Unable to assign 0 as NULL to void** local

Reporting a bug

• I have tried using the latest released version of Numba (most recent is visible in the change log (https://github.com/numba/numba/blob/master/CHANGE_LOG).
• I have included a self contained code sample to reproduce the problem. i.e. it’s possible to run as ‘python bug.py’.

Pointers can be set to NULL by assigning 0. This appears not to work with pointers to pointers.

import numba

# Succeeds
@numba.cfunc("void()", locals = {"bar": numba.types.voidptr})
def foo():
  bar = 0

# Fails
@numba.cfunc("void()", locals = {"bar": numba.types.CPointer(numba.types.voidptr)})
def foo():
  bar = 0

TypingError: Failed in nopython mode pipeline (step: nopython frontend)
No conversion from Literal[int](0) to void** for 'bar', defined at None

I would assume that if this works with pointers generally, it shouldn’t fail for the specific case of a pointer to a pointer.

This is actually blocking progress on something for me at the moment, so any comments on workarounds would be appreciated! I need to pass a NULL void** into a ctypes function from inside a cfunc.

This seems to work fine, but I can’t alter the signature of my cfunc to accommodate so it’s not a solution for me.

@numba.cfunc(numba.types.void(numba.types.CPointer(numba.types.voidptr)))
def foo(a):
  return

foo(0)

The pointer types don’t appear to have casting support.

[stuartarchibald]

That intrinsic trick looks very nice, thanks! I just looked up Numba intrinsics after this and I see that casting to a pointer is actually the example given; I wish I’d managed to turn that page up in my googling! I think this pattern will be quite helpful for a few other things I was having difficulty with too.

No problem. If you know roughly what the LLVM is for what you want, then @intrinsic gives you an escape hatch to do that. clang -S -emit-llvm can help with getting the “rough” LLVM from some C code.

[stuartarchibald]

Something like this might work:

import numba
from numba.extending import intrinsic

@intrinsic
def void_ptrptr(tyctx, arg):
    sig = numba.types.CPointer(numba.types.voidptr)(numba.types.intp)
    def codegen(cgctx, builder, sig, llargs):
        llvalue, = llargs
        argty, = sig.args
        retty = sig.return_type

        llretty = cgctx.get_value_type(retty)
        retptr = builder.alloca(llretty)
        valbuf = builder.alloca(llvalue.type)
        builder.store(llvalue, valbuf)
        valbuf_val = builder.load(valbuf)
        valbuf_asptr = builder.inttoptr(valbuf_val, llretty)
        builder.store(valbuf_asptr, retptr)
        return builder.load(retptr)
    return sig, codegen

import ctypes
libm = ctypes.CDLL('libm.so')
ct_cos = libm.cos
ct_cos.restype = ctypes.c_double
# this is a lie, but acts as a sink such that `bar` isn't optimised away
ct_cos.argtypes = (ctypes.POINTER(ctypes.c_void_p),)


@numba.cfunc("void()")
def foo():
  bar = void_ptrptr(0)
  ct_cos(bar)

print(foo.inspect_llvm())