I have some legacy code that reads like this:
bool has_attribute(int kvidx) const {
void* v = nullptr;
int flag = 0;
MPI_Comm_get_attr(impl_, kvidx, &v, &flag);
return flag != 0;
}
The third argument of MPI_Comm_get_attr takes a .void**
CORRECTION: … it takes void*, but it is void** semantically according to the function documentation. This is the true origin of the warning.
This code now is giving a clang-tidy warning bugprone-multi-level-implicit-pointer-conversion.
Clang-tidy recommends: "multilevel pointer conversion from 'char **' to 'void *', please use explicit cast"
How do I apply this "explicit cast"?
I tried with MPI_Comm_get_attr(impl_, kvidx, static_cast<void**>(&v), &flag); but clang is still complaining.
To be clear, the code is correct AFAIK, I am trying to follow clang-tidy recommendation (without suppressing the warning)
Clang-tidy is correct from its point of view.
The problem arises from the behavior of the API.
According to the MPI_Comm_get_attr ducumentation, this is the signature of the function:
int MPI_Comm_get_attr(MPI_Comm comm, int comm_keyval, void *attribute_val, int *flag)
The 3rd parameter is a void*, not a void**, so Clang-tidy rightfully detected that a multi-level conversion is needed.
However the MPI_Comm_get_attr ducumentation also states that:
Even though the attr_value argument is declared as void *, it is really the address of a void pointer. See the rationale in the standard for more details.
So you were right to pass &v as an argument.
The only solution I see is to suppress Clang-tidy for this part of the code.