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linkergstreamercc

cc linker gives no error, but the resulting library is not linked to libgstvideo-1.0

I'm using cmake to generate a gstreamer library. In the end cmake uses the following command for linking:

/usr/bin/cc -fPIC -shared -Wl,-soname,libmacq-gstmelpi.so -o libmacq-gstmelpi.so <OBJECT_FILES> -lmacq-melpi -lmacq-icar-tools -lmacq-gstmecimeta -lgstreamer-1.0 -lgobject-2.0 -lglib-2.0 -lgstvideo-1.0

Note the -lgstvideo-1.0 flag at the end. The command runs fine, no error is produced, and the resulting library is created just fine. However when I use the library, I get undefined symbol error. So I use ldd to check; and amongst all the output of ldd; libgstvideo-1.0.so is not to be found.

This problem occurs on Ubuntu 14.04 on a armhf architecture. The problem does not occur on opensuse 13.1 (i586) nor on opensuse 13.1 (armv7hl), since in that case ldd libmacq-gstmelpi.so | grep gstvideo gives:

libgstvideo-1.0.so.0 => /usr/lib/libgstvideo-1.0.so.0 (0xb715f000)

EDIT :

I have another library, very similar where a very similar command works just fine; the resulting library is correctly linked to libgstvideo-1.0.so

/usr/bin/cc -fPIC -shared -Wl,-soname,libmacq-gstplugins.so -o libmacq-gstplugins.so <OBJECT_FILES> -lmacq-icar-tools -lmacq-gstmecimeta -lgstapp-1.0 -lgstbase-1.0 -lgstreamer-1.0 -lgobject-2.0 -lglib-2.0 -lgstvideo-1.0

Some remarks on what I have checked and tried:

  • /usr/lib/arm-linux-gnueabihf/libgstvideo-1.0.so exists. (the other libraries in /usr/lib/arm-linux-gnueabihf/ are found without problem, libmacq-gstmelpi.so is linked to /usr/lib/arm-linux-gnueabihf/libgstreamer-1.0.so.0 without problem)
  • I tried changing the order, and putting the -lgstvideo-1.0 flag before all other -l flags; no succes.
  • replacing cc with c++ or gcc; the commands work, but output is the same
  • removing -lgstvideo-1.0 on a system where the build worked. The resulting library builds (links) without error; yet upon execution I have the same undefined symbol error as on Ubuntu. This proves that the missing symbol is in libgstvideo-1.0.so, and that I need it.
Solution

  • Are you actually using symbols from that particular library or do you just want to link to it to avoid linking it in the application that uses the library later on?

    It could be a default compiler behavior that it skips linking of libraries when no symbol from these are actually used.

    Try -Wl,--no-as-needed to your flags. In this case the library should get linked - not checking whether its symbols are actually used or not.

    EDIT: After chatting it turned out that the actual desired symbols are in gstbase-1.0 and not gstvideo-1.0. Since gstvideo-1.0 pulls in gstbase-1.0 as a dependency this worked but would cause problem as the linker may remove this dependency since no symbols from this particular library are being used. Linking directly to gstbase-1.0 seemed to solve all issues.