Log Message:
-----------
Make the second, section number argument of .Xr mandatory.
In fact, we have been requiring it for many years.
The only reason to not warn when it was missing
was excessive traditionalism - it was optional in 4.4BSD.

Modified Files:
--------------
    mdocml:
        mandoc.1
        mandoc.h
        mdoc.7
        mdoc_validate.c
        read.c

Revision Data
-------------
Index: mandoc.1
===================================================================
RCS file: /home/cvs/mdocml/mdocml/mandoc.1,v
retrieving revision 1.164
retrieving revision 1.165
diff -Lmandoc.1 -Lmandoc.1 -u -p -r1.164 -r1.165
--- mandoc.1
+++ mandoc.1
@@ -1147,6 +1147,13 @@ macro is immediately followed by an
 .Ic \&Re
 macro on the next input line.
 Such an empty block does not produce any output.
+.It Sy "missing section argument"
+.Pq mdoc
+An
+.Ic \&Xr
+macro lacks its second, section number argument.
+The first argument, i.e. the name, is printed, but without subsequent
+parantheses.
 .It Sy "missing -std argument, adding it"
 .Pq mdoc
 An
Index: mdoc.7
===================================================================
RCS file: /home/cvs/mdocml/mdocml/mdoc.7,v
retrieving revision 1.258
retrieving revision 1.259
diff -Lmdoc.7 -Lmdoc.7 -u -p -r1.258 -r1.259
--- mdoc.7
+++ mdoc.7
@@ -2714,14 +2714,13 @@ Link to another manual
 .Pq Qq cross-reference .
 Its syntax is as follows:
 .Pp
-.D1 Pf \. Sx \&Xr Ar name Op section
+.D1 Pf \. Sx \&Xr Ar name section
 .Pp
 Cross reference the
 .Ar name
 and
 .Ar section
-number of another man page;
-omitting the section number is rarely useful.
+number of another man page.
 .Pp
 Examples:
 .Dl \&.Xr mandoc 1
@@ -3033,7 +3032,7 @@ then the macro accepts an arbitrary numb
 .It Sx \&Ux  Ta    Yes      Ta    Yes      Ta    n
 .It Sx \&Va  Ta    Yes      Ta    Yes      Ta    n
 .It Sx \&Vt  Ta    Yes      Ta    Yes      Ta    >0
-.It Sx \&Xr  Ta    Yes      Ta    Yes      Ta    >0
+.It Sx \&Xr  Ta    Yes      Ta    Yes      Ta    2
 .It Sx \&br  Ta    \&No     Ta    \&No     Ta    0
 .It Sx \&sp  Ta    \&No     Ta    \&No     Ta    1
 .El
Index: mdoc_validate.c
===================================================================
RCS file: /home/cvs/mdocml/mdocml/mdoc_validate.c,v
retrieving revision 1.309
retrieving revision 1.310
diff -Lmdoc_validate.c -Lmdoc_validate.c -u -p -r1.309 -r1.310
--- mdoc_validate.c
+++ mdoc_validate.c
@@ -103,6 +103,7 @@ static	void	 post_sh_authors(POST_ARGS);
 static	void	 post_sm(POST_ARGS);
 static	void	 post_st(POST_ARGS);
 static	void	 post_std(POST_ARGS);
+static	void	 post_xr(POST_ARGS);
 
 static	v_post mdoc_valids[MDOC_MAX] = {
 	NULL,		/* Ap */
@@ -145,7 +146,7 @@ static	v_post mdoc_valids[MDOC_MAX] = {
 	post_st,	/* St */
 	NULL,		/* Va */
 	NULL,		/* Vt */
-	NULL,		/* Xr */
+	post_xr,	/* Xr */
 	NULL,		/* %A */
 	post_hyph,	/* %B */ /* FIXME: can be used outside Rs/Re. */
 	NULL,		/* %D */
@@ -1807,6 +1808,21 @@ post_sh_head(POST_ARGS)
 }
 
 static void
+post_xr(POST_ARGS)
+{
+	struct roff_node *n, *nch;
+
+	n = mdoc->last;
+	nch = n->child;
+	if (nch->next == NULL) {
+		mandoc_vmsg(MANDOCERR_XR_NOSEC, mdoc->parse,
+		    n->line, n->pos, "Xr %s", nch->string);
+		return;
+	}
+	assert(nch->next == n->last);
+}
+
+static void
 post_ignpar(POST_ARGS)
 {
 	struct roff_node *np;
@@ -2002,7 +2018,7 @@ post_dt(POST_ARGS)
 			}
 	}
 
-	/* Mandatory second argument: section. */
+	/* Mandatory second argument: section. */
 
 	if (nn != NULL)
 		nn = nn->next;
Index: mandoc.h
===================================================================
RCS file: /home/cvs/mdocml/mdocml/mandoc.h,v
retrieving revision 1.210
retrieving revision 1.211
diff -Lmandoc.h -Lmandoc.h -u -p -r1.210 -r1.211
--- mandoc.h
+++ mandoc.h
@@ -107,6 +107,7 @@ enum	mandocerr {
 	MANDOCERR_BF_BADFONT, /* unknown font type, using \fR: Bf font */
 	MANDOCERR_PF_SKIP, /* nothing follows prefix: Pf arg */
 	MANDOCERR_RS_EMPTY, /* empty reference block: Rs */
+	MANDOCERR_XR_NOSEC, /* missing section argument: Xr arg */
 	MANDOCERR_ARG_STD, /* missing -std argument, adding it: macro */
 	MANDOCERR_OP_EMPTY, /* missing option string, using "": OP */
 	MANDOCERR_UR_NOHEAD, /* missing resource identifier, using "": UR */
Index: read.c
===================================================================
RCS file: /home/cvs/mdocml/mdocml/read.c,v
retrieving revision 1.154
retrieving revision 1.155
diff -Lread.c -Lread.c -u -p -r1.154 -r1.155
--- read.c
+++ read.c
@@ -150,6 +150,7 @@ static	const char * const	mandocerrs[MAN
 	"unknown font type, using \\fR",
 	"nothing follows prefix",
 	"empty reference block",
+	"missing section argument",
 	"missing -std argument, adding it",
 	"missing option string, using \"\"",
 	"missing resource identifier, using \"\"",
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