ctypes always uses the included libffi implementation
(which should be updated to current GCC HEAD). Pleae
add an option to build and link against a libffi, which
is installed on the system.

attached is a hack to just always build using the
installed libffi. I'm unsure how to find the correct
ffi implementation (maybe check for LIBFFI_H defined in
the ffi header?), and where to implement this check
(configure.ac --with-system-ffi=/usr?).

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Why is it desirable to link with the system ffi? That just
adds an external dependency, for a negligable space saving
(on x86, libffi.so.4.0.1 is 7840 bytes).

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It's not an external dependency, if it's not found. The size
is not the argument. For distributors it's an additional
maintainance burden to update copies of libraries. Security
advisories for zlib (and pcre?) show this.

patch updated.

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I tend to accept this patch.  Martin, any objections?

Matthias: do any of the buildbot slaves have a system libffi
installed?

About the code:
And, shouldn't it be enough to let the compiler find ffi.h?
 I assume that the presence of this file should indicate
that libffi.so is installed - why do we have to scan this
file for '#define LIBFFI_H', and why do we have to search
for the ffi shared library?

Would updating the included libffi code to GCC HEAD bring
any advantages?  The currently included code has been
patched in a few places, to avoid some of the compiler
warnings, and to allow compilation on OS X 10.3 (which does
not know the 'live_support' segment attribute, see Python
bug 1460514).

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> do any of the buildbot slaves have a system libffi
> installed?

the Debian and Ubuntu bots have libffi from gcc-4.1 installed.

I think in the past, there was a complaint about a
conflicting ffi.h, or maybe was this ffitarget.h? not sure
if the check for LIBFFI_H really helps with this. yes, if
gcc and ffi.h are installed from the same source / into the
same path, then the check for include dirs and libraries are
obsolete. the check for the different library names comes
from the observation that libgcj is linked against libffi as
a convenience library (for performance reasons?). These are
checked first so that not another external dependency is
introduced, if these libraries are available (not installed
by a standard gcc installation).

updating the code would at least make the library buildable
on hppa (or maybe this is just the failed (it's really a
32bit runtime, but hppa64-linux is detected).

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I tried your patch on an AMD64 ubuntu 5.10, after I
installed the 4.0.1-4ubuntu9 libffi package.  It crashes in
ctypes/test/test_python_api.py, test_PyOS_snprintf.  IIRC,
libffi did not support varargs functions, but Andreas Degert
added support for them (for x86_64), and it works fine.

Ok, new features in libffi might not be the norm, but I see
no way how I can check whether a system-installed libffi
supports this feature or not.

Do *you* see a solution for that?

For the actual changes to setup.py, if the patch really is a
good idea:  Since I don't even know what a convience library
is I'll trust you fully that your patch does the right thing.

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maybe a configure switch --with-system-ffi could do it?
How to pass this to the setup.py file?

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I do not really know.  Maybe using
distutils.sysconfig.get_config_var() ?

A pragmatic approach would be to parse pyconfig.h itself in
the setup script.

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If this configuration needs an operator decision, I think
this decision is best made in Modules/Setup.local. Anybody
who wants to use the local libffi installation can do so, by
writing a Setup.local that lists the source files, and gives
-lffi as a library option. Setup.dist could provide a
commented version of such a configuration.

setup.py *should* then skip over building _ctypes, as it
should find out that _ctypes was already built through Makefile.

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I'm astonished how flexible the build-system is :-), once
you leave Windows.

setup.py needs a change, though, so that the _ctypes/libffi
configure script is not run when not needed.

One possible advange of Modules/Setup.local is that one can
define additional compilation flags for ctypes - for
example, if ctypes would support it, to enable/disable
certain features (varargs function calls, for example).

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Setup.local has the disadvantage that you have to edit the
file. I updated the patch to only have an effect, if
configure is passed --with-system-ffi.

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The code is fine, but it lacks documentation. There should
be some way to learn about --with-system-ffi, preferably by
either reading ./configure --help output, or reading README.

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documentation added.

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Thanks for the patch. Committed as r45278.