[PATCH] kbuild: if_changed: document single use per target limitation

[PATCH] kbuild: if_changed: document single use per target limitation

[Dirk Gouders]

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@gouders.net>
---
 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

Re: [PATCH] kbuild: if_changed: document single use per target limitation

[Masahiro Yamada]

2018-07-18 18:13 GMT+09:00 Dirk Gouders <dirk@gouders.net>:
> 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@gouders.net>
> ---

Applied.  Thanks!


>  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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-- 
Best Regards
Masahiro Yamada