The setup.py helper function grep_headers_for() is used by ctypes build step to search header files for functions. The function kinda works on most platforms, but only by accident. On my system it opens all header files that match /usr/include/*.h and returns true if any (!) header file contains the string (!) "ffi_prep_cif_var". The check would also match "ffi_prep_cif_var" in a comment of an unrelated header file.

We cannot limit the search to "ffi.h" only, because that would break multiarch builds with a header file like this:

    #if defined(__i386__)
    #include "ffi-i386.h"
    #elif defined(__powerpc64__)
    #include "ffi-ppc64.h"
    ...

Solutions:

1) Use "AC_EGREP_HEADER" autoconf macro. It runs a header file through the C preprocessor
2) Use AC_LINK_IFELSE autoconf macro. This macro compiles and links code to detect if a library exports a function.
3) Re-implement any of the macros in pure Python (ugh!)
4) Just assume that libffi supports ffi_prep_cif_var, ffi_prep_closure_loc, and ffi_closure_alloc on our target platforms. According to [1] the functions have been around for over a decade.

I favor (4).

Ned, what about USING_APPLE_OS_LIBFFI? Is it still relevant?

[1] https://github.com/libffi/libffi/blame/48bdb02867edb7e9f3785ccb4bdff1087fb44246/include/ffi.h.in#L309

If we remove grep_headers_for(), then we can also close bpo-42325

We could look in distro patch trackers to see they patch CPython’s setup.py to evaluate the impact of removing this function.

> Ned, what about USING_APPLE_OS_LIBFFI? Is it still relevant?

It was just added a year ago as part of the support for macOS 11 Big Sur and Apple Silicon systems.