start: set LXC_HOOK_VERSION

doc/lxc.container.conf.sgml.in

@@ -1667,6 +1667,11 @@ Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA
           conditional on the value of <option>lxc.hook.version</option>. If it
           is set to 1 then LXC_HOOK_SECTION will be set.
           </para></listitem>
+          <listitem><para> LXC_HOOK_VERSION: the version of the hooks. This
+          value is identical to the value of the container's
+          <option>lxc.hook.version</option> config item. If it is set to 0 then
+          old-style hooks are used. If it is set to 1 then new-style hooks are
+          used. </para></listitem>
           <listitem><para> LXC_LOG_LEVEL: the container's log level. </para></listitem>
           <listitem><para> LXC_NAME: is the container's name. </para></listitem>
           <listitem><para> LXC_ROOTFS_MOUNT: the path to the mounted root filesystem. </para></listitem>

src/lxc/start.c

@@ -612,6 +612,7 @@ on_error:
 
 int lxc_init(const char *name, struct lxc_handler *handler)
 {
+	int ret;
 	const char *loglevel;
 	struct lxc_conf *conf = handler->conf;
 
@@ -656,6 +657,13 @@ int lxc_init(const char *name, struct lxc_handler *handler)
 	loglevel = lxc_log_priority_to_string(lxc_log_get_level());
 	if (setenv("LXC_LOG_LEVEL", loglevel, 1))
 		SYSERROR("Failed to set environment variable LXC_LOG_LEVEL=%s", loglevel);
+
+	if (conf->hooks_version == 0)
+		ret = setenv("LXC_HOOK_VERSION", "0", 1);
+	else
+		ret = setenv("LXC_HOOK_VERSION", "1", 1);
+	if (ret < 0)
+		SYSERROR("Failed to set environment variable LXC_HOOK_VERSION=%u", conf->hooks_version);
 	/* End of environment variable setup for hooks. */
 
 	TRACE("set environment variables");