[Cython] Syntax for writing C macros in a Cython .pxd

[Stefan Behnel]

Hi,

Martin C. Martin to-posted:
> Stefan Behnel wrote:
>> since William brought up this topic lately, I think there are a couple
>> of very valid cases where you want to define a macro in C and use it as a
>> function in Cython.
>>
>> I think this is so common that it would help if you could write inline
>> C code in a .pxd file and have it written into your generated C file
>> automatically. I could imagine a syntax like this:
>>
>>     # macros.pxd
>>     MACROS = """
>>         #define _isString(obj)   (PyString_CheckExact(obj)  || \
>>                                   PyUnicode_CheckExact(obj) || \
>>                                   PyObject_TypeCheck(obj,
>> &PyBaseString_Type))
>>
>>         #define _fqtypename(o)   (((PyTypeObject*)o)->ob_type->tp_name)
>>     """
>>
>>     cdef inline from MACROS:
>>         cdef int _isString(object obj)
>>         cdef char* _fqtypename(object t)
>
> Macros in C have a lot of problems, so I think it's best to avoid them
> whenever possible.  In particular:
> 
> - The symbols they define don't show up in the debugger, memory
> profiler, or other tools.

Well, in most cases macros are rather short code snippets, not function
replacing code blocks. So I don't care about their symbols.


> - They work at the textual level, so it's easy to get incorrect
> behaviour if you forget to add extra parenthesis that aren't needed in
> normal C.
> - When using them to define an inline "function," they can't always be
> used all places a function can.

Sure, that's why you have to take care when you use them. In my first
sentence, I said "I think there are a couple of very valid cases", I think
that makes the focus clear.


> In your example:
> 
>  #define _fqtypename(o)   (((PyTypeObject*)o)->ob_type->tp_name)
> 
> _fqtypename(a + b) expands to:
> 
> (((PyTypeObject*)a + b)->ob_type->tp_name)

And that's definitely not a valid use case of _fqtypename().


> If you want an inline function, why not just declare it inline?

Because for small code snippets that exist solely for performance reasons, the
overhead of a function defined in Cython may be too high.


> The use cases discussed on the list recently were to get around the fact
> that Cython doesn't know all that much about C or C++.  Why not just
> teach it more?

Does that imply a 'real' syntax for C macros? I would deny the need. As you
noted, macros are defined at the text level.

Stefan