How to Work with Memory-Mapped Files in Go

Go does not have a standard library function for memory-mapped files; you must use the syscall package or a third-party library like golang.org/x/exp/mmap. Use syscall.Mmap to map a file into memory, then access the returned byte slice directly.

import (
	"os"
	"syscall"
)

func mmapFile(path string) ([]byte, error) {
	f, err := os.Open(path)
	if err != nil {
		return nil, err
	}
	defer f.Close()

	info, err := f.Stat()
	if err != nil {
		return nil, err
	}

	data, err := syscall.Mmap(int(f.Fd()), 0, int(info.Size()), syscall.PROT_READ, syscall.MAP_PRIVATE)
	if err != nil {
		return nil, err
	}

	return data, nil
}

Remember to call syscall.Munmap(data) when you are done to free the memory.

Memory-mapped files let your program read a file directly from the disk into your computer's memory without copying data. This is like opening a book and reading it directly instead of photocopying every page first. You use this technique when you need to process very large files quickly without using up too much memory.