segfault when calling Python from C thread

I'm running into a problem when trying to perform a callback to a
Python function from a C extension. Specifically, the callback is
being made by a pthread that seems to cause the problem. If I call
the callback from the parent process, it works fine. The PyObject is
static, and holds the same value in both Parent and thread, so I'm at
a loss as to what would be different in the pthread from the parent
that would cause a segfault on the callback. The machine specifics
are an x86 intel processor with RedHat linux.
/* calling callback */
void callback(char * str) {
PyObject *arglist;
PyObject *result;
if(str == NULL)
return;

if(my_callback == NULL) {
printf("no callback function provided, returning...\n" );
return;
}

/* Time to call the callback */
arglist = Py_BuildValue(" (s)", str);
result = PyEval_CallObje ct(my_callback, arglist);
Py_DECREF(argli st);
if(result == NULL)
return;
Py_DECREF(resul t);
}

Your callback function needs to hold the Python GIL (and have a vaild
threadstate) before it calls any Python C-API functions. Change the
last part of it to:

PyGILState_STAT E state;

/* ... */

/* Time to call the callback */

state = PyGILState_Ensu re();

arglist = Py_BuildValue(" (s)", str);
result = PyEval_CallObje ct(my_callback, arglist);
Py_DECREF(argli st);
if(result == NULL)
return;
Py_DECREF(resul t);

PyGILState_Rele ase(state);
}

Greg Chapman wrote:

Your callback function needs to hold the Python GIL (and have a
vaild threadstate) before it calls any Python C-API functions.
Change the last part of it to:

PyGILState_STAT E state;

/* ... */

/* Time to call the callback */

state = PyGILState_Ensu re();

arglist = Py_BuildValue(" (s)", str);
result = PyEval_CallObje ct(my_callback, arglist);
Py_DECREF(argli st);
if(result == NULL)
return;
Py_DECREF(resul t);

PyGILState_Rele ase(state);
}

you might wish to make sure you release the GIL even if the callback
raises an exception...

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