What is the difference between DEFS and CPPFLAGS in autoconf and automake

Question

I already read about the difference between CFLAGS and CPPFLAGS. But my Makefile.am currently uses both DEFS and CPPFLAGS and I am not sure about the difference.

DEFS += -DLOCALEDIR=\"$(localedir)\" -DDATADIR=\"$(datadir)\" -DPKGDATADIR=\"$(pkgdatadir)\"

and:

src_foo_CPPFLAGS = \
    $(AM_CPPFLAGS) \
    -I$(top_builddir)/src \
    -DDATADIR='"$(datadir)"' \
    -DMODULEDIR='"$(moduledir)"' \
    -DLIBEXECDIR='"$(libexecdir)"'

Both the CPPFLAGS and the DEFS seems to create defines with the -D option. So whats the difference. Can I remove DEFS and just add the missing defines (PKGDATADIR and LOCALEDIR) to CPPFLAGS?

Answer 1

DEFS is defined in autoconf as follows:

-D options to pass to the C compiler. If AC_CONFIG_HEADERS is called, configure replaces ‘@DEFS@’ with -DHAVE_CONFIG_H instead (see Configuration Headers). This variable is not defined while configure is performing its tests, only when creating the output files. See Setting Output Variables, for how to check the results of previous tests.

When make is executed with the -p flag (e.g. make -p > rules). We can examine the resulting rules file to find out what make make will actually do.

Assuming the AC_CONFIG_HEADERS macro is called then DEFS is initially defined as follows:

DEFS = -DHAVE_CONFIG_H

Assuming we define DEFS as follows:

DEFS += \
    -DLOCALEDIR=\"$(localedir)\" \
    -DDATADIR=\"$(datadir)\" \
    -DPKGDATADIR=\"$(pkgdatadir)\"

then DEFS is going to look like this:

DEFS = -DHAVE_CONFIG_H -DLOCALEDIR=\"$(localedir)\" -DDATADIR=\"$(datadir)\"  \
       -DPKGDATADIR=\"$(pkgdatadir)\"

Next lets look at the way DEFS is used in the Makefiles that are generated by automake, the compile rule in the Makefile will look like this:

COMPILE = $(CC) $(DEFS) $(DEFAULT_INCLUDES) $(INCLUDES) $(AM_CPPFLAGS) \
          $(CPPFLAGS) $(AM_CFLAGS) $(CFLAGS)

As we can see, DEFS appears first, followed by some other variables and the AM_CPPFLAGS Every further occurrence of DEFS is always followed by another occurrence of AM_CPPFLAGS and its variants such as foo_CPPFLAGS.

So to conclude this question, I think its clear that DEFS can be removed and instead its content can be put into CPPFLAGS. I also asked this question on IRC in the autotools channel. It was suggested to me, not to use DEFS and only use CPPFLAGS.