fopencookie - Linux


fopencookie is a utility used to create and manipulate file-like objects using a custom function instead of the standard file system operations. It is useful for abstracting away the physical location and access methods of data.


fopencookie(cookie_func, close_func, read_func, seek_func, write_func)


  • cookie_func: A function that returns an opaque pointer to the data source.
  • close_func: A function that closes the data source.
  • read_func: A function that reads data from the data source.
  • seek_func: A function that seeks to a specified offset in the data source.
  • write_func: A function that writes data to the data source.




Creating a file-like object from a custom function:

# Function to read from a string
read_func = lambda cookie, buf, size: buf.write("Hello, world!")

# Create a file-like object using the custom function
fp = fopencookie(None, None, read_func, None, None)

# Read data from the file-like object  # Output: Hello, world!

Using a file-like object with other Linux commands:

# Create a file-like object from a custom function
# (As shown in the previous example)

# Redirect the output of a command to the file-like object
cat /proc/cpuinfo | tee >(fp)

# Send data from the file-like object to a command
fp | grep -i "model name"

Common Issues

  • Ensure custom functions meet the required interfaces: The provided functions must conform to the expected behavior of the corresponding file operations (e.g., read, seek).
  • Handle resource cleanup properly: Remember to close the file-like object to release any associated resources.
  • Potential thread safety issues: If multiple threads access the file-like object concurrently, consider implementing appropriate synchronization mechanisms.


fopencookie can be integrated with:

  • Custom data sources: Create file-like objects from any data source, such as databases, network protocols, or even other processes.
  • Virtual file systems: Implement custom file systems that use custom functions to interact with data.
  • Filters and transformations: Use file-like objects as intermediate streams to process data before writing it to a file or sending it to another command.

Related Commands

  • fdopen: Creates a file-like object from a file descriptor.
  • popen: Creates a pipe to and from a spawned process.
  • mmap: Maps a file to memory for efficient access.