How to build the gcc Fortran cross-compiler for Android (ARM and x86)

If you need to cross-compile for Android a program written in Fortran, you know already that the official Android NDK does not come with the gfortran compiler, and if like me you need to port to Android code that depends on Fortran (such as the lapack libraries), you are out of luck.
Fortunately I managed to compile the gcc Fortran cross-compiler with the help of  Mike Long's blog. As an added bonus, I managed to build gfortran 4.8.0 not only for the ARM but also for the x86 toolchain, as well as update the script to the latest Android NDK (currently r8d).
What you need to do is the following:

1. download the fortran4android shell script.
2. download the ndk-r8d fortran patch.
3. run the script in your directory:
   
   $ fortran4android
4. wait, wait, wait, ... and you will see in the android-ndk-r8d/toolchains directory two new toolchains called arm-linux-androideabi-4.8.0 and x86-4.8.0
5. Enjoy your new gfortran compiler.

Note that I've successfully tested the script only on Ubuntu 12.04.

Addendum of August 30, 2013

I now have available a patch for the newer android-ndk-r9. To use this instead of the patch above:

1. download the ndk-r9-fortran-patch
2. change the fortran4android script by replacing "r8d" with "r9" everywhere
3. run the fortran4android script in your directory.

 Note that r9 of the NDK already comes with a 4.8 gcc toolchain and to keep it separated, the fortran4android script will still generate in output a "4.8.0" toolchain.

How to inspect expanded C macros with gcc/gcc+

Sometimes you get compilation errors in gcc/g++ and you'd like to inspect the output from the C compiler pre-processor to figure out why certain macros were expanded in such as way to cause an error.
The solution is simply to remove the -o and -c options from your gcc/g++ command line and replace it with

gcc/g++ -E -dD

this will generate on your terminal only the C pre-processor output that is eventually fed to the C/C++ compiler.

For example you are trying to compile Mozilla SpiderMonkey 1.7 and one of  the compiler lines is the following:

/usr/bin/gcc  -Djs_1_7_EXPORTS -DXP_UNIX -DSVR4 -DSYSV -D_BSD_SOURCE -DPOSIX_SOURCE -DHAVE_LOCALTIME_R -DX86_LINUX -D_IEEE_LIBM -DJS_EDITLINE -DEDITLINE -DHAVE_VA_COPY -DVA_COPY=va_copy -DPIC -DANSI_ARROWS -DHAVE_TCGETATTR -DHIDE -DUSE_DIRENT -DSYS_UNIX -DHAVE_STDLIB -DUNIQUE_HISTORY -O3 -DNDEBUG -fPIC -I../js-1.7  -fPIC -o CMakeFiles/js-1.7.dir/jsarena.c.o   -c js-1.7/jsarena.c

To see the expanded C macros run the following command:

 /usr/bin/gcc  -E -dD -Djs_1_7_EXPORTS -DXP_UNIX -DSVR4 -DSYSV -D_BSD_SOURCE -DPOSIX_SOURCE -DHAVE_LOCALTIME_R -DX86_LINUX -D_IEEE_LIBM -DJS_EDITLINE -DEDITLINE -DHAVE_VA_COPY -DVA_COPY=va_copy -DPIC -DANSI_ARROWS -DHAVE_TCGETATTR -DHIDE -DUSE_DIRENT -DSYS_UNIX -DHAVE_STDLIB -DUNIQUE_HISTORY -O3 -DNDEBUG -fPIC -I../js-1.7  -fPIC  js-1.7/jsarena.c

Python For Android (Py4A)

A better solution for cross-compiling Python for Android is to use the Py4A project which is made to be used together with SL4A (Scripting Layer For Android). If you are only interested in the Python interpreter and the runtime Python library, you can also use it standalone.
Get a local copy of the source code using  the following command:

   $ hg clone https://code.google.com/p/python-for-android/

Just focus on the python-build subdirectory and make sure the  python-build/python-src subdirectory is not present (remove it if it came with the Mercurial repository, or else the compilation will fail).
Set up your environment so that the python-for-android build script can pick up the ndk-build script from the Android NDK:

  $ export ANDROID_NDK_ROOT=/home/<your-directory>/android-ndk-r8
  $ export PATH=$ANDROID_NDK_ROOT:$PATH

Finally build Python for Android by issuing the following command:

  $ cd python-for-android/python-build

  $ rm -rf python-src
  $ bash build.sh

Note that on my Ubuntu 12.04 machine I had initially the following compilation error:

Traceback (most recent call last):
  File "build.py", line 161, in <module>
    os.path.join(pwd, 'output.temp', 'usr'))
  File "build.py", line 89, in zipup
    zip_file = zipfile.ZipFile(out_path, 'w', compression=zipfile.ZIP_DEFLATED)
  File "/home/danilo/python-for-android/python-build/host/lib/python2.6/zipfile.py", line 660, in __init__
    "Compression requires the (missing) zlib module"
RuntimeError: Compression requires the (missing) zlib module


I identified the problem in having the zlib library in my system installed under /lib/x86_64-linux-gnu/ instead of one of the traditional lib directories covered by the Python setup.py script. Also on my system I only had libz.so.1 and not libz.so. So to fix both problems I just created a symlink in the standard /usr/lib directory as follows:

  $ cd /usr/lib

  $ sudo ln -s /lib/x86_64-linux-gnu/libz.so.1 libz.so

With this fix the build.sh script was able to successfully build the zlib module for the host environment and create the following zipped files:

• python_extras_r14.zip
• python-lib_r16.zip
• python_r16.zip
• python_scripts_r13.zip

Of these I only used the python_r16.zip which contains the stripped python interpreter and the runtime libraries, and the python-lib_r16.zip which contains the include header files such as Python.h that can be used to compile Python bindings at development time.

Problems with new version of rpmbuild

The Problem

With the new version of rpmbuild installed on CentOS 6.x, if you try to use an old RPM spec file, you will get an error like the following:

error: File not found: <path>/BUILDROOT/<product>-6.7.x86_64/<filename>

Previously rpmbuild would install and look for files under the rpm_top/BUILD directory, while now it looks under the new BUILDROOT directory.

The Solution

This is what I ended up doing to solve the problem. When I call rpmbuild I now define a new variable, for example:

$ centos_release=$(lsb_release -rs | sed 's/\.//')

$ rpmbuild --define "centos_release $centos_release" ... -bb specfile 

then I changed my spec file to contain something like the following:

%if %{centos_release} >= 60

%setup -q -c -n %{buildroot}/usr/local/ 
cp -a -r %{buildroot}/usr/local/ /usr/
 

%else
 

%setup -q -c -n usr/local/
%install
cp -a -r $PWD /usr/
 

%endif

Another related problem

Suppose you have a single tar ball and you want to create two or more RPMs using different spec files. The new version of rpmbuild automatically cleans the BUILDROOT directory after completing the targets for a given spec file. If you want to save time and have the second spec file just look for the BUILDROOT created by the first spec it won't find it because of the automatic clean. So it seems like you are forced to untar and install under the BUILDROOT your file over and over again.
A simple solution to this problem is to add a %clean directive to your spec file and do nothing to disable to automatic cleaning done by rpmbuild. Add the following line at the bottom of your spec file:

%clean