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Building a Terminal Todo App with C: A Deep Dive into Ncurses and Libcurl

Embarking on a terminal user interface (TUI) project often leads developers towards modern toolkits in languages like Python, Rust, or even JavaScript. However, exploring the path less traveled – using the venerable C programming language – offers unique challenges and learning opportunities. This article delves into the experience of reimplementing a Todoist-like application entirely within the terminal using C, focusing on the core libraries and techniques involved.

The Allure and Challenge of C for a TUI

Why choose C for a task where many higher-level tools exist? The motivation often stems from a desire to move beyond abstractions, engage directly with lower-level system interactions, and confront the fundamental challenges of software development, such as manual memory management and explicit state control. While modern frameworks offer convenience, building something substantial in C provides a profound understanding of what happens “under the hood.”

However, C’s low-level nature presents significant hurdles for UI development:

Verbosity: Tasks that are trivial in other languages require considerable code. For instance, creating a simple JSON object involves careful allocation, checking for null pointers, and explicitly adding each member, often spanning numerous lines.
Manual Management: C grants direct memory control, which is powerful but unforgiving. Developers are responsible for allocating and freeing memory, making errors like segmentation faults a common part of the development cycle.
Lack of Built-in Conveniences: Unlike modern languages with rich standard libraries, C often requires relying on external libraries or implementing basic data structures and functionalities from scratch.

Core Libraries: The Building Blocks

Constructing this TUI relied heavily on a few key C libraries, each with its own characteristics and learning curve.

Ncurses: Crafting the Terminal Interface

Ncurses is a foundational library for building text-based interfaces in Unix-like systems. While powerful for its era (dating back decades), it demands meticulous control from the developer. Every UI update, every state change, down to the specific row and column, must be explicitly managed.

Ncurses works by maintaining an internal buffer representing the state of the terminal screen (or a specific WINDOW within it). Developers manipulate this buffer using functions like printw (print to window) and then explicitly call refresh() to update the actual terminal display efficiently.

#include <ncurses.h>

int main()
{
    initscr();          /* Start curses mode          */
    printw("Hello World !!!");  /* Print Hello World          */
    refresh();          /* Print it on to the real screen */
    getch();            /* Wait for user input */
    endwin();           /* End curses mode        */

    return 0;
}
// Basic Ncurses "Hello World"

Ncurses provides components like menus and forms, analogous to HTML elements. However, using them involves significant setup. Rendering a simple menu requires substantial code, and making it interactive demands careful handling of user input and state updates within an event loop.

cJSON: Handling JSON Data

Given that the application interacts with the Todoist API, which uses JSON, a library for parsing and creating JSON objects is essential. cJSON, though technically a single-header file library, comprises thousands of lines of code and plays a crucial role. It provides functions to create JSON objects, arrays, strings, numbers, and to parse JSON text into these structures, albeit with the typical C requirement of careful error checking and memory management.

    // Example: Creating a simple JSON object with cJSON
    cJSON *monitor = cJSON_CreateObject();
    if (monitor == NULL) { /* Handle error */ }

    cJSON *name = cJSON_CreateString("Awesome 4K");
    if (name == NULL) { /* Handle error */ }
    cJSON_AddItemToObject(monitor, "name", name);
    // ... additional error checks and memory management needed ...

Libcurl: Making HTTP Requests

To communicate with the Todoist API, Libcurl is the go-to library in C for handling HTTP requests. Like Ncurses, it’s powerful and flexible but requires explicit configuration for almost everything. Setting up headers, specifying the request method, handling response data – each step involves specific function calls.

// Example: Setting up HTTP headers with Libcurl
struct curl_slist *requestHeaders = NULL;
requestHeaders = curl_slist_append(requestHeaders, "Authorization: Bearer YOUR_API_TOKEN");
requestHeaders = curl_slist_append(requestHeaders, "Content-Type: application/json");
requestHeaders = curl_slist_append(requestHeaders, "Accept: application/json");
// ... then associate these headers with a curl handle ...

A key aspect of using Libcurl efficiently is reusing the CURL easy handle for multiple requests rather than creating a new one each time. This necessitates carefully setting and clearing options for each request to avoid unintended carry-over effects.

Documentation: A Different Landscape

Developers accustomed to modern, interactive documentation sites (like those for React or Django) will find the documentation for older C libraries like Ncurses and Libcurl quite different. Often consisting of extensive man pages or text-heavy websites, the information is dense and requires careful reading. While comprehensive, it often lacks the interactive examples and guided tutorials common in newer ecosystems. Finding specific information can sometimes feel like searching through reference manuals rather than learning guides.

Understanding the Todoist Model

Before diving into the implementation, it’s helpful to understand the basic structure of Todoist:

Projects: Containers for tasks.
Tasks (Todos/Items): Actionable items that can belong to a project.
Due Dates: Tasks can have deadlines.
Today View: A special view showing all tasks due today, regardless of their project.

Implementation Walkthrough

Disclaimer: The code presented is illustrative and reflects a learning process. It may contain inefficiencies or potential memory issues. It’s intended to demonstrate concepts, not serve as production-ready code.

Initial Setup and Fetching Projects

The main() function initializes Ncurses (initscr(), raw(), noecho(), keypad()) and Libcurl (curl_easy_init()). Error checking is paramount after initialization.

int main(void) {
  // Ncurses initialization
  initscr();
  raw();
  noecho();
  keypad(stdscr, TRUE);
  printw("Loading...\n");
  refresh();

  // Libcurl initialization
  CURL *curl = curl_easy_init();
  curl_global_init(CURL_GLOBAL_DEFAULT);

  if (!curl) {
    // Handle initialization error
    endwin();
    return 1;
  }

  // ... rest of the program ...

  curl_easy_cleanup(curl);
  curl_global_cleanup();
  endwin();
  return 0;
}

Authentication involves retrieving the Todoist API token (e.g., from an environment variable) and constructing the Authorization: Bearer header using curl_slist_append.

The URL for fetching projects is constructed, and a helper function, makeRequest, encapsulates the logic for setting Libcurl options (CURLOPT_URL, CURLOPT_HTTPHEADER, CURLOPT_WRITEFUNCTION, CURLOPT_CUSTOMREQUEST, etc.), performing the request (curl_easy_perform), and parsing the JSON response using cJSON.

Crucially, the “Today” view, not being a standard project in the API, is added manually as a cJSON object to the list of projects fetched from the API before displaying the menu.

Displaying Projects and the Main Loop

A function like renderMenuFromJson takes the cJSON array of projects, extracts the “name” field from each, and creates an array of Ncurses ITEM pointers. This array is used to create a MENU.

// Simplified concept of renderMenuFromJson
MENU *renderMenuFromJson(cJSON *jsonArray, const char *displayField) {
    int count = cJSON_GetArraySize(jsonArray);
    ITEM **items = (ITEM **)malloc((count + 1) * sizeof(ITEM *));
    // ... check malloc result ...

    for (int i = 0; i < count; ++i) {
        cJSON *itemJson = cJSON_GetArrayItem(jsonArray, i);
        cJSON *displayValue = cJSON_GetObjectItem(itemJson, displayField);
        // ... check JSON access results ...
        items[i] = new_item(displayValue->valuestring, /* description */ "");
        // ... associate metadata (like ID) using set_item_userptr ...
    }
    items[count] = NULL; // Null-terminate the item array

    MENU *menu = new_menu(items);
    // ... configure menu options ...
    return menu;
}

After creating the menu, it’s posted (post_menu()) and the screen is refreshed (refresh()) to make it visible.

An event loop using getch() waits for user input. Keys like ‘j’ and ‘k’ drive the menu navigation (menu_driver with REQ_DOWN_ITEM / REQ_UP_ITEM), ‘q’ quits the program (triggering cleanup), and ‘l’ (or Enter) selects a project.

Viewing and Managing Tasks

When a project (or the “Today” view) is selected, a new function, projectPanel, is called. This function often creates a new Ncurses WINDOW and PANEL to overlay the project list, effectively creating a new screen for tasks.

Inside projectPanel, another API call is made using makeRequest to fetch tasks associated with the selected project ID (or tasks due today). The resulting JSON array of tasks is processed similarly using renderMenuFromJson to display the task list.

A critical technique here involves storing essential task metadata (like the task ID, content, priority) with each Ncurses menu item. Since new_item only accepts a name and description, the set_item_userptr() function is used to attach a pointer to a custom struct containing this metadata. This metadata is retrieved later using item_userptr() when performing actions on a task. Remember to free() this metadata during cleanup.

The projectPanel has its own event loop:
* ‘j’/’k’: Navigate tasks.
* ‘h’/’q’: Go back to the project list (break the loop, cleanup panel resources).
* ‘p’: Close (complete) the selected task.
* ‘o’: Reopen the selected task.
* ‘d’: Delete the selected task (usually with confirmation).
* ‘a’: Add a new task.

Actions like closing, reopening, or deleting involve:
1. Getting the current ITEM.
2. Retrieving the associated metadata (task ID) using item_userptr().
3. Constructing the appropriate API endpoint URL and potentially a JSON payload.
4. Calling makeRequest with the correct HTTP method (POST for close/reopen, DELETE for delete).
5. Refreshing the task list if the action was successful (either by re-fetching or manually removing/updating the item in the Ncurses menu).

Creating a New Task: Input Handling

Creating a task requires getting input from the user. Ncurses provides FORM and FIELD functionalities for this. A helper function displayInputField can be created:
1. Creates a FIELD for text input.
2. Creates a FORM containing that field.
3. Posts the form and refreshes the screen.
4. Enters a loop using getch() to capture keystrokes.
5. Uses form_driver() to handle input (e.g., REQ_NEXT_CHAR, REQ_DEL_PREV). Basic characters are added, backspace removes characters.
6. The loop exits when Enter is pressed.
7. The content of the field buffer (field_buffer()) is retrieved and returned.

Once the task content is obtained, an API request (POST to the tasks endpoint) is made using makeRequest. The most complex part here is updating the UI without necessarily re-fetching the entire list. This might involve:
1. Getting the current items from the menu.
2. Calculating the new size needed.
3. Using realloc (or malloc for a new array and copying) to create a larger ITEM array.
4. Creating a new_item for the newly added task.
5. Retrieving the ID and other details from the API response for the new task.
6. Creating and attaching the metadata struct using set_item_userptr() to the new item.
7. Adding the NULL terminator to the item array.
8. Updating the menu using set_menu_items() and re-posting the menu.

UI Limitations: Styling and Color

While Ncurses provides basic attributes (like underline, bold), fine-grained color control, especially on a per-item basis within standard menus, can be cumbersome or limited compared to modern UI toolkits. Achieving complex visual styling often requires significant effort or custom drawing routines. For simplicity, this project often relies on the terminal emulator’s theme for visual appearance.

Conclusion: A Learning Journey

Building this Todoist-like TUI in C was primarily an exercise in understanding low-level interface programming, API interaction, and manual resource management. It’s a proof of concept rather than a polished, production-ready application. Many excellent TUI alternatives for Todoist exist, often built with more modern and developer-friendly tools in languages like Rust or Python. This project served its purpose as a deep dive into the intricacies of C, Ncurses, and Libcurl, reinforcing the appreciation for the abstractions provided by higher-level languages and frameworks while proving the feasibility of tackling complex tasks even with fundamental tools.

Navigating the complexities of C development, API integration, and custom interface design, as explored in building this terminal application, highlights the need for deep technical expertise. At Innovative Software Technology, we specialize in tackling such challenges. Whether you require robust system-level programming in C/C++, seamless API integrations for your business processes, performance optimization for critical applications, or bespoke software solutions including terminal applications tailored to unique requirements, our experienced team delivers high-performance, reliable results. Leverage our expertise at Innovative Software Technology to transform your complex technical visions into robust, efficient software reality.