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A custom dynamic linker for Android programs that adds a few interesting
features compared to /system/bin/linker:
- Support changing the library search path. The system linker, when used
inside Android applications, is limited to the value of LD_LIBRARY_PATH
at boot time, that only looks into system directories, not application
This linker allows the client to add application paths before the
default system ones, this has two benefits:
- Library dependencies are loaded automatically in the right order.
- Libraries from the application paths are favored over system ones.
This avoids conflicts when one of your application's libraries
has the same name than a system one (which happens randomly
on certain devices due to system application bundling).
(Note: The system linker issue above has been fixed in Android 4.3).
- Supports any number of shared libraries. On older Android platforms,
the system linker will refuse to load more than 64 or 128 libraries
in a single process (Note: Fixed in Android 4.3).
- Supports loading a library at an explicit (page-aligned) memory
address. The system linker always randomizes the address. Note that
this is generally a _bad_ idea for security reasons. Consider using
this only when using shared RELROs (see below).
- Supports loading a library from an explicit (page-aligned) file
offset. This can be useful to load a library directly from an .apk,
provided that it is uncompressed and at a page-aligned offset.
- Support sharing of RELRO sections. When two processes load the same
library at exactly the same address, the content of its RELRO section
is identical. By default, each instance uses private RAM pages to host
it, but it is possible to use a single ashmem region to share the same
data instead.
See include/crazy_linker.h for the API and its documentation.
See LICENSE file for full licensing details (hint: BSD)
A few notes:
- Do not use this if you don't know what you're doing. Read the API
documentation first, and look at the test programs for usage examples.
- The crazy linker will always use the system linker to load NDK-exposed
system libraries (e.g. and others). This avoids having two
instances of the same library in the same process, and correctly
resolving any symbols from system libraries.
- Any library loaded by the crazy linker, and which uses functions of will continue to work. However, calls to dlopen(), dlsym(),
et al will be redirected to the crazy linker's own wrappers.
This ensures that if a library is loaded by the crazy linker, any of
its dependencies will be loaded by it too.
- Libraries loaded with the crazy linker are visible to GDB, or Breakpad,
and stack unwinding / C++ exception propagation should just work.
You can't call the crazy_linker code directly from Java in your Android
application (it's a static library). You need to put it into your own
shared library, loaded with System.loadLibrary() instead (or alternatively,
inside your NativeActivity's shared library).
Also, libraries loaded with the crazy linker are not visible to the system
one. In practice, it means that lazy native method lookup will not work. I.e.:
The first time you call a native method like:
The VM will look into existing native libraries with dlsym() for a
function symbol named like:
This will not work if the symbol is inside a library loaded with the
To work-around this, register the native methods explicitely
in your JNI_OnLoad() by calling env->RegisterNatives() with the
appropriate parameters.
Usage instructions:
1/ Add the following to your module definition in your project's
2/ Also Include the top-level crazy_linker, as in:
include /path/to/crazy_linker/
3/ In your C or C++ source:
#include <crazy_linker.h>
Read the header for API documentation.
If your library implements native methods, it must explicitely register
them with env->RegisterNatives() before they become usable.
- Libraries loaded by the crazy linker are not automatically closed when
the process exits.
- dlopen() when called inside a library loaded by the crazy linker doesn't
If you modify this code, check your changes by running the test suite using:
cd $NDK
tests/ crazy_linker
See DESIGN.TXT for an overview of the library's design.