Ctypes error Attribute symbol Error not found, OS X 10.7.5

Question

I have a simple test function in C ++:

#include <stdio.h> #include <string.h> #include <stdlib.h> #include <locale.h> #include <wchar.h> char fun() { printf( "%i", 12 ); return 'y'; } 

compilation:

 gcc -o test.so -shared -fPIC test.cpp 

and using it in python with ctypes:

 from ctypes import cdll from ctypes import c_char_p lib = cdll.LoadLibrary('test.so') hello = lib.fun hello.restype = c_char_p print('res', hello()) 

but then I get an error:

 Traceback (most recent call last): File "./sort_c.py", line 10, in <module> hello = lib.fun File "/System/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/ctypes/__init__.py", line 366, in __getattr__ func = self.__getitem__(name) File "/System/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/ctypes/__init__.py", line 371, in __getitem__ func = self._FuncPtr((name_or_ordinal, self)) AttributeError: dlsym(0x100979b40, fun): symbol not found 

Where is the problem?

via:

Mac Os X 10.7.5 and Python 2.7

Answer 1

Your first problem is C ++ name change. If you run nm in your .so file, you will get something like this:

 nm test.so 0000000000000f40 T __Z3funv U _printf U dyld_stub_binder 

If you mark it as a C style when compiling with C ++:

 #ifdef __cplusplus extern "C" char fun() #else char fun(void) #endif { printf( "%i", 12 ); return 'y'; } 

nm gives:

 0000000000000f40 T _fun U _printf U dyld_stub_binder 

The second problem is that python will die using Segmentation fault: 11 (on OS X). C ++ returns a char , while you mark it in python as a pointer to a char. Using:

 hello.restype = c_char 

instead (change your import statement to match).

EDIT: as @eryksun pointed out, you should not use gcc , you should use g++ . Otherwise, the correct C ++ runtime will not be bound. To test OS X :

 otool -L test.so 

( ldd , a tool commonly used on UNIX / Linux, is not distributed with OS X)