COP4600 -  P2: POSIX Standards & Bindings  - Solved

POSIX Standards & Bindings 

 

Prologue
Your work in the Legion has been highly praised, but you’ve had a creeping concern since. You’d always assumed that, as a faithful soldier, you would be rewarded and be permitted to serve as a vassal upon the subjugation of Earth. However, through the metadata you’ve analyzed, you’ve discovered that ultimately “lesser creatures” – including humans such as yourself – will likely be eliminated once Earth is conquered. As you continue your investigation, you take on another task with hopes you’ll still be considered a useful pawn on the board. This time, you’re tasked with creating a program capable of printing out the contents of specified files. Realizing the importance of the documents being stored on Sky Skink, the Legion’s cloud system, you’ll also create a backdoor that connects the file’s contents to a GUI for readability for later access. It might come in handy down the road.

Overview
Modern operating systems are often built in layers with specific goals and limited privileges. Layering also facilitates the implementation of recognized standards by separating hardware and OS-specific implementations from generalized API calls. Often these layers are written in different languages and permit access via a binding layer to lower level functionality. In Android, POSIX functions can be called from Java applications via the Native Development Kit (NDK).

 

In this project, you will build a C++ language function that calls POSIX system library procedures to read a text file and return it as a C-style string (null-terminated character array). You will also build two programs that use your C++ functions – a simple program at the Android application layer that uses the NDK binding to call your C++ functions to read and display a text file in an application window, and another version that will print the text file to the screen in Ubuntu. We will provide a program that exercises your new call and program on the command line. You’ll then create a short video to demonstrate your code. You’ll submit the project via Canvas.

Structure
The project is broken into four main parts:

 

1)      Create a C++ function that reads a text file via POSIX calls and returns a pointer to its contents

2)      Create a short program that uses the function above to display a file via the command line

3)      Create a simple Android GUI application that accepts a text file name as input and has a text box

4)      Bind the function via the NDK to that the application to display the contents of the file in the text box

 

Figure 1: A function is called from a text program, and separately, bound to the GUI application. 

 

While exact implementation may vary, the library functions must match the signatures laid out in this document.

 

Specification
Students will write several sections of code according the following specifications.

 

File Reader Library  

The files read_file.h / read_file.cpp will contain declaration / definition of this function:

 

char *read_file(const char *filename) 

Makes POSIX calls to read the contents of filename from disk to be stored in a null-terminated character array (C-style string). The array should be allocated dynamically. A pointer to the array will be returned. The caller will be expected to free the memory allocated for the array. If the file is not found, it should return nullptr.

 

 

Text Program 

The text program, once built, should have the name displayfile and should take exactly one command line argument – the filename of the file to be displayed on the screen. It should use the read_file() function. If a file cannot be found, the text program should print “Error: File Note Found”:

 

$ ./displayfile /sdcard/example.txt 

Contents of

                Hello world!                                            /sdcard/example.txt 

                This is the last line of the file.         Hello world!      

                 $                                          This is the last line of the file.         

                   

$ ./displayfile nope.txt 

                Error: File Not Found                         Note that nope.txt does not exist. 

                $                                           

 

 

GUI Program 

The GUI program skeleton provides three GUI elements (without functional code). Students must add code as necessary to elicit the following behavior:

 

1)      One-line text box where the file to be displayed will be typed by the user, named filenameBox

2)      Button to submit the filename (which should trigger the read and display), named submitButton

3)      Multi-line text box where the file will be displayed when the button is pressed, named displayBox

 

Students should modify the Java source to invoke the C++ functions. A simple example of transmitting a string from C++ to Java is included in the project base code. You must use the exact same source files created in the File Reader Library for this task to receive credit, comments and includes are no exception. As in the text program, if a file cannot be found, then the GUI must display the error message in the text box (“Error: File Not Found”).

 

NOTE: You will need to add any new C++ source files to the CMakeLists.txt build file! When accessing files, it must be the absolute path starting with /mnt/ubuntu/<abs_path>, do NOT prepend this in your code!