🔹 1. Sequential Access
📌 Definition:
Accesses data in a file in order, one record after another, from beginning to end.
🛠 How It Works:
- Reading starts at the beginning.
- Read or write moves the file pointer forward.
- No skipping or jumping between data blocks.
📖 Use Cases:
- Text editors
- Log files
- Audio/video streaming
✅ Advantages:
- Simple to implement.
- Efficient for processing large volumes of data.
❌ Disadvantages:
- Inefficient for searching or updating specific records.
🧪 Example:
while (fgets(buffer, sizeof(buffer), file) != NULL) {
printf("%s", buffer);
}
🔹 2. Direct Access (Random Access)
📌 Definition:
Also called relative access, this method allows jumping to any location in the file to read or write data.
🛠 How It Works:
- Uses byte offset or record number to access data.
- File is treated like an array of fixed-size blocks.
📖 Use Cases:
- Databases
- Compilers
- Large binary files (images, executable files)
✅ Advantages:
- Fast access to any part of the file.
- Ideal for read-heavy operations.
❌ Disadvantages:
- More complex to implement.
- Requires known structure (e.g., fixed record size).
🧪 Example:
fseek(file, 100, SEEK_SET); // Jump to byte 100
fread(buffer, sizeof(char), 20, file); // Read 20 bytes
🔹 3. Indexed Access
📌 Definition:
Accesses records using an index—a data structure that maps keys to file locations (like a book index).
🛠 How It Works:
- An index is built containing key and pointer pairs.
- The pointer tells where in the file the record is located.
📖 Use Cases:
- Large database systems
- Key-value storage systems (e.g., NoSQL)
- Library catalogues
✅ Advantages:
- Fast search and retrieval using keys.
- Supports both sequential and random access.
❌ Disadvantages:
- Index maintenance overhead.
- Extra storage required for the index.
🧪 Conceptual Example:
Index Table:
Key | File Offset
--------------------
1001 | 0
1002 | 128
1003 | 256
To retrieve key 1002 → Go directly to byte 128 in the file.
📊 Comparison Table:
| Access Method | Flexibility | Speed (Random) | Use Case | Complexity |
|---|---|---|---|---|
| Sequential | Low | Slow | Logs, media files | Low |
| Direct (Random) | High | Fast | Databases, executables | Medium |
| Indexed | Very High | Very Fast | Large search systems | High |
🔚 Conclusion:
- The Operating System may support one or more access methods depending on file type and application needs.
- Indexed access is the most powerful but also the most resource-intensive.
- Sequential access remains popular for simplicity and stream processing.