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] Docs@Python, Extending Python with C or C++

[2] Docs@Python, Python/C API Reference Manual

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

[4] user1667191@StackOverflow, Compilation fails with “relocation R_X86_64_32 against `.rodata.str1.8' can not be used when making a shared object”