ffi.h: No such file or directory (solutions included)

[AndreMiras]

Problem summary

On Gentoo Linux cffi recipe host install fails with the error below:

c/_cffi_backend.c:13:17: fatal error: ffi.h: No such file or directory

Detailed description

This is because ffi.h is installed in /usr/lib64/libffi-3.2.1/include/ffi.h which is not part of gcc default include paths.
Here is where ffi.h is installed (equery f is the Gentoo equivalent to Debian-like dpkg -L):

$ equery f dev-libs/libffi | grep ffi.h
/usr/lib64/libffi-3.2.1/include/ffi.h

Or with dpkg-config:

$ pkg-config --cflags libffi
-I/usr/lib64/libffi-3.2.1/include

And here are the gcc default include paths:

$ echo | gcc -E -Wp,-v -
#include "..." search starts here:
#include <...> search starts here:
 /usr/lib/gcc/x86_64-pc-linux-gnu/5.4.0/include
 /usr/lib/gcc/x86_64-pc-linux-gnu/5.4.0/include-fixed
 /usr/include
End of search list.

Complete error output

Below is the detailed buildozer android debug (cffi) output log:

[INFO]:    Installing cffi into site-packages                                                                                                                                                                                                                     [66/1768]
[INFO]:    -> directory context /home/andre/Progz/PyWallet/.buildozer/android/platform/build/build/other_builds/cffi-python2/armeabi-v7a/cffi                                                                                                                              
[INFO]:    -> running python.host setup.py install -O2                                                                                                                                                                                                                     
[INFO]:    Rebuilding compiled components in cffi                                                                                                                                                                                                                          
[INFO]:    -> running hostpython setup.py clean --all                                                                                
[INFO]:    -> running hostpython setup.py build_ext -v                                                                               
Exception in thread background thread for pid 19037:                                                                                 
Traceback (most recent call last):                                                                                                   
  File "/usr/lib64/python3.4/threading.py", line 911, in _bootstrap_inner                                                            
    self.run()                                                                                                                       
  File "/usr/lib64/python3.4/threading.py", line 859, in run                                                                         
    self._target(*self._args, **self._kwargs)                                                                                        
  File "/home/andre/.local/lib64/python3.4/site-packages/sh.py", line 2170, in background_thread                                     
    handle_exit_code(exit_code)                                                                                                      
  File "/home/andre/.local/lib64/python3.4/site-packages/sh.py", line 1929, in fn                                                    
    return self.command.handle_command_exit_code(exit_code)                                                                          
  File "/home/andre/.local/lib64/python3.4/site-packages/sh.py", line 672, in handle_command_exit_code                               
    raise exc                                                                                                                        
sh.ErrorReturnCode_1:                                                                                                                
                                                                                                                                     
  RAN: /home/andre/Progz/PyWallet/.buildozer/android/platform/build/build/other_builds/hostpython2/desktop/hostpython2/hostpython setup.py build_ext -v
                                                                                                                                     
  STDOUT:                                                                                                                            
running build_ext                                                                                                                    
building '_cffi_backend' extension
creating build
creating build/temp.linux-x86_64-2.7
creating build/temp.linux-x86_64-2.7/c
gcc -pthread -fno-strict-aliasing -g -O2 -DNDEBUG -g -fwrapv -O3 -Wall -Wstrict-prototypes -fPIC -DUSE__THREAD -I/home/andre/Progz/PyWallet/.buildozer/android/platform/build/build/other_builds/hostpython2/desktop/hostpython2/Include -I/home/andre/Progz/PyWallet/.buildozer/android/platform/build/build/other_builds/hostpython2/desktop/hostpython2 -c c/_cffi_backend.c -o build/temp.linux-x86_64-2.7/c/_cffi_backend.o
c/_cffi_backend.c:13:17: fatal error: ffi.h: No such file or directory
compilation terminated.

Solutions / workarounds

Below are few solutions/workarounds I had in mind.

Quick'n dirty

Symlink libffi includes to gcc default include path:

ln -sfn /usr/lib64/libffi-3.2.1/include/ffi.h /usr/include/
ln -sfn /usr/lib64/libffi-3.2.1/include/ffitarget.h /usr/include/

While it works it's an unsatisfying solution.

dpkg-config when possible (i.e. host)

We could update current disable-pkg-config.patch to try to run, but except silently. That way if dpkg-config is available on the host, we can use it, but it won't fail if not.

dpkg-config recipe

Since cffi is not the first package, nor the last needing dpkg-config, we could create a recipe for it to make it available for the target architecture.
https://autotools.io/pkgconfig/cross-compiling.html

Install libffi recipe for the host

I'm not too sure about that one, but the idea would be to update the libffi recipe so it also makes the library/headers available for the host so we could point to it. But it could be a mess.

Final word

I have a preference for the "dpkg-config when possible (i.e. host)" solution. It seems easy enough to implement and it would fix the issue. For the long run I prefer if we had dpkg-config available for the target, but I'm not sure how achievable it is.
I would be glad to contribute more to python-for-android, so please let me know which solution you prefer and I could work on a pull request when I get time.

[AndreMiras]

For what it worth, the problem could also exist on Arch Linux since libffi headers are not installed in gcc default include paths:

$ pkg-config --cflags libffi
-I/usr/lib/libffi-3.2.1/include