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.

5 files changed, 30 insertions, 6 deletions

diff --git a/mandoc.1 b/mandoc.1
index c8ed310f..65616272 100644
--- a/mandoc.1
+++ b/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
diff --git a/mandoc.h b/mandoc.h
index 90ebe9c3..79756aec 100644
--- a/mandoc.h
+++ b/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 */
diff --git a/mdoc.7 b/mdoc.7
index bf5d9afd..a283254d 100644
--- a/mdoc.7
+++ b/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 number of arguments.
 .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
diff --git a/mdoc_validate.c b/mdoc_validate.c
index 97985558..f5f73df0 100644
--- a/mdoc_validate.c
+++ b/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;
diff --git a/read.c b/read.c
index 851e9153..9bfe2bda 100644
--- a/read.c
+++ b/read.c
@@ -150,6 +150,7 @@ static	const char * const	mandocerrs[MANDOCERR_MAX] = {
 	"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 \"\"",