Continued from the last time, I decided to implement the strings and files in C on Linux first. And I learnt a lot from this.
One thing is that how to extend C. So the entrance of the C program should be PyMODINIT_FUNC PyInit_modulename(void), python setuptools will recognize this, and then in the function, you should return an object called PyModule_Create(&modulename). the object is defined with a data structure called static struct PyModuleDef. In this, you will need to define the wrapper of python, the function table (similar as the call table in Linux kernel). This is really a briliant design since it considers all the object-oriented idea: encapsulation, inheritance and polymorphism. But the thing needs to notice is that there is a significant difference between python2 and python3 in terms of the implementation of this, so I decide to implement python3 version first since python2 is almost deprecated.
Another thing is the real implementation of strings and file. There are two difficulties that I have addressed. The first one is reading file. There is a slight difference between python and C. In C, firstly you need to get the file size and allocate memory for the array. And this could be dangerous since the memory might leak (maybe you forgot to free it after use). The second one is how do you iterate it after reading. This is trival but important. At first, I simply used strlen(buffer), but this is not correct. it should be the file length you got before. Another one important knowledge I learnt is the difference between char * and char array. We know that you could modify an array but not a char pointer. The reason is that char  will allocate a string on stack while char * will only create a pointer on stack and the string stores in data segment.