# Python Fcntl Module

The `fcntl` module performs file control and I/O control on file descriptors. It is an interface to the `fcntl()` and `ioctl()` Unix routines.

*Note: This module is only available on Unix.*

## Importing the Module

```python
import fcntl
```

## File Locking

One of the most common uses of `fcntl` is for file locking using `flock()`. This allows you to prevent multiple processes from accessing a file simultaneously.

### Operations

*   `fcntl.LOCK_SH`: Acquire a shared lock.
*   `fcntl.LOCK_EX`: Acquire an exclusive lock.
*   `fcntl.LOCK_UN`: Unlock.
*   `fcntl.LOCK_NB`: Bitwise OR with any of the above to avoid blocking if the lock cannot be acquired immediately.

### Example

```python
import fcntl
import time

with open('example.txt', 'w') as f:
    try:
        # Acquire an exclusive lock
        fcntl.flock(f, fcntl.LOCK_EX | fcntl.LOCK_NB)
        print("Lock acquired")
        
        f.write("Writing to file...")
        time.sleep(5) # Simulate work
        
    except BlockingIOError:
        print("File is locked by another process")
    finally:
        # Unlock
        fcntl.flock(f, fcntl.LOCK_UN)
        print("Lock released")
```

## File Control

The `fcntl()` function performs the operation `cmd` on file descriptor `fd`.

```python
import fcntl
import os

fd = os.open('example.txt', os.O_RDWR | os.O_CREAT)

# Get flags
flags = fcntl.fcntl(fd, fcntl.F_GETFL)
print(f"Flags: {flags}")

# Set flags (e.g., add O_APPEND)
fcntl.fcntl(fd, fcntl.F_SETFL, flags | os.O_APPEND)

os.close(fd)
```

[[programming/python/python]]