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authorDirk Gouders <dirk@gouders.net>2018-07-18 11:13:36 +0200
committerMasahiro Yamada <yamada.masahiro@socionext.com>2018-07-21 06:50:32 +0900
commitbb81955fd4a49fffdd86d50afd0c1f2eea044c05 (patch)
tree1498493d163bdb67b59538c82b1275e78a467683 /Documentation/kbuild
parent0df57d90bfd6bb94831e70f4bcb4b7af784066cf (diff)
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 <dirk@gouders.net> Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Diffstat (limited to 'Documentation/kbuild')
-rw-r--r--Documentation/kbuild/makefiles.txt6
1 files changed, 6 insertions, 0 deletions
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.