fixing warning message related to mpd_setminalloc

Question

I am constantly seeing

context.c:55: warning: mpd_setminalloc: ignoring request to set MPD_MINALLOC a second time

after every call to the below function in runtime that calls a python function in c++

This function uses Python.h as explained in https://docs.python.org/2/extending/extending.html

void process_string(string text)
{
    //cout<<text<<endl;
    PyObject *pName, *pModule, *pDict, *pFunc;
    PyObject *pArgs, *pValue;

    Py_Initialize();
    PyRun_SimpleString("import sys");
    PyRun_SimpleString("import os");
    PyRun_SimpleString("sys.path.append( os.path.dirname(os.getcwd()) )");

    pName = PyUnicode_FromString("python_files.strings");
    pModule = PyImport_Import(pName);
    Py_DECREF(pName);

    if (pModule != nullptr) {
        pFunc = PyObject_GetAttrString(pModule, "process_string");
        if (pFunc && PyCallable_Check(pFunc)) {
            pArgs = PyTuple_New(1);
            pValue = PyUnicode_FromString(text.c_str());
            cout<<_PyUnicode_AsString(pValue)<<endl;
            if (!pValue) {
                Py_DECREF(pArgs);
                Py_DECREF(pModule);
                fprintf(stderr, "Cannot convert argument\n");
            }
            PyTuple_SetItem(pArgs, 0, pValue);
            pValue = PyObject_CallObject(pFunc, pArgs);
            Py_DECREF(pArgs);
            if (pValue != NULL)
            {
                //cout<<_PyUnicode_AsString(pValue)<<endl;
                Py_DECREF(pValue);
            } else {
                Py_DECREF(pFunc);
                Py_DECREF(pModule);
                PyErr_Print();
                fprintf(stderr, "Call failed\n");
            }
        } else {
            if (PyErr_Occurred())
                PyErr_Print();
            fprintf(stderr, "Cannot find function \"%s\"\n", "process_string");
        }
        Py_XDECREF(pFunc);
        Py_DECREF(pModule);
    } else {
        PyErr_Print();
        fprintf(stderr, "Failed to load \"%s\"\n", "python_files.strings");
    }
    Py_Finalize();
}

The problem is in the c++ side of the code as even if I change the python function to simply return the input, I see the warning on the console.

The string.py file(a example) :

import os
import sys
import warnings

def process_string(text):
    if not sys.warnoptions:
        warnings.simplefilter("ignore")
    return text

I tried disabling warning print on the python side with no advantage.

Answer 1

I cannot reproduce the behaviour you describe but this is very likely due to the fact that you call Py_Initialize several times.

Each time after the first one, the decimal module is initialized and calls mpd_setminalloc with minalloc_is_set == 1, hence the warning message.

void
mpd_setminalloc(mpd_ssize_t n)
{
    static int minalloc_is_set = 0;

    if (minalloc_is_set) {
        mpd_err_warn("mpd_setminalloc: ignoring request to set "
                     "MPD_MINALLOC a second time\n");
        return;
    }
    if (n < MPD_MINALLOC_MIN || n > MPD_MINALLOC_MAX) {
        mpd_err_fatal("illegal value for MPD_MINALLOC"); /* GCOV_NOT_REACHED */
    }
    MPD_MINALLOC = n;
    minalloc_is_set = 1;
}

You should have the initialization of the Python interpreter in a separate function, and call it once only. For example, if you want to put it in main:

int
main()
{
    int i;    

    Py_Initialize();

    for( i=0; i < 5; i++)
        process_string("A nice string");
    process_string("Another nice string");

    Py_Finalize();
    return 0;
}

Note you could also put the imports there, as the Python interpreter will remain alive until your program finishes.

For reference the compiling command I use is:

g++ -c -I/usr/include/python3.6m -Wno-unused-result -Wsign-compare -march=x86-64 -mtune=generic -O2 -pipe -fstack-protector-strong -fno-plt -DNDEBUG -g -fwrapv -O3 -Wall <yourprogram.cpp> -lpython3.6m

And the linking command is:

g++ -g -L/usr/lib -lpython3.6m -lpthread -ldl  -lutil -lm  -Xlinker -export-dynamic -o <yourexecutable> <yourobjectfile.o>