Evaluate a python expression from C?

[Kragen Sitaker]

David Brady <daves_spam_dodging_account at yahoo.com> writes:
> Could the problem be the NULL args I pass in?  For
> simple evaluation, what should I populate them with
> instead?

There were several problems; that was one of them.  Here's an
apparently-working program, in C:

#include <stdio.h>
#include <Python.h>

void PyEval(char *data) 
{ 
  char *buf;
  PyObject *globals, *locals, *pObj;

  Py_Initialize(); 

  globals = PyDict_New();
  if (!globals) return;
  locals = PyDict_New();
  if (!locals) {
    Py_DECREF(globals);
    return;
  }
 
  // The following line doesn't access violate. 
  pObj = PyRun_String(data, Py_eval_input, globals, locals); 
 
  PyArg_ParseTuple(pObj, "s", &buf); 
  printf("%s", buf);
  Py_DECREF(pObj);
  Py_DECREF(globals);
  Py_DECREF(locals);
} 

int main(int argc, char **argv) {
  PyEval(argv[1]);
}

I compiled it with 

cc -g pyeval.c -Wall -I/usr/local/include/python2.1 -lpython2.1 -L/usr/local/lib/python2.1/config -o pyeval -ldl -lpthread -lm -lutil

which is somewhat system-dependent, and run it with

./pyeval '("hi\n",)'

(because, after all, PyArg_ParseTuple is expecting a tuple containing
a string)