Why is ffmpeg linking with too many libraries? - ffmpeg

Question

I'm trying to build ffmpeg shared libraries for decoding video on Linux systems. The build is done on Arch Linux but the resulting shared libraries need to be as portable as possible.
Fortunately, the functionality built into ffmpeg is sufficient, so I don't want to link with any libraries that might or might not be present on the target system. I'm taking libvdpau as an example of an unwanted dependency here, but there are many more.
Here's what I tried:
$ git clone https://git.ffmpeg.org/ffmpeg.git --branch n4.3.1 --depth 1
$ cd ffmpeg
$ mkdir build
$ cd build
$ ../configure \
--enable-shared \
--disable-programs --disable-doc \
--disable-avdevice --disable-postproc --disable-avfilter \
--disable-autodetect
$ make -j9
Notice --disable-autodetect which should prevent autodetection of which dependencies happen to be present on the build system.
And yet, when I check the resulting libraries, all of them have dependencies that I didn't ask for. For example:
$ ldd libavcodec/libavcodec.so
linux-vdso.so.1 (0x00007ffcd73cd000)
libswresample.so.3 => /usr/lib/libswresample.so.3 (0x00007fba1e45c000)
libavutil.so.56 => /usr/lib/libavutil.so.56 (0x00007fba1e1a7000)
libm.so.6 => /usr/lib/libm.so.6 (0x00007fba1e061000)
libpthread.so.0 => /usr/lib/libpthread.so.0 (0x00007fba1e03f000)
libc.so.6 => /usr/lib/libc.so.6 (0x00007fba1de76000)
libsoxr.so.0 => /usr/lib/libsoxr.so.0 (0x00007fba1ddf4000)
libva-drm.so.2 => /usr/lib/libva-drm.so.2 (0x00007fba1dded000)
libva.so.2 => /usr/lib/libva.so.2 (0x00007fba1ddc0000)
libva-x11.so.2 => /usr/lib/libva-x11.so.2 (0x00007fba1ddb8000)
libvdpau.so.1 => /usr/lib/libvdpau.so.1 (0x00007fba1ddb3000) <- Why is this here?
libX11.so.6 => /usr/lib/libX11.so.6 (0x00007fba1dc72000)
libdrm.so.2 => /usr/lib/libdrm.so.2 (0x00007fba1dc5d000)
libmfx.so.1 => /usr/lib/libmfx.so.1 (0x00007fba1dc4d000)
libdl.so.2 => /usr/lib/libdl.so.2 (0x00007fba1dc47000)
/usr/lib64/ld-linux-x86-64.so.2 (0x00007fba1f971000)
libgomp.so.1 => /usr/lib/libgomp.so.1 (0x00007fba1dc06000)
libXext.so.6 => /usr/lib/libXext.so.6 (0x00007fba1dbf1000)
libXfixes.so.3 => /usr/lib/libXfixes.so.3 (0x00007fba1dbe8000)
libxcb.so.1 => /usr/lib/libxcb.so.1 (0x00007fba1dbbc000)
libstdc++.so.6 => /usr/lib/libstdc++.so.6 (0x00007fba1d9df000)
libgcc_s.so.1 => /usr/lib/libgcc_s.so.1 (0x00007fba1d9c5000)
libXau.so.6 => /usr/lib/libXau.so.6 (0x00007fba1d9c0000)
libXdmcp.so.6 => /usr/lib/libXdmcp.so.6 (0x00007fba1d9b8000)
I also tried listing all disable flags explicitly, such as --disable-vdpau, but this didn't change anything; libvdpau.so.1 is still listed in ldd output.
Maybe they are just being linked, but not actually used? readelf -d seems to point in that direction:
$ ls */*.so
libavcodec/libavcodec.so libavutil/libavutil.so libswscale/libswscale.so
libavformat/libavformat.so libswresample/libswresample.so
$ readelf -d */*.so | grep 'Shared library:' | sort -u
0x0000000000000001 (NEEDED) Shared library: [libavcodec.so.58]
0x0000000000000001 (NEEDED) Shared library: [libavutil.so.56]
0x0000000000000001 (NEEDED) Shared library: [libc.so.6]
0x0000000000000001 (NEEDED) Shared library: [libm.so.6]
0x0000000000000001 (NEEDED) Shared library: [libpthread.so.0]
0x0000000000000001 (NEEDED) Shared library: [libswresample.so.3]
This is a nice and tight set of dependencies, the way I actually want them to be.
So maybe the build system is stupid and always adds all detected libraries to the linker command line anyway? To suppress this, I tried adding --extra-ldflags=-Wl,--as-needed, but that doesn't seem to have any effect either.
The resulting linker command line looks like this (abridged):
$ make -n libavcodec/libavcodec.so.58
gcc \
-shared \
-Wl,-soname,libavcodec.so.58 \
-Wl,-Bsymbolic \
-Wl,--version-script,libavcodec/libavcodec.ver \
-Llibavcodec -Llibavdevice -Llibavfilter -Llibavformat -Llibavresample -Llibavutil -Llibpostproc -Llibswscale -Llibswresample \
-Wl,--as-needed \
-Wl,-z,noexecstack \
-Wl,--warn-common \
-Wl,-rpath-link=:libpostproc:libswresample:libswscale:libavfilter:libavdevice:libavformat:libavcodec:libavutil:libavresample \
-Wl,--as-needed \
-o libavcodec/libavcodec.so.58 \
[...all .o files...] \
-lswresample -lavutil -pthread -lm -lm -pthread -lm
It looks like -Wl,--as-needed was already added anyway.
Interestingly, -Llibavdevice is also listed, even though I passed --disable-avdevice --disable-postproc --disable-avfilter to the configure script. That might be a clue: it would pick up my system's libavdevice, which of course comes with far more dependencies. Still I'd expect -Wl,--as-needed to trim these unbuilt dependencies out as well.
Where do these dependencies come from, and how do I get rid of them?

Answer 1

Of course: when running ldd, any dependencies on other ffmpeg libraries are resolved to those installed on my system. And those have many more dependencies. So the problem wasn't with the build system, but rather with the way I was inspecting the resulting output.
Setting LD_LIBRARY_PATH reveals the true list of transitive dependencies:
$ LD_LIBRARY_PATH=libavcodec:libavformat:libavutil:libswresample:libswscale ldd libavcodec/libavcodec.so
linux-vdso.so.1 (0x00007fffaab0f000)
libswresample.so.3 => libswresample/libswresample.so.3 (0x00007f9a8f213000)
libavutil.so.56 => libavutil/libavutil.so.56 (0x00007f9a8ef6c000)
libm.so.6 => /usr/lib/libm.so.6 (0x00007f9a8ede0000)
libpthread.so.0 => /usr/lib/libpthread.so.0 (0x00007f9a8edbe000)
libc.so.6 => /usr/lib/libc.so.6 (0x00007f9a8ebf5000)
/usr/lib64/ld-linux-x86-64.so.2 (0x00007f9a906e1000)