From bb81955fd4a49fffdd86d50afd0c1f2eea044c05 Mon Sep 17 00:00:00 2001 From: Dirk Gouders Date: Wed, 18 Jul 2018 11:13:36 +0200 Subject: kbuild: if_changed: document single use per target limitation 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 Signed-off-by: Masahiro Yamada --- Documentation/kbuild/makefiles.txt | 6 ++++++ 1 file changed, 6 insertions(+) (limited to 'Documentation/kbuild/makefiles.txt') diff --git a/Documentation/kbuild/makefiles.txt b/Documentation/kbuild/makefiles.txt index 63655c1a3ad6..766355b1d221 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. -- cgit