Append compile flags to CFLAGS and CXXFLAGS while configuration/make

Question

The project that I am trying to build has default flags

CFLAGS = -Wall -g -O2

CXXFLAGS = -g -O2

I need to append a flag -w to both these variables (to remove: 'consider all warnings as errors')

I have a method to work it out, give

make 'CFLAGS=-Wall -g -O2 -w'; 'CXXFLAGS=-g -O2 -w'

OR

Run ./configure and statically modify Makefile

But I want to append my options with the existing options while running configure or make

The post Where to add a CFLAG, such as -std=gnu99, into an autotools project conveniently uses a macro to achieve this.

Solution

You almost have it right; why did you add the semicolon?

To do it on the configure line:

 ./configure CFLAGS='-g -O2 -w' CXXFLAGS='-g -O2 -w'

To do it on the make line:

 make CFLAGS='-g -O2 -w' CXXFLAGS='-g -O2 -w'

However, that doesn't really remove consider all warnings as errors; that removes all warnings. So specifying both -Wall and -w doesn't make sense. If you want to keep the warnings but not have them considered errors, use the -Wall -Wno-error flags.

Alternatively, most configure scripts which enable -Werror by default also have a flag such as --disable-werror or similar. Run ./configure --help and see if there's something like that.

Comments

[Vigneshwaren]

Is there a way for me to NOT specify this statitcally evertime, like, CFLAGS=' $CFLAGS <my-options>', to retain whatever is already configured and without knowing it as well.

[MadScientist]

No, that's not possible (at least not without modifying the makefile). According to the autoconf coding standards the default value of CFLAGS is always only enabling debugging/optimization, so it should normally always be -g -O2 for most systems. It's not correct (according to the coding standards) to add other important flags such as -I, -D, etc. into CFLAGS. So it should be safe to always just override it.

[William Pursell]

If you cannot assign CFLAGS without retaining the package defaults, then the software packaging contains a bug which should be reported to the package maintainer.