Re: question on Makefile first flavor assignment
Bahman Movaqar <[email protected]>
| Newsgroups | gmane.comp.gnu.make.general |
|---|---|
| Message-ID | <[email protected]> |
I believe a Makefile is worth a thousand words! Try out the attached Makefile by simply running `make'. I hope it clearly demonstrates why extra care should be taken when using recursive variables. Let me know if you need more clarification. -- Bahman On Wed, 2023-09-13 at 16:48 -0700, Tom Ma wrote: > Hi gnu make friend, > > I am trying to understand the first flavor of variable - recursively > expanded variable assignment disadvantages mentioned in gnu make > manual, > > Another disadvantage is that any functions (see Functions for > Transforming > Text > <https://www.gnu.org/software/make/manual/html_node/Functions.html>) > referenced in the definition will be executed every time the variable > is > expanded. This makes make run slower; worse, it causes the wildcard > and > shell functions to give unpredictable results because you cannot > easily > control when they are called, or even how many times. > > For the recursively expanded variable assignment on Transforming > Text > functions, it will be executed every time the variable is expanded, > which > will cause make run slower. > Can you give a simple example to make it a little bit more clearer? > > Thanks, > Tom
Makefile
(text/x-makefile, 1.3 KB)
################################################################# # Demonstrating recursive variables - Bahman Movaqar ################################################################# SHELL := /usr/bin/env -S bash -o pipefail .DEFAULT_GOAL := all ################################################################# define a-recursive-var = $(info 💡 a-recursive-var is computed.)$(strip a-recursive-var) endef define a-simple-var := $(info 💡 a-simple-var is computed.)$(strip a-simple-var) endef .PHONY : demonstrating-text-functions demonstrating-text-functions : @echo \ ; echo $(@) \ ; echo ' $(a-recursive-var)' \ ; echo ' $(a-recursive-var)' \ ; echo ' $(a-recursive-var)' \ ; echo ' $(a-simple-var)' \ ; echo ' $(a-simple-var)' \ ; echo ' $(a-simple-var)' \ ; echo ################################################################# timestamp = $(shell perl -MTime::HiRes=time -E'say time()') .PHONY : demonstrating-shell-function demonstrating-shell-function : @echo \ ; echo $(@) \ ; export ts1='$(timestamp)' \ ; export ts2='$(timestamp)' \ ; [[ "$$ts1" == "$$ts2" ]] \ && echo " ✅ ts1 = ts2" \ || echo " ❌ ts1 ≠ ts2" \ ; echo ################################################################# .PHONY : all all : demonstrating-text-functions all : demonstrating-shell-function