Python/C API

C Codes

  • demo.c

    int add(int x, int y){
        return x + y;
    }
    
    //int main(void){
    //    printf("%d", add(1, 2));
    //    return 0;
    //}
    
    #include<Python.h>
    
    static PyObject* W_add(PyObject* self, PyObject* args){
        int x;
        int y;
        if(!PyArg_ParseTuple(args, "i|i", &x, &y)){
            return NULL;
        } else {
            return Py_BuildValue("i", add(x, y));
        }
    }
    
    static PyMethodDef ExtendMethods[] = {
        {"add", W_add, METH_VARARGS, "a function from C"},
        {NULL, NULL, 0, NULL},
    };
    
    //initxxx, xxx should be this file name
    //if spam.c => spam.so
    //when import spam will raise ImportError
    //ImportError: dynamic module does not define init function (initspam)
    //-o xxx.so, xxx also should be this file name
    PyMODINIT_FUNC initdemo(){
        Py_InitModule("demo", ExtendMethods);
    }
    
  • Compilation and Linkage

    • Command

      gcc demo.c -I /usr/include/python2.6 -shared -o demo.so
      
      • Ubuntu 12.04.5 LTS raise error

        $ gcc demo.c -I /usr/include/python2.7 -shared -o demo.so
        /usr/bin/ld: /tmp/cceeXldr.o: relocation R_X86_64_32 against `.rodata' can not be used when making a shared object; recompile with -fPIC
        /tmp/cceeXldr.o: could not read symbols: Bad value
        collect2: ld returned 1 exit status
        
        • Recompile with -fPIC works

          gcc demo.c -I /usr/include/python2.6 -shared -fPIC -o demo.so
          
      • Mac OS X raise error

         $ gcc demo.c -I /usr/include/python2.7 -shared -o demo.so
         Undefined symbols for architecture x86_64:
           "_PyArg_ParseTuple", referenced from:
               _W_add in demo-00439c.o
           "_Py_BuildValue", referenced from:
               _W_add in demo-00439c.o
           "_Py_InitModule4_64", referenced from:
               _initdemo in demo-00439c.o
         ld: symbol(s) not found for architecture x86_64
         clang: error: linker command failed with exit code 1 (use -v to see invocation)
        
  • Tips

    • if note off main function

      $ gcc demo.c -I /usr/include/python2.7 -shared -fPIC -o demo.so
      demo.c: In function ‘main’:
      demo.c:6:5: warning: incompatible implicit declaration of built-in function ‘printf’ [enabled by default]
      
    • and not note off printf works

      int main(void){
      //    printf("%d", add(1, 2));
          return 0;
      }
      

Python Building Extension

  • Setup.py

    • https://github.com/urwid/urwid/blob/master/setup.py

      building 'urwid.str_util' extension
      creating build/temp.macosx-10.9-intel-2.7
      creating build/temp.macosx-10.9-intel-2.7/source
      cc -fno-strict-aliasing -fno-common -dynamic -arch x86_64 -arch i386 -g -Os -pipe -fno-common -fno-strict-aliasing -fwrapv -DENABLE_DTRACE -DMACOSX -DNDEBUG -Wall -Wstrict-prototypes -Wshorten-64-to-32 -DNDEBUG -g -fwrapv -Os -Wall -Wstrict-prototypes -DENABLE_DTRACE -arch x86_64 -arch i386 -pipe -I/System/Library/Frameworks/Python.framework/Versions/2.7/include/python2.7 -c source/str_util.c -o build/temp.macosx-10.9-intel-2.7/source/str_util.o
      cc -bundle -undefined dynamic_lookup -arch x86_64 -arch i386 -Wl,-F. build/temp.macosx-10.9-intel-2.7/source/str_util.o -o build/lib.macosx-10.9-intel-2.7/urwid/str_util.so
      

References

[1] [email protected], Extending Python with C or C++

[2] [email protected], Python/C API Reference Manual

[3] O'Reilly's CD bookshelf, 24.1 Extending Python with Python's C API

[4] [email protected], Compilation fails with “relocation R_X86_64_32 against `.rodata.str1.8' can not be used when making a shared object”

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