tomat.c

#include <stdio.h>
#include <unistd.h>

int
tomat(int length, FILE *stdout_)
{
    int remain = 0;
    while ((remain = (length -= 1)) != 0)
    {
        fprintf(stdout_, "00:%d\n", remain);
        sleep(1);
    }
    return 0;
}

tomat_test.py

import sys
import ctypes
import unittest

class TomatTestCase(unittest.TestCase):
    def setUp(self):
        self.lib = ctypes.CDLL('./tomat.so')

    def test_prints_every_second(self):
        seconds = ['00:1', '00:2', '00:2', '00:3', '00:4', '00:5',
                   '00:6', '00:7', '00:8', '00:9']
        self.lib.tomat(10, sys.stdout.fileno())
        self.assertEqual(output, seconds[::-1])