Users of if_changed could easily feel invited to use it to divide a
recipe into parts like:
a: prereq FORCE
$(call if_changed,do_a)
$(call if_changed,do_b)
But this is problematic, because if_changed should not be used more
than once per target: in the above example, if_changed stores the
command-line of the given command in .a.cmd and when a is up-to-date
with respect to prereq, the file .a.cmd contains the command-line for
the last command executed, i.e. do_b.
When the recipe is then executed again, without any change of
prerequisites, the command-line check for do_a will fail, do_a will be
executed and stored in .a.cmd. The next check, however, will still see
the old content (the file isn't re-read) and if_changed will skip
do_b, because the command-line test will not recognize a change. On
the next execution of the recipe the roles will flip: do_a is OK but
do_b not and it will be executed. And so on...
Signed-off-by: Dirk Gouders <dirk@xxxxxxxxxxx>
---
Documentation/kbuild/makefiles.txt | 6 ++++++
1 file changed, 6 insertions(+)
diff --git a/Documentation/kbuild/makefiles.txt b/Documentation/kbuild/makefiles.txt
index 048fc39a6b91..01fae5a01ce7 100644
--- a/Documentation/kbuild/makefiles.txt
+++ b/Documentation/kbuild/makefiles.txt
@@ -1105,6 +1105,12 @@ When kbuild executes, the following steps are followed (roughly):
target: source(s) FORCE
#WRONG!# $(call if_changed, ld/objcopy/gzip/...)
+ Note: if_changed should not be used more than once per target.
+ It stores the executed command in a corresponding .cmd
+ file and multiple calls would result in overwrites and
+ unwanted results when the target is up to date and only the
+ tests on changed commands trigger execution of commands.
+
ld
Link target. Often, LDFLAGS_$@ is used to set specific options to ld.
--
2.16.1
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