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infra: add comment describing single/double dollar-sign rules

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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@gmail.com>
Acked-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Reviewed-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
This commit is contained in:
Thomas De Schampheleire 2014-06-11 21:12:25 +02:00 committed by Thomas Petazzoni
parent 54456cc698
commit 7742d073a4
1 changed files with 23 additions and 0 deletions
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23
package/pkg-generic.mk
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@ -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