bpo-39632: Fix ctypes variadic function call convention #18560
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The build is failing on macOS because ctypes uses an obsolete libffi version bundled into Modules/_ctypes/libffi_osx for this platform. This old version of libffi is missing I don't know why libffi is bundled only for mac but I guess that we could just bundle the latest release of libffi (currently libffi-3.3 for every target. Since this is my very first (rather ambitious) contribution to CPython, I think I will need some guidance moving forward. Thanks |
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On armhf and for variadic functions (and contrary to non-variadic functions), the VFP co-processor registers are not used for float argument parameter passing. This specificity was completely disregarded by ctypes which used `ffi_prep_cif` systematically to prepare the parameter passing of a function while it should use `ffi_prep_cif_var` for variadic functions instead. As such variadic function call with float arguments through ctypes was broken on armhf targets. This change fixes and improves ctypes variadic function support. Functions that take a variable number of arguments should now be declared using an Ellipsis (...) as their last argument (just as a prototype for a C function). ctypes variadic functions declared this way automatically perform the default argument promotion. Small integer types are promoted to c_int and c_float is promoted to c_double. For reference/see also: - https://sourceware.org/ml/libffi-discuss/2016/msg00043.html - https://bugs.python.org/issue17580 - spacepy/spacepy#20 - https://en.cppreference.com/w/c/language/conversion#Default_argument_promotions
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@ndessart, are you interested in this? Could you fix merge conflicts? |
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On armhf and for variadic functions (and contrary to non-variadic
functions), the VFP co-processor registers are not used for float
argument parameter passing. This specificity was completely
disregarded by ctypes which used
ffi_prep_cifsystematicallyto prepare the parameter passing of a function while it should
use
ffi_prep_cif_varfor variadic functions instead.As such variadic function call with float arguments through ctypes
was broken on armhf targets.
This change fixes and improves ctypes variadic function support.
Functions that take a variable number of arguments should now be
declared using an Ellipsis (...) as their last argument (just as
a prototype for a C function). ctypes variadic functions declared
this way automatically perform the default argument promotion.
Small integer types are promoted to c_int and c_float is promoted
to c_double.
I am new to CPython development but I think that without the
introduction of the Ellipsis in functions argtypes this could be
backported to the 3.7 and 3.8 maintenance branches.
For reference/see also:
https://bugs.python.org/issue39632