Bug report

Bug description:

On macOS, the configure script does not provide a way to link _ctypes against a non-system libffi.

Although LIBFFI_CFLAGS and LIBFFI_LIBS are documented as configuration variables for libffi, configure first checks for the system libffi from Apple's SDK and if found overwrites these variables.
The pkg-config based check, which does preserve these variables, is never run when the system libffi is detected.

The confusing --with-system-ffi option, removed in Python 3.12 (#100544), provided a way to invoke the pkg-config based configuration on macOS, bypassing the detection of the SDK libffi.

I ran into this while updating python-build-standalone to use the standard extension configuration in 3.12+ (astral-sh/python-build-standalone#1180). Other than patching configure the best option seemed to be to use ac_cv_lib_ffi_ffi_call=no to force the check to fail.

I also found that conda-forge patches configure to get around this issue when building CPython.

Related issues

• How to link with a non-system libffi? #58732
• gh-100540: Remove obsolete '--with-system-ffi' configure option #100544

CPython versions tested on:

3.12, 3.13, 3.14, 3.15, CPython main branch

Operating systems tested on:

macOS

Linked PRs

• gh-155813: Stop overwriting LIBFFI_CFLAGS and LIBFFI_LIBS on macOS #156486