Re: exported vs command line variables

[Pietro]

Paul Smith <address@hidden> writes:

> On Tue, 2016-09-20 at 15:28 +0000, Pietro wrote:
>> Hi,
>> 
>> I have noticed that there is a difference between the two scenarios
>> listed below:
>> 
>> i) make  CC=arm-linux-gnueabihf-gcc CPPFLAGS=[..]
>> 
>> ii) export CPPFLAGS=[..] [RET]
>>     make CC=arm-linux-gnueabihf-gcc
>> 
>> I was told that the difference is that in the second case Make
>> will append stuff to the CPPFLAGS variable exported while in the
>> first case the command line argument will override the settings
>> contained in the makefile itself.
>
> That is definitely not true.  Whether or not the values here replace or
> append to any value of CPPFLAGS in the makefile depends entirely on how
> the variable is set in the makefile.
>
> GNU make has a hierarchy of precedence when it comes to how variables
> are set.  The weakest setting is values that are built-in to make.
>  Next highest is values taken from the environment.  Those are
> overridden by values set in the makefile itself.  And finally values
> set on the command line have the highest precedence.
>
> So if you have a makefile like this:
>
>   CPPFLAGS = -Wall
>
>   all: ; echo 'CPPFLAGS=$(CPPFLAGS)'
>
> Then if you run:
>
>   $ CPPFLAGS=-Werror make
>   CPPFLAGS=-Wall
>
> and the makefile setting overrides the environment setting.  But:
>
>   $ make CPPFLAGS=-Werror
>   CPPFLAGS=-Werror
>
> command line settings take precedence over makefile settings.  In
> neither case will the value in the environment or command line be
> appended to the value in the makefile: it always replaces the value.
>
> There are ways you can manipulate the priority, using the -e command
> line option and the override specifier on variable assignment (see the
> GNU make manual for this).  
>
>> The following example, literally pasted from another thread could
>> clarify the matter :
>> 
>> var := $(shell echo "echo hi" >say_hi.sh; chmod +x say_hi.sh;
>> say_hi.sh)
>> all: ; @echo $(var)
>
> This actually just adds a lot of confusion and doesn't clarify it at
> all, because (a) you're using PATH environment variable here which is
> "special", at least nominally and (b) you're using $(shell ...) not
> recipes, and the behavior of the shell function with respect to the
> environment is different from that of recipes.  This example seems to
> me to be completely different than what you asked about above, such
> that you can't really compare them.
>
>> I have posted a question at the general make mailing list and I was
>> told that this might be a bug, I would love to have a clarification -
>> is it a bug ?
>
> I think this is more a matter of you wanted to do something and you
> followed a path and got stuck, so you asked about what got you stuck.
>
> I think you should back up and ask about what you are really trying to
> do in the first place, with an example, because likely the path you
> followed is not the one you want so discussing where you're stuck won't
> really help.
>
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Thanks a lot, really.

I was trying to cross-compile a library and reading a forum about
the library itself I have read this sentence:

------------------------------
Try this instead:

CPPFLAGS=-I/home/rafael/mobi/ArmLibs/include \ 
LDFLAGS=-L/home/rafael/mobi/ArmLibs/lib \
make HAS_PKG_CONFIG=false CC=arm-xilinx-linux-gnueabi-gcc
CXX=arm-xilinx-linux-gnueabi-g++ \
LD=arm-xilinx-linux-gnueabi-gcc LDXX=arm-xilinx-linux-gnueabi-g++ \
AR=arm-xilinx-linux-gnueabi-ar

*CPPFLAGS and LDFLAGS must be defined as environment variables instead of
command line arguments, as we're adding to them.*

-----------------------------

At the beginning I thought there was something under the hood taking
place I was not aware of, but in fact I now realize my understanding of
the hierarchy of variable was ... poor: so given the rules you have
explained me that is the result of how that *specific* makefile is
written.

When asking the same question on the general mailing list I was
suggested that might be a bug that is why I posted a question here.

There is not bug whatsoever :-)
Thanks,
P.