The Stream Abstraction

In C, we do not interact directly with files. Instead, we interact with Streams. A stream is a uniform interface for reading and writing data, regardless of whether that data is coming from a hard drive, a keyboard, or a network socket.

The Standard Library provides three streams by default:

• stdin: Standard Input (Keyboard)
• stdout: Standard Output (Console)
• stderr: Standard Error (Console, unbuffered)

Working with FILE *

To work with a custom file, we use a FILE pointer. This pointer manages a buffer and tracks the current position in the file.

1. Opening a File

FILE *fp = fopen("filename.txt", "mode");

Common Modes:

• "r": Read (fails if file doesn’t exist).
• "w": Write (overwrites existing file).
• "a": Append (writes to the end).
• "rb", "wb": Binary modes (prevents OS from altering line endings).

2. Error Handling

Opening a file can fail (missing file, permission denied). Always check if the pointer is NULL.

Reading and Writing

There are three ways to move data through a stream:

Position and Random Access

A file pointer maintains an internal “cursor.” You can move this cursor manually:

• fseek(fp, offset, origin): Move the cursor.
• ftell(fp): Get the current cursor position.
• rewind(fp): Jump back to the start.

char buffer[100];
FILE *fp = fopen("data.txt", "r");
if (fp != ) {
    fgets(buffer, 100, fp);
    fclose(fp);
}

Buffered I/O

For performance, C does not write every byte to disk immediately. It stores them in a Buffer and writes them in chunks.

• fflush(fp): Forces the buffer to be written to disk immediately.
• fclose(fp): Automatically flushes and closes the stream.

Binary vs. Text

In Text Mode, some systems automatically convert \n to \r\n and vice-versa. In Binary Mode ("rb", "wb"), the data is moved exactly as-is, which is crucial for images, executables, and custom data structures.