📂 Go – Read/Write File Without Truncation (os.OpenFile Usage Guide)

🧲 Introduction – Why Read/Write Without Truncating?

By default, opening a file in write mode may truncate (clear) it. To preserve existing content while allowing read/write access, you need to explicitly configure file access using os.OpenFile with the correct flags.

🎯 In this guide, you’ll learn:

• How to open a file for reading and writing without truncation
• Use os.OpenFile with proper mode and flags
• Append vs overwrite vs preserve contents
• Best practices for safe read/write operations

---

✅ Example – Read and Write Without Truncating

package main

import (
    "fmt"
    "os"
)

func main() {
    file, err := os.OpenFile("example.txt", os.O_RDWR, 0644)
    if err != nil {
        fmt.Println("Error opening file:", err)
        return
    }
    defer file.Close()

    // Read from file
    buf := make([]byte, 100)
    n, _ := file.Read(buf)
    fmt.Println("Original content:")
    fmt.Println(string(buf[:n]))

    // Move cursor to end of file and write
    file.WriteString("\nAppended line without truncating.")
}

✅ This opens example.txt for both reading and writing while preserving its content.

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🔑 os.OpenFile Flags Breakdown

Flag	Description
os.O_RDONLY	Open for read-only
os.O_WRONLY	Open for write-only
os.O_RDWR	Open for read & write
os.O_APPEND	Always write to the end (append mode)
os.O_CREATE	Create the file if it doesn’t exist
os.O_TRUNC	Truncate file when opening (⚠️ avoid)

✅ To preserve contents, do not use os.O_TRUNC.

---

🔁 Append Without Overwriting

file, err := os.OpenFile("example.txt", os.O_APPEND|os.O_CREATE|os.O_WRONLY, 0644)
file.WriteString("Appended content\n")

✅ os.O_APPEND ensures data is added at the end, preserving all existing content.

---

⚙️ Read, Write & Create If Not Exists (Safe Mode)

file, err := os.OpenFile("data.txt", os.O_RDWR|os.O_CREATE, 0644)

✅ This:

• Reads and writes
• Creates the file if it doesn’t exist
• Does not truncate the file

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🧠 Best Practices

Tip	Why It Helps
✅ Use `os.O_RDWR	os.O_CREATE`
❌ Avoid os.O_TRUNC if preserving	Prevents accidental data loss
✅ Always defer file.Close()	Ensures file is properly closed
✅ Check cursor with Seek()	To control where writing happens

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📌 Summary – Recap & Next Steps

Go allows you to safely read and write files without losing data—simply configure the right OpenFile flags. This is crucial for logs, caches, and state files where content must persist between operations.

🔍 Key Takeaways:

• Use os.O_RDWR or os.O_APPEND for writing without truncating
• Avoid os.O_TRUNC unless you intend to clear the file
• Combine os.O_CREATE to create the file if missing
• Use Seek() to manage write position when needed

⚙️ Next: Explore Go File Copying, Truncation Safety, or Buffered Writing with bufio.

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❓ FAQs – Go Read/Write Without Truncating

❓ How do I prevent Go from truncating my file?
✅ Avoid using the os.O_TRUNC flag with os.OpenFile.

❓ Can I read and write to the same file?
✅ Yes, use os.O_RDWR when opening the file.

❓ How do I append data instead of overwriting?
✅ Use os.O_APPEND when opening the file.

❓ What if the file doesn’t exist?
✅ Add os.O_CREATE to your flag set to create it if missing.

❓ How can I overwrite part of a file without truncating it all?
✅ Use file.Seek(offset, whence) to reposition the write cursor manually.

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