Automake/libtool doesn't build shared library - automake

Question

Please consider the following project.
configure.ac:
AC_PREREQ([2.69])
AC_INIT([dummy-project], [0.0.1])
AM_INIT_AUTOMAKE([foreign])
LT_INIT([disable-static])
AC_PROG_CXX
AC_CONFIG_FILES([Makefile])
AC_OUTPUT
Makefile.am:
noinst_LTLIBRARIES=something.la
something_la_SOURCES=source.cpp
something_la_LDFLAGS=-module
source.cpp is just an empty file.
When I run:
/tmp/scratch$ autoreconf -i
libtoolize: Consider adding 'AC_CONFIG_MACRO_DIRS([m4])' to configure.ac,
libtoolize: and rerunning libtoolize and aclocal.
libtoolize: Consider adding '-I m4' to ACLOCAL_AMFLAGS in Makefile.am.
/tmp/scratch$ ./configure
checking for a BSD-compatible install... /usr/bin/install -c
checking whether build environment is sane... yes
checking for a thread-safe mkdir -p... /bin/mkdir -p
checking for gawk... gawk
checking whether make sets $(MAKE)... yes
checking whether make supports nested variables... yes
checking build system type... x86_64-pc-linux-gnu
checking host system type... x86_64-pc-linux-gnu
checking how to print strings... printf
checking for style of include used by make... GNU
checking for gcc... gcc
checking whether the C compiler works... yes
checking for C compiler default output file name... a.out
checking for suffix of executables...
checking whether we are cross compiling... no
checking for suffix of object files... o
checking whether we are using the GNU C compiler... yes
checking whether gcc accepts -g... yes
checking for gcc option to accept ISO C89... none needed
checking whether gcc understands -c and -o together... yes
checking dependency style of gcc... gcc3
checking for a sed that does not truncate output... /bin/sed
checking for grep that handles long lines and -e... /bin/grep
checking for egrep... /bin/grep -E
checking for fgrep... /bin/grep -F
checking for ld used by gcc... /usr/bin/ld
checking if the linker (/usr/bin/ld) is GNU ld... yes
checking for BSD- or MS-compatible name lister (nm)... /usr/bin/nm -B
checking the name lister (/usr/bin/nm -B) interface... BSD nm
checking whether ln -s works... yes
checking the maximum length of command line arguments... 1572864
checking how to convert x86_64-pc-linux-gnu file names to x86_64-pc-linux-gnu format... func_convert_file_noop
checking how to convert x86_64-pc-linux-gnu file names to toolchain format... func_convert_file_noop
checking for /usr/bin/ld option to reload object files... -r
checking for objdump... objdump
checking how to recognize dependent libraries... pass_all
checking for dlltool... no
checking how to associate runtime and link libraries... printf %s\n
checking for ar... ar
checking for archiver #FILE support... #
checking for strip... strip
checking for ranlib... ranlib
checking command to parse /usr/bin/nm -B output from gcc object... ok
checking for sysroot... no
checking for a working dd... /bin/dd
checking how to truncate binary pipes... /bin/dd bs=4096 count=1
checking for mt... mt
checking if mt is a manifest tool... no
checking how to run the C preprocessor... gcc -E
checking for ANSI C header files... yes
checking for sys/types.h... yes
checking for sys/stat.h... yes
checking for stdlib.h... yes
checking for string.h... yes
checking for memory.h... yes
checking for strings.h... yes
checking for inttypes.h... yes
checking for stdint.h... yes
checking for unistd.h... yes
checking for dlfcn.h... yes
checking for objdir... .libs
checking if gcc supports -fno-rtti -fno-exceptions... no
checking for gcc option to produce PIC... -fPIC -DPIC
checking if gcc PIC flag -fPIC -DPIC works... yes
checking if gcc static flag -static works... yes
checking if gcc supports -c -o file.o... yes
checking if gcc supports -c -o file.o... (cached) yes
checking whether the gcc linker (/usr/bin/ld -m elf_x86_64) supports shared libraries... yes
checking whether -lc should be explicitly linked in... no
checking dynamic linker characteristics... GNU/Linux ld.so
checking how to hardcode library paths into programs... immediate
checking whether stripping libraries is possible... yes
checking if libtool supports shared libraries... yes
checking whether to build shared libraries... yes
checking whether to build static libraries... no
checking for g++... g++
checking whether we are using the GNU C++ compiler... yes
checking whether g++ accepts -g... yes
checking dependency style of g++... gcc3
checking how to run the C++ preprocessor... g++ -E
checking for ld used by g++... /usr/bin/ld -m elf_x86_64
checking if the linker (/usr/bin/ld -m elf_x86_64) is GNU ld... yes
checking whether the g++ linker (/usr/bin/ld -m elf_x86_64) supports shared libraries... yes
checking for g++ option to produce PIC... -fPIC -DPIC
checking if g++ PIC flag -fPIC -DPIC works... yes
checking if g++ static flag -static works... yes
checking if g++ supports -c -o file.o... yes
checking if g++ supports -c -o file.o... (cached) yes
checking whether the g++ linker (/usr/bin/ld -m elf_x86_64) supports shared libraries... yes
checking dynamic linker characteristics... (cached) GNU/Linux ld.so
checking how to hardcode library paths into programs... immediate
checking that generated files are newer than configure... done
configure: creating ./config.status
config.status: creating Makefile
config.status: executing depfiles commands
config.status: executing libtool commands
/tmp/scratch$ make
/bin/bash ./libtool --tag=CXX --mode=compile g++ -DPACKAGE_NAME=\"dummy-camera\" -DPACKAGE_TARNAME=\"dummy-camera\" -DPACKAGE_VERSION=\"0.0.1\" -DPACKAGE_STRING=\"dummy-camera\ 0.0.1\" -DPACKAGE_BUGREPORT=\"\" -DPACKAGE_URL=\"\" -DPACKAGE=\"dummy-camera\" -DVERSION=\"0.0.1\" -DSTDC_HEADERS=1 -DHAVE_SYS_TYPES_H=1 -DHAVE_SYS_STAT_H=1 -DHAVE_STDLIB_H=1 -DHAVE_STRING_H=1 -DHAVE_MEMORY_H=1 -DHAVE_STRINGS_H=1 -DHAVE_INTTYPES_H=1 -DHAVE_STDINT_H=1 -DHAVE_UNISTD_H=1 -DHAVE_DLFCN_H=1 -DLT_OBJDIR=\".libs/\" -I. -g -O2 -MT source.lo -MD -MP -MF .deps/source.Tpo -c -o source.lo source.cpp
libtool: compile: g++ -DPACKAGE_NAME=\"dummy-camera\" -DPACKAGE_TARNAME=\"dummy-camera\" -DPACKAGE_VERSION=\"0.0.1\" "-DPACKAGE_STRING=\"dummy-camera 0.0.1\"" -DPACKAGE_BUGREPORT=\"\" -DPACKAGE_URL=\"\" -DPACKAGE=\"dummy-camera\" -DVERSION=\"0.0.1\" -DSTDC_HEADERS=1 -DHAVE_SYS_TYPES_H=1 -DHAVE_SYS_STAT_H=1 -DHAVE_STDLIB_H=1 -DHAVE_STRING_H=1 -DHAVE_MEMORY_H=1 -DHAVE_STRINGS_H=1 -DHAVE_INTTYPES_H=1 -DHAVE_STDINT_H=1 -DHAVE_UNISTD_H=1 -DHAVE_DLFCN_H=1 -DLT_OBJDIR=\".libs/\" -I. -g -O2 -MT source.lo -MD -MP -MF .deps/source.Tpo -c source.cpp -fPIC -DPIC -o .libs/source.o
mv -f .deps/source.Tpo .deps/source.Plo
/bin/bash ./libtool --tag=CXX --mode=link g++ -g -O2 -module -o something.la source.lo
libtool: link: ar cru .libs/something.a .libs/source.o
ar: `u' modifier ignored since `D' is the default (see `U')
libtool: link: ranlib .libs/something.a
libtool: link: ( cd ".libs" && rm -f "something.la" && ln -s "../something.la" "something.la" )
For some reason, it creates .libs/something.a (pic compiled for static linking to another so), but not something.so like I need it to.
I'm doing something wrong here, but I don't know what.

Answer 1

There are two parts to the answer. One is on the automake level, and the other is on the libtool level.
On the automake level, the moment a library is defined under the noinst_LTLIBRARIES rule, it is assumed to be a "convenience" library. Such a library is used internally by your build system, not intended to be installed at runtime.
On the libtool level, since a convenience library will not be installed, there is no need to build a shared object version of it. The build for shared object libtool does is to compile a .o file (static linked) with -fpic so that it can be statically linked into another shared object.
The mechanism by which libtool knows this is a convenience library is by seeing whether an rpath was provided. If no rpath was provided, the output is assumed to be a convenience library, and no shared object is generated.