port exosip to android

Port exosip to android platform isn't a difficult task because exosip doesn't rely on any special system calls that aren't available on android. It only requires osip to compile, which can also be easily ported.
As an example, I created two applications to run against exosip lib. One is a native application that can run in android shell, the other is an java application that interact with exosip through jni. The dependency relationship between modules is:

The diagram below depicts the organization of files. They're organized this way so that they can be compiled through android ndk build system.

exosip_root (NDK_MODULE_PATH environment variable points here)

• libosip

• Android.mk
• source files

• libexosip

• Android.mk
• source files

• sip_exe

• jni

• Android.mk
• Application.mk
• source file

• sip_jni

• jni

• Android.mk
• source files

• libs

• armeabi

• libosip.so
• libexosip.so

• src

• java source files

• AndroidManifest.xml

To comply with ndk build system's requirements, we create a directory named jni under sip_jni and sip_exe module, and place actual source file there. The Android.mk and Application.mk (optional) are placed in the jni directory as well. Keeping files this way, we can issue ndk-build command right in sip_jni or sip_exe directories to compile applications.
Note that we don't create jni directory for libosip and libexosip, their Android.mk is placed directly under libosip and libexosip directories. This is because they are dependent on by application modules. We don't need compile them directly, instead, the build system will build them automatically while building applications. In order to help build system find libosip and libexosip, we must set NDK_MODULE_PATH environment variable to the directory that directly containing  libosip and libexosip. The build system will search for them based on directory name, so their names matter.

To port exosip to android, the essential task is to create the Android.mk file, which specifies source files, c flags, ld flags, dependent libraries. We define HAVE_TIME_H and HAVE_SYS_SELECT_H to compile exosip successfully. And we define ENABLE_TRACE and OSIP_MT to enable logging and multi-threading. In the last line, $ ( call import - module , libosip ) tells the build system that exosip depends on osip, and all header files exported by osip ( LOCAL_EXPORT_C_INCLUDES := $ ( LOCAL_C_INCLUDES ) )  will be included by exosip automatically. 
import-module is a feature that isn't available in the build system in android source tree. It enables us organizing our projects in arbitrary manner. For example, we can place libosip and libexosip directories in another directory. The build system can still find them as long as NDK_MODULE_PATH is set to the directory containing them. It's much more flexible than specifying dependency relationship with relative path.

Sample:
The sample for this post is available at:
http://code.google.com/p/rxwen-blog-stuff/source/browse/trunk/android/exosip_sample/
It shows how to:

1. use ndk build system
2. use stl in c++ code
3. create a very basic exosip application
4. call native code from java
5. call java code from native code

References:
ndk document
exosip user manual