Hello World with C Extension

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The following C source file (which we will call hello.c for demonstration purposes) produces an extension module named hello that contains a single function greet():

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

#if PY_MAJOR_VERSION >= 3
#define IS_PY3K
#endif

static PyObject *hello_greet(PyObject *self, PyObject *args)
{
    const char *input;
    if (!PyArg_ParseTuple(args, "s", &input)) {
        return NULL;
    }
    printf("%s", input);
    Py_RETURN_NONE;
}

static PyMethodDef HelloMethods[] = {
    { "greet", hello_greet, METH_VARARGS, "Greet the user" },
    { NULL, NULL, 0, NULL }
};

#ifdef IS_PY3K
static struct PyModuleDef hellomodule = {
    PyModuleDef_HEAD_INIT, "hello", NULL, -1, HelloMethods
};

PyMODINIT_FUNC PyInit_hello(void)
{
    return PyModule_Create(&hellomodule);
}
#else
PyMODINIT_FUNC inithello(void)
{
    (void) Py_InitModule("hello", HelloMethods);
}
#endif

To compile the file with the gcc compiler, run the following command in your favourite terminal:

gcc /path/to/your/file/hello.c -o /path/to/your/file/hello

To execute the greet() function that we wrote earlier, create a file in the same directory, and call it hello.py

import hello          # imports the compiled library
hello.greet("Hello!") # runs the greet() function with "Hello!" as an argument

Found a mistake? Have a question or improvement idea? Let me know.

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