1 files changed, 35 insertions, 1 deletions
diff --git a/unit-tests/deptgt-begin.mk b/unit-tests/deptgt-begin.mk
index c6ca2f4aa3c7..b71d78f371ed 100644
--- a/unit-tests/deptgt-begin.mk
+++ b/unit-tests/deptgt-begin.mk
@@ -1,4 +1,4 @@
-# $NetBSD: deptgt-begin.mk,v 1.3 2020/08/29 17:34:21 rillig Exp $
+# $NetBSD: deptgt-begin.mk,v 1.5 2020/11/15 22:28:08 rillig Exp $
 #
 # Tests for the special target .BEGIN in dependency declarations,
 # which is a container for commands that are run before any other
@@ -7,6 +7,40 @@
 .BEGIN:
 	: $@
 
+# To register a custom action to be run at the beginning, the simplest way is
+# to directly place some commands on the '.BEGIN' target.  This doesn't scale
+# though, since the ':' dependency operator prevents that any other place may
+# add its commands after this.
+#
+# There are several ways to resolve this situation, which are detailed below.
+.BEGIN:
+	: Making another $@.
+
+# One way to run commands at the beginning is to define a custom target and
+# make the .BEGIN depend on that target.  This way, the commands from the
+# custom target are run even before the .BEGIN target.
+.BEGIN: before-begin
+before-begin: .PHONY .NOTMAIN
+	: Making $@ before .BEGIN.
+
+# Another way is to define a custom target and make that a .USE dependency.
+# For the .BEGIN target, .USE dependencies do not work though, since in
+# Compat_Run, the .USE and .USEBEFORE nodes are expanded right after the
+# .BEGIN target has been run, which is too late.
+.BEGIN: use
+use: .USE .NOTMAIN
+	: Making $@ from a .USE dependency.
+
+# Same as with .USE, but run the commands before the main commands from the
+# .BEGIN target.
+#
+# For the .BEGIN target, .USEBEFORE dependencies do not work though, since in
+# Compat_Run, the .USE and .USEBEFORE nodes are expanded right after the
+# .BEGIN target has been run, which is too late.
+.BEGIN: use-before
+use-before: .USEBEFORE .NOTMAIN
+	: Making $@ from a .USEBEFORE dependency.
+
 all:
 	: $@