	if (buf[0] == '\0' || *ep != '\0')
		return(0);
	if ((errno == ERANGE && 
			(lval == LONG_MAX || lval == LONG_MIN)) ||
			(lval > INT_MAX || lval < 0))
		return(0);

	*res = (unsigned int)lval;
	return(1);
}


/* ARGSUSED */
static enum rofferr
roff_cblock(ROFF_ARGS)
{

	/*
	 * A block-close `..' should only be invoked as a child of an
	 * ignore macro, otherwise raise a warning and just ignore it.
	 */

	if (NULL == r->last) {
		if ( ! (*r->msg)(MANDOCERR_NOSCOPE, r->data, ln, ppos, NULL))
			return(ROFF_ERR);
		return(ROFF_IGN);
	}

	switch (r->last->tok) {
	case (ROFF_am):
		/* FALLTHROUGH */
	case (ROFF_ami):
		/* FALLTHROUGH */
	case (ROFF_am1):
		/* FALLTHROUGH */
	case (ROFF_de):
		/* FALLTHROUGH */
	case (ROFF_dei):
		/* FALLTHROUGH */
	case (ROFF_de1):
		/* FALLTHROUGH */
	case (ROFF_ig):
		break;
	default:
		if ( ! (*r->msg)(MANDOCERR_NOSCOPE, r->data, ln, ppos, NULL))
			return(ROFF_ERR);
		return(ROFF_IGN);
	}

	if ((*bufp)[pos])
		if ( ! (*r->msg)(MANDOCERR_ARGSLOST, r->data, ln, pos, NULL))
			return(ROFF_ERR);

	roffnode_pop(r);
	roffnode_cleanscope(r);
	return(ROFF_IGN);

}


static void
roffnode_cleanscope(struct roff *r)
{

	while (r->last) {
		if (--r->last->endspan < 0)
			break;
		roffnode_pop(r);
	}
}


/* ARGSUSED */
static enum rofferr
roff_ccond(ROFF_ARGS)
{

	if (NULL == r->last) {
		if ( ! (*r->msg)(MANDOCERR_NOSCOPE, r->data, ln, ppos, NULL))
			return(ROFF_ERR);
		return(ROFF_IGN);
	}

	switch (r->last->tok) {
	case (ROFF_el):
		/* FALLTHROUGH */
	case (ROFF_ie):
		/* FALLTHROUGH */
	case (ROFF_if):
		break;
	default:
		if ( ! (*r->msg)(MANDOCERR_NOSCOPE, r->data, ln, ppos, NULL))
			return(ROFF_ERR);
		return(ROFF_IGN);
	}

	if (r->last->endspan > -1) {
		if ( ! (*r->msg)(MANDOCERR_NOSCOPE, r->data, ln, ppos, NULL))
			return(ROFF_ERR);
		return(ROFF_IGN);
	}

	if ((*bufp)[pos])
		if ( ! (*r->msg)(MANDOCERR_ARGSLOST, r->data, ln, pos, NULL))
			return(ROFF_ERR);

	roffnode_pop(r);
	roffnode_cleanscope(r);
	return(ROFF_IGN);
}


/* ARGSUSED */
static enum rofferr
roff_block(ROFF_ARGS)
{
	int		sv;
	size_t		sz;

	if (ROFF_ig != tok && '\0' == (*bufp)[pos]) {
		if ( ! (*r->msg)(MANDOCERR_NOARGS, r->data, ln, ppos, NULL))
			return(ROFF_ERR);
		return(ROFF_IGN);
	} else if (ROFF_ig != tok) {
		while ((*bufp)[pos] && ' ' != (*bufp)[pos])
			pos++;
		while (' ' == (*bufp)[pos])
			pos++;
	}

	if ( ! roffnode_push(r, tok, ln, ppos))
		return(ROFF_ERR);

	if ('\0' == (*bufp)[pos])
		return(ROFF_IGN);

	sv = pos;
	while ((*bufp)[pos] && ' ' != (*bufp)[pos] && 
			'\t' != (*bufp)[pos])
		pos++;

	/*
	 * Note: groff does NOT like escape characters in the input.
	 * Instead of detecting this, we're just going to let it fly and
	 * to hell with it.
	 */

	assert(pos > sv);
	sz = (size_t)(pos - sv);

	if (1 == sz && '.' == (*bufp)[sv])
		return(ROFF_IGN);

	r->last->end = malloc(sz + 1);

	if (NULL == r->last->end) {
		(*r->msg)(MANDOCERR_MEM, r->data, ln, pos, NULL);
		return(ROFF_ERR);
	}

	memcpy(r->last->end, *bufp + sv, sz);
	r->last->end[(int)sz] = '\0';

	if ((*bufp)[pos])
		if ( ! (*r->msg)(MANDOCERR_ARGSLOST, r->data, ln, pos, NULL))
			return(ROFF_ERR);

	return(ROFF_IGN);
}


/* ARGSUSED */
static enum rofferr
roff_block_sub(ROFF_ARGS)
{
	enum rofft	t;
	int		i, j;

	/*
	 * First check whether a custom macro exists at this level.  If
	 * it does, then check against it.  This is some of groff's
	 * stranger behaviours.  If we encountered a custom end-scope
	 * tag and that tag also happens to be a "real" macro, then we
	 * need to try interpreting it again as a real macro.  If it's
	 * not, then return ignore.  Else continue.
	 */

	if (r->last->end) {
		i = pos + 1;
		while (' ' == (*bufp)[i] || '\t' == (*bufp)[i])
			i++;

		for (j = 0; r->last->end[j]; j++, i++)
			if ((*bufp)[i] != r->last->end[j])
				break;

		if ('\0' == r->last->end[j] && 
				('\0' == (*bufp)[i] ||
				 ' ' == (*bufp)[i] ||
				 '\t' == (*bufp)[i])) {
			roffnode_pop(r);
			roffnode_cleanscope(r);

			if (ROFF_MAX != roff_parse(*bufp, &pos))
				return(ROFF_RERUN);
			return(ROFF_IGN);
		}
	}

	/*
	 * If we have no custom end-query or lookup failed, then try
	 * pulling it out of the hashtable.
	 */

	ppos = pos;
	t = roff_parse(*bufp, &pos);

	/* If we're not a comment-end, then throw it away. */
	if (ROFF_cblock != t)
		return(ROFF_IGN);

	assert(roffs[t].proc);
	return((*roffs[t].proc)(r, t, bufp, szp, 
				ln, ppos, pos, offs));
}


/* ARGSUSED */
static enum rofferr
roff_block_text(ROFF_ARGS)
{

	return(ROFF_IGN);
}


/* ARGSUSED */
static enum rofferr
roff_cond_sub(ROFF_ARGS)
{
	enum rofft	 t;
	enum roffrule	 rr;
	struct roffnode	*l;

	ppos = pos;
	rr = r->last->rule;

	/* 
	 * Clean out scope.  If we've closed ourselves, then don't
	 * continue. 
	 */

	l = r->last;
	roffnode_cleanscope(r);

	if (l != r->last)
		return(ROFFRULE_DENY == rr ? ROFF_IGN : ROFF_CONT);

	if (ROFF_MAX == (t = roff_parse(*bufp, &pos)))
		return(ROFFRULE_DENY == rr ? ROFF_IGN : ROFF_CONT);

	/*
	 * A denied conditional must evaluate its children if and only
	 * if they're either structurally required (such as loops and
	 * conditionals) or a closing macro.
	 */
	if (ROFFRULE_DENY == rr)
		if ( ! (ROFFMAC_STRUCT & roffs[t].flags))
			if (ROFF_ccond != t)
				return(ROFF_IGN);

	assert(roffs[t].proc);
	return((*roffs[t].proc)(r, t, bufp, szp, 
				ln, ppos, pos, offs));
}


/* ARGSUSED */
static enum rofferr
roff_cond_text(ROFF_ARGS)
{
	char		*ep, *st;
	enum roffrule	 rr;

	rr = r->last->rule;

	/*
	 * We display the value of the text if out current evaluation
	 * scope permits us to do so.
	 */

	st = &(*bufp)[pos];
	if (NULL == (ep = strstr(st, "\\}"))) {
		roffnode_cleanscope(r);
		return(ROFFRULE_DENY == rr ? ROFF_IGN : ROFF_CONT);
	}

	if (ep == st || (ep > st && '\\' != *(ep - 1)))
		roffnode_pop(r);

	roffnode_cleanscope(r);
	return(ROFFRULE_DENY == rr ? ROFF_IGN : ROFF_CONT);
}


static enum roffrule
roff_evalcond(const char *v, int *pos)
{

	switch (v[*pos]) {
	case ('n'):
		(*pos)++;
		return(ROFFRULE_ALLOW);
	case ('e'):
		/* FALLTHROUGH */
	case ('o'):
		/* FALLTHROUGH */
	case ('t'):
		(*pos)++;
		return(ROFFRULE_DENY);
	default:
		break;
	}

	while (v[*pos] && ' ' != v[*pos])
		(*pos)++;
	return(ROFFRULE_DENY);
}


/* ARGSUSED */
static enum rofferr
roff_line(ROFF_ARGS)
{

	return(ROFF_IGN);
}


/* ARGSUSED */
static enum rofferr
roff_cond(ROFF_ARGS)
{
	int		 sv;
	enum roffrule	 rule;

	/* Stack overflow! */

	if (ROFF_ie == tok && r->rstackpos == RSTACK_MAX - 1) {
		(*r->msg)(MANDOCERR_MEM, r->data, ln, ppos, NULL);
		return(ROFF_ERR);
	}

	/* First, evaluate the conditional. */

	if (ROFF_el == tok) {
		/* 
		 * An `.el' will get the value of the current rstack
		 * entry set in prior `ie' calls or defaults to DENY.
	 	 */
		if (r->rstackpos < 0)
			rule = ROFFRULE_DENY;
		else
			rule = r->rstack[r->rstackpos];
	} else
		rule = roff_evalcond(*bufp, &pos);

	sv = pos;

	while (' ' == (*bufp)[pos])
		pos++;

	/*
	 * Roff is weird.  If we have just white-space after the
	 * conditional, it's considered the BODY and we exit without
	 * really doing anything.  Warn about this.  It's probably
	 * wrong.
	 */

	if ('\0' == (*bufp)[pos] && sv != pos) {
		if ((*r->msg)(MANDOCERR_NOARGS, r->data, ln, ppos, NULL))
			return(ROFF_IGN);
		return(ROFF_ERR);
	}

	if ( ! roffnode_push(r, tok, ln, ppos))
		return(ROFF_ERR);

	r->last->rule = rule;

	if (ROFF_ie == tok) {
		/*
		 * An if-else will put the NEGATION of the current
		 * evaluated conditional into the stack.
		 */
		r->rstackpos++;
		if (ROFFRULE_DENY == r->last->rule)
			r->rstack[r->rstackpos] = ROFFRULE_ALLOW;
		else
			r->rstack[r->rstackpos] = ROFFRULE_DENY;
	}

	/* If the parent has false as its rule, then so do we. */

	if (r->last->parent && ROFFRULE_DENY == r->last->parent->rule)
		r->last->rule = ROFFRULE_DENY;

	/*
	 * Determine scope.  If we're invoked with "\{" trailing the
	 * conditional, then we're in a multiline scope.  Else our scope
	 * expires on the next line.
	 */

	r->last->endspan = 1;

	if ('\\' == (*bufp)[pos] && '{' == (*bufp)[pos + 1]) {
		r->last->endspan = -1;
		pos += 2;
	} 

	/*
	 * If there are no arguments on the line, the next-line scope is
	 * assumed.
	 */

	if ('\0' == (*bufp)[pos])
		return(ROFF_IGN);

	/* Otherwise re-run the roff parser after recalculating. */

	*offs = pos;
	return(ROFF_RERUN);
}


/* ARGSUSED */
static enum rofferr
roff_nr(ROFF_ARGS)
{
	const char	*key, *val;
	struct reg	*rg;

	key = &(*bufp)[pos];
	rg = r->regs->regs;

	/* Parse register request. */
	while ((*bufp)[pos] && ' ' != (*bufp)[pos])
		pos++;

	/*
	 * Set our nil terminator.  Because this line is going to be
	 * ignored anyway, we can munge it as we please.
	 */
	if ((*bufp)[pos])
		(*bufp)[pos++] = '\0';

	/* Skip whitespace to register token. */
	while ((*bufp)[pos] && ' ' == (*bufp)[pos])
		pos++;

	val = &(*bufp)[pos];

	/* Process register token. */

	if (0 == strcmp(key, "nS")) {
		rg[(int)REG_nS].set = 1;
		if ( ! roff_parse_nat(val, &rg[(int)REG_nS].v.u))
			rg[(int)REG_nS].v.u = 0;
	}

	return(ROFF_IGN);
}