Calling python hello world function from C, parsing string arguments

Question

I used the code in this answer to create the following file

callpython.c

#include </usr/include/python2.7/Python.h>

int
main(int argc, char *argv[])
{
    PyObject *pName, *pModule, *pDict, *pFunc;
    PyObject *pArgs, *pValue;
    int i;

    if (argc < 3) {
        fprintf(stderr,"Usage: call pythonfile funcname [args]\n");
        return 1;
    }

    Py_Initialize();
    pName = PyString_FromString(argv[1]);
    /* Error checking of pName left out */
    //fprintf(stderr,"pName is %s\n", pName);
    PyRun_SimpleString("import sys");
    PyRun_SimpleString("sys.path.append(\".\")");
    //PySys_SetArgv(argc, argv);

    pModule = PyImport_Import(pName);
    Py_DECREF(pName);

    if (pModule != NULL) {
        pFunc = PyObject_GetAttrString(pModule, argv[2]);
        /* pFunc is a new reference */

        if (pFunc && PyCallable_Check(pFunc)) {
            pArgs = PyTuple_New(argc - 3);
            for (i = 0; i < argc - 3; ++i) {
                pValue = PyInt_FromLong(atoi(argv[i + 3]));
                if (!pValue) {
                    Py_DECREF(pArgs);
                    Py_DECREF(pModule);
                    fprintf(stderr, "Cannot convert argument\n");
                    return 1;
                } 
                /* iValue reference stolen here: */
                PyTuple_SetItem(pArgs, i, pValue);
                //PyTuple_SetItem(pArgs, i, argv[i + 3]);
            }
            pValue = PyObject_CallObject(pFunc, pArgs);
            Py_DECREF(pArgs);
            if (pValue != NULL) {
                printf("Result of call: %ld\n", PyInt_AsLong(pValue));
                Py_DECREF(pValue);
            }
            else {
                Py_DECREF(pFunc);
                Py_DECREF(pModule);
                PyErr_Print();
                fprintf(stderr,"Call failed\n");
                return 1;
            }
        }
        else {
            if (PyErr_Occurred())
                PyErr_Print();
            fprintf(stderr, "Cannot find function \"%s\"\n", argv[2]);
        }
        Py_XDECREF(pFunc);
        Py_DECREF(pModule);
    }
    else {
        PyErr_Print();
        fprintf(stderr, "Failed to load \"%s\"\n", argv[1]);
        return 1;
    }
    Py_Finalize();
    return 0;
}

I created another file in the same directory as helloWorld.py. The contents of this python script are

def helloworldFunc(a):
    print 'Hello '+str(a)

I compile and run callpython.c as below

 g++ -o callpython callpython.c -lpython2.7 -lm -L/usr/lib/python2.7/config && ./callpython helloworld helloworldFunc world

Rather than printing "Hello world", it prints "Hello 0"

Why does it not parse the python function argument as string?

Have you tried to use a debugger? Have you tried to run the program with more arguments? — n. m. could be an AI
– n. m. could be an AI, Commented May 21, 2018 at 3:33

Mark Tolonen · Accepted Answer · 2018-05-21 04:00:15Z

2

The sample code is parsing the arguments as integers, buy you've passed a string. atoi("world") returns 0, so that's the integer you get:

/* Create tuple of the correct length for the arguments. */
pArgs = PyTuple_New(argc - 3);
for (i = 0; i < argc - 3; ++i) {
   /* Convert each C argv to a C integer, then to a Python integer. */
    pValue = PyInt_FromLong(atoi(argv[i + 3]));
    if (!pValue) {
        Py_DECREF(pArgs);
        Py_DECREF(pModule);
        fprintf(stderr, "Cannot convert argument\n");
        return 1;
    } 
    /* iValue reference stolen here: */
    /* Store the Python integer in the tuple at the correct offset (i) */
    PyTuple_SetItem(pArgs, i, pValue);
}

Change the conversion line to the following to handle any string:

pValue = PyString_FromString(argv[i + 3]);

edited May 21, 2018 at 4:00

answered May 21, 2018 at 3:44

Mark Tolonen

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Comments

user13107 · Accepted Answer · 2018-05-21 03:40:39Z

0

Solved the issue. The culprit was this line

pValue = PyInt_FromLong(atoi(argv[i + 3]));

It was parsing each argument to python script as an integer.

When replaced with the following line, it parses each argument as a string

pValue = PyString_FromString(argv[i+3]);

I haven't really understood how pValue works, but this solves the problem for now.

answered May 21, 2018 at 3:40

user13107

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4 Comments

Mark Tolonen Over a year ago

Ha, you answered while I was prepping mine.

user13107 Over a year ago

@MarkTolonen Thanks, if, in your answer, you could add an explanation of PyTuple_SetItem(pArgs, i, pValue); that would be great!

Mark Tolonen Over a year ago

PyTuple_New() creates a tuple equal in size to the number of arguments to the function. PyTuple_SetItem() sets the corresponding element of the tuple to the pValue object generated for each argument. See PyTuple_SetItem in the documentation for more details.

Mark Tolonen Over a year ago

Updated my answer as well.

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