File Modes

When you open a file in C using `fopen()`, the 'mode' string you provide is a critical parameter that dictates how your program can interact with the file. Choosing the correct mode is essential for preventing accidental data loss and ensuring your program behaves as expected. The basic modes are: `"r"` (read): Opens an existing file for reading. The file pointer is positioned at the beginning of the file. If the file does not exist, `fopen` returns `NULL`. `"w"` (write): Opens a file for writing. If the file does not exist, it is created. If the file already exists, its contents are truncated (erased) before writing. The file pointer starts at the beginning. `"a"` (append): Opens a file for appending. Data is written to the end of the file. If the file does not exist, it is created. The original contents are preserved. In addition to these, there are 'update' modes that allow both reading and writing: `"r+"`: Opens an existing file for both reading and writing, starting at the beginning. `"w+"`: Opens a file for both reading and writing. It creates the file if it doesn't exist and truncates it if it does. `"a+"`: Opens a file for reading and appending. It creates the file if it doesn't exist. Reading starts from the beginning, but writing (appending) always happens at the end. For each of these modes, you can also add a `b` character (e.g., `"rb"`, `"wb"`) to specify that you are working with a **binary file**. On some systems like Windows, this is important as it prevents the translation of newline characters (`\n`) to carriage return/line feed combinations (`\r\n`). On UNIX-like systems, there is no difference between text and binary modes.