The open statement establishes a connection between the program and a file on the storage device. It specifies the file name and the mode in which the file is to be accessed (e.g., read, write, append). The read statement retrieves data from an opened file and stores it in a variable or data structure. The write statement stores data from a variable or data structure into an opened file. Finally, the close statement releases the resources associated with the opened file, ensuring that any buffered data is written to the disk and the file handle is freed.

It is crucial to use the close statement because:

• Data Integrity: Ensures that all data written to the file is actually saved to the storage device. Without closing, data might remain in a buffer and be lost if the program terminates unexpectedly.
• Resource Management: Releases system resources (file handles) that are tied to the opened file. Failing to close files can lead to resource exhaustion and program instability.
• Preventing Corruption: Properly closing files can help prevent data corruption, especially when dealing with binary files.

A program accesses data stored in an external file through a series of steps involving the operating system (OS). Here's a breakdown:

1. File Path Specification: The program specifies the location of the file using a file path. This path can be either:

   • Absolute Path: A complete path that specifies the exact location of the file, starting from the root directory (e.g., C:\Users\Username\Documents\data.txt on Windows, or /home/username/documents/data.txt on Linux/macOS).
   • Relative Path: A path specified relative to the program's current working directory (e.g., data.txt if the file is in the same directory as the program, or data/info.txt if the file is in a subdirectory named "info").

2. Operating System Interaction: The program uses system calls (provided by the OS) to request access to the file. These calls include specifying the file path and the desired access mode (read, write, etc.).
3. File System Management: The OS's file system manages the storage and retrieval of files on the storage device. It translates the file path into a physical location on the disk.
4. File Opening: The OS opens the file, allocating resources (memory buffers, etc.) to allow the program to access the data.
5. Data Retrieval/Modification: The program then reads from or writes to the file using the specified access mode.
6. File Closing: Finally, the program closes the file, releasing the allocated resources. This is important to ensure data integrity and prevent resource leaks.

The OS plays a vital role in security and permissions. It checks if the program has the necessary permissions to access the file based on user accounts and file system settings. Without the correct permissions, the program will be denied access.

Write mode (often denoted as 'w') opens a file for writing. If the file already exists, its contents are overwritten. If the file does not exist, a new file is created.

Append mode (often denoted as 'a') opens a file for writing. New data is added to the end of the existing file content without overwriting it. If the file does not exist, a new file is created.

Example of 'w' mode: You would use 'w' mode when you want to replace the entire content of a file with new content. For instance, overwriting a configuration file with a new set of settings.

Example of 'a' mode: You would use 'a' mode when you want to add new data to the end of an existing file. For example, logging events to a log file, where each new event is appended to the end of the log.