What is the purpose of MkRules.local.in and MkRules.rules ?

[AndreMikulec]

@jeroen

I am not finding the files MkRules.local.in and MkRules.rules anywhere in the docs

https://cran.r-project.org/doc/manuals/r-patched/R-admin.html

Are either meant to prepend/overwrite/append to specific entries or the entire file MkRules.local within the build process?

What does the ?= mean (of specifically) in the file MkRules.rules and the line

EOPTS ?= -mfpmath=sse -msse2

Is MkRules.rules read-in last?

Thanks,
Andre

[jeroen]

MkRules.local.in gets copied into MkRules.local after some substitutions:

base/scripts/build.bat

Lines 47 to 48 in d9107a3

	sed -e "s|@win@|%WIN%|" -e "s|@home@|%XR_HOME%|" -e "s|@home32@|%XHOME32%|"^
	%SOURCEDIR%\files\MkRules.local.in > %R_HOME%/src/gnuwin32/MkRules.local

What does the ?= mean (of specifically) in the file MkRules.rules and the line
EOPTS ?= -mfpmath=sse -msse2

This is standard gnu make syntax that sets a variable if it has not been set yet. So you could override it by defining EOPTS in MkRules.local, but if unset, EOPTS will default to -mfpmath=sse -msse2

[AndreMikulec]

Great answer! Thanks!