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Calling Python macro from ctypes

0 votes

Is it possible to call a Python macro from ctypes? For example, Python 3.3 introduces some new macros for querying the internal representation of strings:

So I try this in 3.3:

py> import ctypes
py> ctypes.pythonapi.PyUnicode_MAX_CHAR_VALUE
Traceback (most recent call last):
 File "", line 1, in 
 File "/usr/local/lib/python3.3/ctypes/", line 366, in __getattr__
 func = self.__getitem__(name)
 File "/usr/local/lib/python3.3/ctypes/", line 371, in __getitem__
 func = self._FuncPtr((name_or_ordinal, self))
AttributeError: python3.3: undefined symbol: PyUnicode_MAX_CHAR_VALUE
posted Aug 12, 2013 by Amit Parthsarthi

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1 Answer

+1 vote

That's not possible. It may look like a function, but a preprocessor replaces the C macro in the C source before compilation. An example of very bad usage of macros, just to drive the point home:

$ cat macro.c 
#define IF(expr) if (expr) {
#define ENDIF ;}

 printf("It workedn")

And here's what the compiler sees:

$ gcc -E -P macro.c

 if (1>0) {
 printf("It workedn")
answer Aug 12, 2013 by Majula Joshi
To elaborate a bit more, Python can only see those symbols that are put into the shared library They can be functions, and they can be "values," but they don't include macros, which are processed by the preprocessor, before the real C compiler even starts. C Macros are actually text-substitution rules. They can look like functions, but those functions do not end up in the shared library.

In Windows, you can use dumpbin to examine a DLL and see what symbols it exports. I don't remember the syntax; it's been years.

I assume there's a similar tool for Linux to examine a shared library (typically an .so file). Perhaps "readelf" and/or "nm" is such a tool, but I don't really know. Although I've been using Python and C++ in Linux in recent years, I haven't used them together, and neither have I had to examine a shared library.

The following link looks interesting.
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