I/O

Table of Contents

Objectives

  • Read in files into a program
  • Create the necessary logic to handle an interface
  • Filter out files by pattern

Getting Started

Use the GitHub Classroom link posted in the Learning Suite for the lab to accept the assignment. Next, ssh into your Raspberry Pi using VSCode and clone the repository in your home directory. This lab should be done in VSCode on your Raspberry Pi. Make sure the lab is top level folder of your VSCode editor.

Overview

Learning to interact with the peripherals of the Pi Z2W is what allows us to take a simple single-board computer and turn it into a system with a more directed purpose. You may have noticed by looking at the future lab names of this class that the culmination of this project is to make a smart doorbell system. To that end, a HAT with an LCD screen and some buttons has been provided. Its purpose is to provide a way for a user of your smart doorbell to directly interact with the computer that is powering it.

In the last lab you were able to draw practically anything to the LCD screen by utilizing the provided vendor libraries. You were also able to correlate these drawing events with button presses. In this lab we will take this practice and put it towards a more practical use: creating a photo and log viewer for your doorbell.

The photo and log viewer of this lab consists of several components: scanning the folder where the intended files live, filtering out any files that the photo and log viewer doesn’t care about, and displaying the intended content of each file.

Scanning the Folder

In this lab you are expected to scan through a current folder and make a list of strings that indicate the contents of that folder. While this is as trivial as a simple ls -al command in the shell, it becomes more challenging when trying to achieve the same objective in C. Implement the following directions from the sections below to create an array of strings that contain all of the files of interest inside the viewer folder. Your scanning function should filter out anything that doesn’t end in .bmp or .log.

DIR Pointer

Before we can read in the contents of a directory in C, we will need a variable that holds some information about the directory that we are reading. For this, we use a DIR pointer. This object will allow us to look at the location of where our directory lives and then use this a pointer to iterate through files in a directory and (for our purposes) retrieve those file names. To make this pointer:

DIR *dp;    // Creates a directory pointer dp

Folder operations

With our newly allocated DIR pointer, we can use the following functions that will help us interface with the directory in our code.

  • opendir() and closedir()

    In order to open a directory while using the DIR pointer, you can do the following:

      DIR *dp;
  dp = opendir("my-folder");

  if (dp == NULL) {
      // Folder does not exist
  }

  // Use dp to read the contents of the folder

    This opens the my-folder directory into code and puts the dp pointer at the top of this directory. When we are done iterating through the directory, you will need to close it by using the following call to closedir():

      closedir(dp);

Parsing through the Folder

While iterating through a directory, you will need to have an object that corresponds to the files that are inside the directory. This object is known as the struct dirent pointer and is the return value of readdir(). In other words, when we call readdir() on a DIR pointer, it will iterate through a linked-list that returns the files in the form of a dirent struct pointer. Once we have finished iterating through all of the files in a DIR pointer, readdir() will return NULL:

  • readdir() example:

      struct dirent * entry;    // Current folder object

  while ((entry = readdir(dp)) != NULL) {
      // Do something with the directory...
  }

  • d_name The d_name value inside of the dirent struct holds the file name of the file we are looking at. To see what the name of the current file is from a readdir() operation, do the following:

      entry = readdir(dp);                      // Get the current file object
  printf("Item name: %s\n", entry->d_name); // Print out name

Filtering Files

While reading in your files using the folder operations, you may want to filter out some folders or files that are not of interest to you such as the . and .. files that exist in every directory. To do this, you can check the name of the filename with the strncmp(). If the files match a certain value (i.e. the last 3 characters of the name are bmp or log), you can add them to a list, else you can ignore it. More reading on how to use strncmp() is found in the Explore More! section below.

Reading Files

In this lab, you will be expected to read both .bmp and .log files from the viewer folder. Then you must show them on the LCD screen of your Pi Z2W. In order for your program to input these files in code so you can interact with them, you will need to become familiar with some basic C constructs such as FILE streams and their associated functions. The information in the following section should help you learn about reading files into C and writing them to a different destination (i.e. in this case, to your LCD screen).

FILE Pointers

Interacting with a file will require you to make a FILE stream pointer. This pointer represents the current position of where a read or a write action is inside of a file. For example, if you open a file to read for the first time, the pointer will be at the beginning of the document. As you read words by advancing the pointer position with functions such as fscanf() or fread(), the pointer value will increment. For this lab, we will use FILE pointers only in the context of the functions needed for the file operations described below. To declare a FILE pointer:

FILE *fp;    // This declares a file pointer that is read to be used in file operations

File Operations

There are a few essential file operations that exist in the stdio.h library. The following is a selection of functions that will help you with to input files into a location in your code.

  • fopen() and fclose()

    This function takes in a FILE stream pointer, a file path, and some special strings to indicate the mode as arguments for opening a file. For example:

      FILE *fp;    // File pointer for interacting with file.
  fp = fopen("fancy_output.log", "r");

    This code will create a FILE stream pointer called fp. We then use fopen to set the location of fp to the beginning of the fancy_output.log file in the current directory with the read mode (look at the Explore More! section for different file modes).

    Once we have done everything that we want to with the file, we must be responsible and release the pointer back to the system. This is done by closing the file using fclose(). To do this, simply do the following:

      fclose(fp);    // Closes the file that was attached to the fp pointer

  • fread()

    Once we have a file open, how do we use the FILE stream pointer to get data? This is where functions like fread come into play. This function reads data from a file pointer. Specifically, it takes a buffer to read data into, the size of the data, the length of the data, and the file pointer.

      char data[256];            // Creates a string that can hold up to 256 characters 
  fread(data, 1, 256, fp);   // Reads up to 256 bytes (or characters) from the file

Requirements

In this lab you should accomplish the following:

  • Create a simple menu that lists all of the files in the viewer folder:

    • Filter out any file that does not end in .bmp or .log.

    • File names should be drawn in 8pt font.

    • A selection bar should highlight which file you are about to open. When the up and down buttons are pressed, the selection bar should move up and down to the next or previous entry. The selection bar should scroll around from the top to the bottom and the bottom to the top. This means if your selection bar is at the top of the list and you press up, it should move to the bottom of the list. The same thing should be true for the bottom of the list and pressing down.

  • When the center button is pressed do the following:

    • If the file is a .bmp image, display the corresponding image for 2 seconds and then go back to the menu view with the highlight bar over the selected item.

    • If the file is a .log file, display the contents of the file for 2 seconds and then go back to the menu view with the highlight bar over the selected item.

  • You only need to show up to 8 items on the display. You can ignore the rest of the items if there are more than 8.

  • To perform these tasks, we have given you three functions that you must implement and use in your main function: get_entries, draw_menu, and draw_file. Also, use the #defined variables at the top of the file to help implement these functions.

Here is a demo showing the different features of the lab:

Submission

  • Answer the questions in the README.md.

  • To pass off to a TA, demonstrate your doorbell running your program that fulfills all of the requirements outlined above.

  • To successfully submit your lab, you will need to follow the instructions in the Lab Setup page, especially the Committing and Pushing Files section.

Explore More!