Initial, still somewhat experimental implementation to leverage

less(1) -T and :t ctags(1)-like functionality to jump to the
definitions of various terms inside manual pages.
To be polished in the tree, so bear with me and report issues.

Technically, if less(1) is used as a pager, information is collected
by the mdoc(7) terminal formatter, first stored using the ohash
library, then ultimately written to a temporary file which is passed
to less via -T.  No change intended for other output formatters or
when running without a pager.

Based on an idea from Kristaps using feedback from many, in particular
phessler@ nicm@ millert@ halex@ doug@ kspillner@ deraadt@.

9 files changed, 292 insertions, 29 deletions

diff --git a/Makefile b/Makefile
index 5007315a..8f466d02 100644
--- a/Makefile
+++ b/Makefile
@@ -93,6 +93,7 @@ SRCS		 = att.c \
 		   roff.c \
 		   soelim.c \
 		   st.c \
+		   tag.c \
 		   tbl.c \
 		   tbl_data.c \
 		   tbl_html.c \
@@ -160,6 +161,7 @@ DISTFILES	 = INSTALL \
 		   soelim.1 \
 		   st.in \
 		   style.css \
+		   tag.h \
 		   tbl.3 \
 		   tbl.7 \
 		   term.h \
@@ -234,6 +236,7 @@ BASE_OBJS	 = $(MANDOC_HTML_OBJS) \
 		   main.o \
 		   manpath.o \
 		   out.o \
+		   tag.o \
 		   tree.o
 
 MAIN_OBJS	 = $(BASE_OBJS)
diff --git a/Makefile.depend b/Makefile.depend
index 52e95807..1481d1cd 100644
--- a/Makefile.depend
+++ b/Makefile.depend
@@ -22,7 +22,7 @@ eqn_html.o: eqn_html.c config.h mandoc.h out.h html.h
 eqn_term.o: eqn_term.c config.h mandoc.h out.h term.h
 html.o: html.c config.h mandoc.h mandoc_aux.h out.h html.h manconf.h main.h
 lib.o: lib.c config.h roff.h mdoc.h libmdoc.h lib.in
-main.o: main.c config.h mandoc_aux.h mandoc.h roff.h mdoc.h man.h main.h manconf.h mansearch.h
+main.o: main.c config.h mandoc_aux.h mandoc.h roff.h mdoc.h man.h tag.h main.h manconf.h mansearch.h
 man.o: man.c config.h mandoc_aux.h mandoc.h roff.h man.h libmandoc.h roff_int.h libman.h
 man_hash.o: man_hash.c config.h roff.h man.h libman.h
 man_html.o: man_html.c config.h mandoc_aux.h roff.h man.h out.h html.h main.h
@@ -42,7 +42,7 @@ mdoc_hash.o: mdoc_hash.c config.h roff.h mdoc.h libmdoc.h
 mdoc_html.o: mdoc_html.c config.h mandoc_aux.h roff.h mdoc.h out.h html.h main.h
 mdoc_macro.o: mdoc_macro.c config.h mandoc.h roff.h mdoc.h libmandoc.h roff_int.h libmdoc.h
 mdoc_man.o: mdoc_man.c config.h mandoc_aux.h mandoc.h roff.h mdoc.h man.h out.h main.h
-mdoc_term.o: mdoc_term.c config.h mandoc_aux.h mandoc.h roff.h mdoc.h out.h term.h main.h
+mdoc_term.o: mdoc_term.c config.h mandoc_aux.h mandoc.h roff.h mdoc.h out.h term.h tag.h main.h
 mdoc_validate.o: mdoc_validate.c config.h mandoc_aux.h mandoc.h roff.h mdoc.h libmandoc.h roff_int.h libmdoc.h
 msec.o: msec.c config.h mandoc.h libmandoc.h msec.in
 out.o: out.c config.h mandoc_aux.h mandoc.h out.h
@@ -51,6 +51,7 @@ read.o: read.c config.h mandoc_aux.h mandoc.h roff.h mdoc.h man.h libmandoc.h ro
 roff.o: roff.c config.h mandoc.h mandoc_aux.h roff.h libmandoc.h roff_int.h libroff.h predefs.in
 soelim.o: soelim.c config.h compat_stringlist.h
 st.o: st.c config.h roff.h mdoc.h libmdoc.h st.in
+tag.o: tag.c compat_ohash.h mandoc_aux.h tag.h
 tbl.o: tbl.c config.h mandoc.h mandoc_aux.h libmandoc.h libroff.h
 tbl_data.o: tbl_data.c config.h mandoc.h mandoc_aux.h libmandoc.h libroff.h
 tbl_html.o: tbl_html.c config.h mandoc.h out.h html.h
diff --git a/main.c b/main.c
index 61aba7ae..75f6e4e0 100644
--- a/main.c
+++ b/main.c
@@ -39,6 +39,7 @@
 #include "roff.h"
 #include "mdoc.h"
 #include "man.h"
+#include "tag.h"
 #include "main.h"
 #include "manconf.h"
 #include "mansearch.h"
@@ -497,7 +498,9 @@ out:
 
 	if (pager_pid != 0 && pager_pid != 1) {
 		fclose(stdout);
+		tag_write();
 		waitpid(pager_pid, NULL, 0);
+		tag_unlink();
 	}
 
 	return((int)rc);
@@ -959,10 +962,50 @@ spawn_pager(void)
 	char		*argv[MAX_PAGER_ARGS];
 	const char	*pager;
 	char		*cp;
+	size_t		 cmdlen;
 	int		 fildes[2];
 	int		 argc;
 	pid_t		 pager_pid;
 
+	pager = getenv("MANPAGER");
+	if (pager == NULL || *pager == '\0')
+		pager = getenv("PAGER");
+	if (pager == NULL || *pager == '\0')
+		pager = "more -s";
+	cp = mandoc_strdup(pager);
+
+	/*
+	 * Parse the pager command into words.
+	 * Intentionally do not do anything fancy here.
+	 */
+
+	argc = 0;
+	while (argc + 4 < MAX_PAGER_ARGS) {
+		argv[argc++] = cp;
+		cp = strchr(cp, ' ');
+		if (cp == NULL)
+			break;
+		*cp++ = '\0';
+		while (*cp == ' ')
+			cp++;
+		if (*cp == '\0')
+			break;
+	}
+
+	/* Read all text right away and use the tag file. */
+
+	if ((cmdlen = strlen(argv[0])) >= 4) {
+		cp = argv[0] + cmdlen - 4;
+		if (strcmp(cp, "less") == 0 ||
+		    strcmp(cp, "more") == 0) {
+			tag_init();
+			argv[argc++] = mandoc_strdup("+G1G");
+			argv[argc++] = mandoc_strdup("-T");
+			argv[argc++] = tag_filename();
+		}
+	}
+	argv[argc] = NULL;
+
 	if (pipe(fildes) == -1) {
 		fprintf(stderr, "%s: pipe: %s\n",
 		    progname, strerror(errno));
@@ -998,32 +1041,6 @@ spawn_pager(void)
 	}
 	close(fildes[0]);
 
-	pager = getenv("MANPAGER");
-	if (pager == NULL || *pager == '\0')
-		pager = getenv("PAGER");
-	if (pager == NULL || *pager == '\0')
-		pager = "more -s";
-	cp = mandoc_strdup(pager);
-
-	/*
-	 * Parse the pager command into words.
-	 * Intentionally do not do anything fancy here.
-	 */
-
-	argc = 0;
-	while (argc + 1 < MAX_PAGER_ARGS) {
-		argv[argc++] = cp;
-		cp = strchr(cp, ' ');
-		if (cp == NULL)
-			break;
-		*cp++ = '\0';
-		while (*cp == ' ')
-			cp++;
-		if (*cp == '\0')
-			break;
-	}
-	argv[argc] = NULL;
-
 	/* Hand over to the pager. */
 
 	execvp(argv[0], argv);
diff --git a/man.1 b/man.1
index db4c86e9..0f756c4e 100644
--- a/man.1
+++ b/man.1
@@ -360,6 +360,13 @@ Any non-empty value of the environment variable
 .Ev MANPAGER
 will be used instead of the standard pagination program,
 .Xr more 1 .
+If
+.Xr less 1
+is used, the interactive
+.Ic :t
+command can be used to go to the definitions of various terms, for
+example command line options, command modifiers, internal commands,
+and environment variables.
 .It Ev MANPATH
 The standard search path used by
 .Nm
diff --git a/mdoc_term.c b/mdoc_term.c
index a2ce71fc..95790806 100644
--- a/mdoc_term.c
+++ b/mdoc_term.c
@@ -34,6 +34,7 @@
 #include "mdoc.h"
 #include "out.h"
 #include "term.h"
+#include "tag.h"
 #include "main.h"
 
 struct	termpair {
@@ -117,6 +118,7 @@ static	int	  termp_skip_pre(DECL_ARGS);
 static	int	  termp_sm_pre(DECL_ARGS);
 static	int	  termp_sp_pre(DECL_ARGS);
 static	int	  termp_ss_pre(DECL_ARGS);
+static	int	  termp_tag_pre(DECL_ARGS);
 static	int	  termp_under_pre(DECL_ARGS);
 static	int	  termp_ud_pre(DECL_ARGS);
 static	int	  termp_vt_pre(DECL_ARGS);
@@ -145,7 +147,7 @@ static	const struct termact termacts[MDOC_MAX] = {
 	{ termp_bold_pre, NULL }, /* Cm */
 	{ NULL, NULL }, /* Dv */
 	{ NULL, NULL }, /* Er */
-	{ NULL, NULL }, /* Ev */
+	{ termp_tag_pre, NULL }, /* Ev */
 	{ termp_ex_pre, NULL }, /* Ex */
 	{ termp_fa_pre, NULL }, /* Fa */
 	{ termp_fd_pre, termp_fd_post }, /* Fd */
@@ -1049,6 +1051,7 @@ static int
 termp_fl_pre(DECL_ARGS)
 {
 
+	termp_tag_pre(p, pair, meta, n);
 	term_fontpush(p, TERMFONT_BOLD);
 	term_word(p, "\\-");
 
@@ -1330,6 +1333,7 @@ static int
 termp_bold_pre(DECL_ARGS)
 {
 
+	termp_tag_pre(p, pair, meta, n);
 	term_fontpush(p, TERMFONT_BOLD);
 	return(1);
 }
@@ -2252,3 +2256,19 @@ termp_under_pre(DECL_ARGS)
 	term_fontpush(p, TERMFONT_UNDER);
 	return(1);
 }
+
+static int
+termp_tag_pre(DECL_ARGS)
+{
+
+	if (n->child != NULL &&
+	    n->child->type == ROFFT_TEXT &&
+	    n->prev == NULL &&
+	    (n->parent->tok == MDOC_It ||
+	     (n->parent->tok == MDOC_Xo &&
+	      n->parent->parent->prev == NULL &&
+	      n->parent->parent->parent->tok == MDOC_It)) &&
+	    ! tag_get(n->child->string, 0))
+		tag_put(n->child->string, 0, p->line);
+	return(1);
+}
diff --git a/tag.c b/tag.c
new file mode 100644
index 00000000..a3610a47
--- /dev/null
+++ b/tag.c
@@ -0,0 +1,183 @@
+/*      $Id$    */
+/*
+ * Copyright (c) 2015 Ingo Schwarze <schwarze@openbsd.org>
+ *
+ * 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.
+ */
+#include <sys/types.h>
+
+#include <stddef.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+#include <unistd.h>
+
+#if HAVE_OHASH
+#include <ohash.h>
+#else
+#include "compat_ohash.h"
+#endif
+
+#include "mandoc_aux.h"
+#include "tag.h"
+
+struct tag_entry {
+	size_t	 line;
+	char	 s[];
+};
+
+static	void	*tag_alloc(size_t, void *);
+static	void	 tag_free(void *, void *);
+static	void	*tag_calloc(size_t, size_t, void *);
+
+static struct ohash	 tag_data;
+static char		*tag_fn = NULL;
+static int		 tag_fd = -1;
+
+
+/*
+ * Set up the ohash table to collect output line numbers
+ * where various marked-up terms are documented and create
+ * the temporary tags file, saving the name for the pager.
+ */
+void
+tag_init(void)
+{
+	struct ohash_info	 tag_info;
+
+	tag_fn = mandoc_strdup("/tmp/man.XXXXXXXXXX");
+	if ((tag_fd = mkstemp(tag_fn)) == -1) {
+		free(tag_fn);
+		tag_fn = NULL;
+		return;
+	}
+
+	tag_info.alloc = tag_alloc;
+	tag_info.calloc = tag_calloc;
+	tag_info.free = tag_free;
+	tag_info.key_offset = offsetof(struct tag_entry, s);
+	tag_info.data = NULL;
+	ohash_init(&tag_data, 4, &tag_info);
+}
+
+char *
+tag_filename(void)
+{
+
+	return(tag_fn);
+}
+
+/*
+ * Return the line number where a term is defined,
+ * or 0 if the term is unknown.
+ */
+size_t
+tag_get(const char *s, size_t len)
+{
+	struct tag_entry	*entry;
+	const char		*end;
+	unsigned int		 slot;
+
+	if (tag_fd == -1)
+		return(0);
+	if (len == 0)
+		len = strlen(s);
+	end = s + len;
+	slot = ohash_qlookupi(&tag_data, s, &end);
+	entry = ohash_find(&tag_data, slot);
+	return(entry == NULL ? 0 : entry->line);
+}
+
+/*
+ * Set the line number where a term is defined.
+ */
+void
+tag_put(const char *s, size_t len, size_t line)
+{
+	struct tag_entry	*entry;
+	const char		*end;
+	unsigned int		 slot;
+
+	if (tag_fd == -1)
+		return;
+	if (len == 0)
+		len = strlen(s);
+	end = s + len;
+	slot = ohash_qlookupi(&tag_data, s, &end);
+	entry = ohash_find(&tag_data, slot);
+	if (entry == NULL) {
+		entry = mandoc_malloc(sizeof(*entry) + len + 1);
+		memcpy(entry->s, s, len);
+		entry->s[len] = '\0';
+		ohash_insert(&tag_data, slot, entry);
+	}
+	entry->line = line;
+}
+
+/*
+ * Write out the tags file using the previously collected
+ * information and clear the ohash table while going along.
+ */
+void
+tag_write(void)
+{
+	FILE			*stream;
+	struct tag_entry	*entry;
+	unsigned int		 slot;
+
+	if (tag_fd == -1)
+		return;
+	stream = fdopen(tag_fd, "w");
+	entry = ohash_first(&tag_data, &slot);
+	while (entry != NULL) {
+		if (stream != NULL)
+			fprintf(stream, "%s - %zu\n", entry->s, entry->line);
+		free(entry);
+		entry = ohash_next(&tag_data, &slot);
+	}
+	ohash_delete(&tag_data);
+	if (stream != NULL)
+		fclose(stream);
+}
+
+void
+tag_unlink(void)
+{
+
+	if (tag_fn != NULL)
+		unlink(tag_fn);
+}
+
+/*
+ * Memory management callback functions for ohash.
+ */
+static void *
+tag_alloc(size_t sz, void *arg)
+{
+
+	return(mandoc_malloc(sz));
+}
+
+static void *
+tag_calloc(size_t nmemb, size_t sz, void *arg)
+{
+
+	return(mandoc_calloc(nmemb, sz));
+}
+
+static void
+tag_free(void *p, void *arg)
+{
+
+	free(p);
+}
diff --git a/tag.h b/tag.h
new file mode 100644
index 00000000..18f7b804
--- /dev/null
+++ b/tag.h
@@ -0,0 +1,27 @@
+/*      $Id$    */
+/*
+ * Copyright (c) 2015 Ingo Schwarze <schwarze@openbsd.org>
+ *
+ * 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.
+ */
+
+__BEGIN_DECLS
+
+void	 tag_init(void);
+char	*tag_filename(void);
+size_t	 tag_get(const char *, size_t);
+void	 tag_put(const char *, size_t, size_t);
+void	 tag_write(void);
+void	 tag_unlink(void);
+
+__END_DECLS
diff --git a/term.h b/term.h
index fc4eda7d..debaab94 100644
--- a/term.h
+++ b/term.h
@@ -53,6 +53,7 @@ struct	termp {
 	struct rofftbl	  tbl;		/* table configuration */
 	int		  synopsisonly; /* print the synopsis only */
 	int		  mdocstyle;	/* imitate mdoc(7) output */
+	size_t		  line;		/* Current output line number. */
 	size_t		  defindent;	/* Default indent for text. */
 	size_t		  defrmargin;	/* Right margin of the device. */
 	size_t		  lastrmargin;	/* Right margin before the last ll. */
diff --git a/term_ascii.c b/term_ascii.c
index d210c812..596c1142 100644
--- a/term_ascii.c
+++ b/term_ascii.c
@@ -70,6 +70,7 @@ ascii_init(enum termenc enc, const struct mchars *mchars,
 	p = mandoc_calloc(1, sizeof(struct termp));
 
 	p->symtab = mchars;
+	p->line = 1;
 	p->tabwidth = 5;
 	p->defrmargin = p->lastrmargin = 78;
 	p->fontq = mandoc_reallocarray(NULL,
@@ -163,6 +164,7 @@ ascii_sepline(void *arg)
 	size_t		 i;
 
 	p = (struct termp *)arg;
+	p->line += 3;
 	putchar('\n');
 	for (i = 0; i < p->defrmargin; i++)
 		putchar('-');
@@ -209,6 +211,7 @@ static void
 ascii_endline(struct termp *p)
 {
 
+	p->line++;
 	putchar('\n');
 }
 
@@ -365,6 +368,7 @@ static void
 locale_endline(struct termp *p)
 {
 
+	p->line++;
 	putwchar(L'\n');
 }