[Buildroot] [git commit] infra: add comment describing single/double dollar-sign rules
Thomas Petazzoni
thomas.petazzoni at free-electrons.com
Sat Jun 14 17:10:01 UTC 2014
commit: http://git.buildroot.net/buildroot/commit/?id=7742d073a434b07a72cdef5bf0eb81572e6c3a8c
branch: http://git.buildroot.net/buildroot/commit/?id=refs/heads/master
As the rules with respect to variable and function references and the need
for single or double dollar signs are not trivial, add a comment in
pkg-generic.mk describing them.
Signed-off-by: Thomas De Schampheleire <thomas.de.schampheleire at gmail.com>
Acked-by: "Yann E. MORIN" <yann.morin.1998 at free.fr>
Reviewed-by: Thomas Petazzoni <thomas.petazzoni at free-electrons.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni at free-electrons.com>
---
package/pkg-generic.mk | 23 +++++++++++++++++++++++
1 files changed, 23 insertions(+), 0 deletions(-)
diff --git a/package/pkg-generic.mk b/package/pkg-generic.mk
index 3821d33..457d873 100644
--- a/package/pkg-generic.mk
+++ b/package/pkg-generic.mk
@@ -269,6 +269,29 @@ endef
# argument 3 is the uppercase package name, without the HOST_ prefix
# for host packages
# argument 4 is the type (target or host)
+#
+# Note about variable and function references: inside all blocks that are
+# evaluated with $(eval), which includes all 'inner-xxx-package' blocks,
+# specific rules apply with respect to variable and function references.
+# - Numbered variables (parameters to the block) can be referenced with a single
+# dollar sign: $(1), $(2), $(3), etc.
+# - pkgdir and pkgname should be referenced with a single dollar sign too. These
+# functions rely on 'the most recently parsed makefile' which is supposed to
+# be the package .mk file. If we defer the evaluation of these functions using
+# double dollar signs, then they may be evaluated too late, when other
+# makefiles have already been parsed. One specific case is when $$(pkgdir) is
+# assigned to a variable using deferred evaluation with '=' and this variable
+# is used in a target rule outside the eval'ed inner block. In this case, the
+# pkgdir will be that of the last makefile parsed by buildroot, which is not
+# the expected value. This mechanism is for example used for the TARGET_PATCH
+# rule.
+# - All other variables should be referenced with a double dollar sign:
+# $$(TARGET_DIR), $$($(2)_VERSION), etc. Also all make functions should be
+# referenced with a double dollar sign: $$(subst), $$(call), $$(filter-out),
+# etc. This rule ensures that these variables and functions are only expanded
+# during the $(eval) step, and not earlier. Otherwise, unintuitive and
+# undesired behavior occurs with respect to these variables and functions.
+#
################################################################################
define inner-generic-package
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