Question

I’m a dummy regarding C build tools, so I have a forked project to which I want to add a dynamically linked library:

https://github.com/iem-projects/ncview/tree/26c3549d165dc6047dc37db252062fd73eb9282c

Basically, what I need is to include liblo. There is all kinds of voodoo going on for the existing libraries of the project (netcdf for example).

I am trying to follow this manual which basically says, I should add stuff to configure.in and Makefile.am, then run autoreconf, autoconf, and automake, then ./configure and finally make.

This I added to configure.in:

# OSC support
PKG_CHECK_MODULES(LIBLO, liblo >= 0.26)

And this I added to Makefile.am:

bin_PROGRAMS = ncview

ncview_LDADD = $(LIBLO_LIBS)

Now configure is at least successfully checking for that library:

checking for LIBLO... yes

But make doesn’t include the library with the linker it seems:

$ make
make  all-recursive
Making all in src
/usr/bin/gcc-4.2 -I/usr/X11/include -g -O2   -L/opt/local/lib -lnetcdf -lSM -lICE \
-L/usr/X11/lib -R/usr/X11/lib -lX11  -L/usr/X11/lib -R/usr/X11/lib  -Wl,-rpath,  -o \
ncview  ncview.o file.o util.o do_buttons.o file_netcdf.o view.o do_print.o \
epic_time.o interface.o x_interface.o dataedit.o display_info.o plot_xy.o utils.o \
range.o printer_options.o overlay.o filesel.o set_options.o plot_range.o udu.o \
SciPlot.o RadioWidget.o cbar.o utCalendar2_cal.o calcalcs.o colormap_funcs.o \
make_tc_data.o stringlist.o handle_rc_file.o   -lm -L/opt/local/lib -lnetcdf -lXaw \
-lXt  -L/usr/X11/lib -R/usr/X11/lib -lSM -lICE -L/usr/X11/lib -R/usr/X11/lib -lX11  \
-L/usr/X11/lib -R/usr/X11/lib  -lpng 

Undefined symbols:
  "_lo_address_new", referenced from:
      _main in ncview.o
  "_lo_send_internal", referenced from:
      _main in ncview.o
ld: symbol(s) not found
collect2: ld returned 1 exit status

So it links the old libraries (netcdf, X11), but doesn’t pick up the one I added (liblo)

This whole makefile business is black magic to me, so any clues as to why the library doesn’t get linked is welcome.

---

Solution:

The the hint of AC_SUBST, and looking again closer at the way the other libraries were integrated, I managed to get it working. Nothing had to be added to Makefile.am. In configure.in (aka configure.ac), the following was added:

# OSC support
PKG_CHECK_MODULES(LIBLO, liblo >= 0.26)
LIBSsave=$LIBS
CFLAGSsave=$CFLAGS
CFLAGS=$LIBLO_CFLAGS
LIBS=$LIBLO_LIBS
# AC_MSG_CHECKING([for liblo OSC library])
# AC_MSG_RESULT()
# AC_CHECK_LIB(LIBLO,lo_address_new,[],[libloWorks=no])
echo "liblo OSC library: $LIBLO_LIBS"
AC_SUBST(LIBLO_CFLAGS)  # si?
AC_SUBST(LIBLO_LIBS)
LIBS+=$LIBSsave
CFLAGS+=$CFLAGSsave

Answer 1

After PKG_CHECK_MODULES, the variables LIBIO_CFLAGS and LIBIO_LIBS should be defined. At this point, you should add:

AC_SUBST(LIBIO_CFLAGS)
AC_SUBST(LIBIO_LIBS)

to the configure.ac file. The Makefile.am then needs to make use of these substitutions:

ncview_CPPFLAGS = $(LIBIO_CFLAGS)
ncview_LDADD = $(LIBIO_LIBS)