Difference between CPPFLAGS and CXXFLAGS in GNU Make

Question

What's the difference between CPPFLAGS and CXXFLAGS in GNU Make?

You can get make to print out its predefined variables and rules database using the invocation `make -p` — Daniele Pallastrelli, Jul 02 '14 at 07:57

score 237 · Accepted Answer · edited Apr 12 '12 at 03:39

237

CPPFLAGS is supposed to be for flags for the C PreProcessor; CXXFLAGS is for flags for the C++ compiler.

The default rules in make (on my machine, at any rate) pass CPPFLAGS to just about everything, CFLAGS is only passed when compiling and linking C, and CXXFLAGS is only passed when compiling and linking C++.

edited Apr 12 '12 at 03:39

Ergwun

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answered Jan 30 '09 at 14:23

Kieron

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it seems like a common practice that CFLAGS would also be passed when compile C++? – Baiyan Huang May 12 '13 at 13:12
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Ha. I get it! the `x` is a `+` turned on it's side because `C++FLAGS` would blow up the compiler. ... I may have arrived to the party late, but that's still better than arriving on time to the wrong party. – Jacksonkr Mar 10 '16 at 15:18
@BaiyanHuang I wouldn't think about it as common or not; you'll run into both conventions. You have to know what your current setup is doing. – James Moore Nov 16 '16 at 23:29
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`CPPFLAGS` is **NOT** for **C** **P**lus **P**lus but `CXXFLAGS` is. – shizhen Feb 20 '19 at 02:51
True but terrible. This feels like we are stuck in the 80s. – shuhalo Sep 19 '19 at 12:50
So damn confusing. At first I thought `CXX` meant "both C++ and C." – Константин Ван Jun 26 '21 at 12:07
The confusion may stem from the fact that the suffix for C++ source files is often `.cpp`, as in `mycode.cpp`. I have actually also seen source files with the `.cxx` suffix. – stackoverflowuser2010 Sep 19 '21 at 00:13

score 35 · Answer 2 · edited Aug 12 '12 at 19:23

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By default, CPPFLAGS will be given to the C preprocessor, while CXXFLAGS will be given to the C++ compiler.

The GNU Make Manual is a good resource for questions like this (see Implicit Variables).

edited Aug 12 '12 at 19:23

Matthew

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answered Jan 30 '09 at 14:16

Christoph

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I was staring right at the manual when I had this exact same question. I typed CPPFLAGS into stackoverflow and got the answer much quicker than searching the manual. – Dan Hook Aug 11 '09 at 18:00

starblue · Answer 3 · 2017-09-28T20:46:51.867

19

CPPFLAGS are for the C preprocessor, while CXXFLAGS are for the C++ compiler.

See here.

edited Sep 28 '17 at 20:46

answered Jan 30 '09 at 14:17

starblue

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score 0 · Answer 4 · answered Sep 28 '17 at 21:27

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By default, they're set to something.

In practice, you need to know what every single project does. Virtually no one uses those defaults built into make, and if you rely on, for example, CPPFLAGS meaning "flags to the C preprocessor" you'll find that the project you care about has used it to mean "flags to the C++ compiler" instead. And does the CFLAGS flag get passed to C++ compile lines? Sometimes. Not always. Etc, etc, etc.

answered Sep 28 '17 at 21:27

James Moore

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Some projects use CPPFLAGS to mean "c++ flags", but those projects are almost definitely doing so out of ignorance of the standard, and it would be better if they used CXXFLAGS. – Score_Under Aug 08 '18 at 09:22
@Score_Under I don't disagree with you, but the world is filled with projects that couldn't care less what anyone thinks the standard is. You _always_ have to investigate. – James Moore Aug 09 '18 at 14:12

Difference between CPPFLAGS and CXXFLAGS in GNU Make

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