Loading shared libs that depend on other shared libs

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According to https://groups.google.com/forum/?fromgroups#!msg/android-ndk/J3lzK4X–bM/4YaijymZy_AJ

Yes, and this is the documented behaviour: you must load libraries in reverse dependency order explicitely. […] It is a limitation of the system.

In a nutshell, the dynamic linker doesn’t know anything about your application (e.g. where its libraries live), it only knows about the LD_LIBRARY_PATH value that was set when the process was created. When you start an Android application, you really fork the Zygote process, you don’t create a new one, so the library search path is the initial one and doesn’t include your app’s /data/data//lib/ directory, where your native libraries live. This means that dlopen(“libfoo.so”) will not work, because only /system/lib/libfoo.so will be searched.

When you call System.loadLibrary(“foo”) from Java, the VM framework knows the application’s directory, so it can translate “foo” into “/data/data//lib/libfoo.so”, then call dlopen() with this full path, which will work.

It libfoo.so references “libbar.so”, then the dynamic linker will not be able to find the latter.

Add to this that even if you update LD_LIBRARY_PATH from native code, the dynamic linker will not see the new value. For various low-level reasons, the dynamic linker contains its own copy of the program’s environment as it was when the process was created (not forked). And there is simply no way to update it from native code. This is by design, and changing this would have drastic security constraints. For the record, this is also how the Linux dynamic linker works, this forces any program that needs a custom library search path to use a wrapper script to launch its executable (e.g. Firefox, Chrome and many others).

I’ve emailed the author asking where this is documented.

Tor Lillqvist goes on to provide a workaround: https://groups.google.com/d/msg/android-ndk/J3lzK4X–bM/n2zUancIFUEJ

To be more verbose, what that lo_dlopen() function does is:

  • Searches where the shared object in question is. It searches a set of directories passed to it by the Java code. The Java code looks at LD_LIBRARY_PATH and adds the app’s lib directory to that.
  • Opens the found shared object file and reads the ELF structures in it . Not all, but just enough to find out what shared objects it needs (the DT_NEEDED ones as displayed by arm-linux-androideabi-readelf -d). It calls itself recursively on the needed shared objects.
  • Only after that, i.e. after making sure that all needed other shared objects have been loaded, it calls the real dlopen() on the found full pathname to the shared object.

You can find his code at http://cgit.freedesktop.org/libreoffice/core/tree/sal/android/lo-bootstrap.c?id=5510127e89d6971a219ce3664e4631d6c6dda2b1

UPDATE: According to http://code.google.com/p/android/issues/detail?id=34416 this code was integrated into Android as of December 2012. Yay! Dependencies are loaded automatically for devices with API level 18 and up. If you are supporting older API levels than that you still need to list the dependencies.

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