On my OS X Yosemite 10.10.3, I got this warning when compiling Modules/_ctypes/libffi_osx/ffi.c.

gcc -Wno-unused-result -Wsign-compare -g -O0 -Wall -Wstrict-prototypes -Werror=declaration-after-statement -I_ctypes/darwin -I/Users/sky/Code/python/cpython/Modules/_ctypes/libffi_osx/include -I/Users/sky/Code/python/cpython/Modules/_ctypes/libffi_osx/powerpc -I./Include -I. -IInclude -I/usr/local/include -I/Users/sky/Code/python/cpython/Include -I/Users/sky/Code/python/cpython -c /Users/sky/Code/python/cpython/Modules/_ctypes/libffi_osx/ffi.c -o build/temp.macosx-10.9-x86_64-3.6-pydebug/Users/sky/Code/python/cpython/Modules/_ctypes/libffi_osx/ffi.o -DMACOSX
/Users/sky/Code/python/cpython/Modules/_ctypes/libffi_osx/ffi.c:108:1: warning: unused function 'struct_on_stack' [-Wunused-function]
struct_on_stack(
^
1 warning generated.

Here is the patch that I think can fix the warning.