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OpenFile currently gives an error if O_CREATE is passed and the file
already exists. This is not how O_CREATE is meant to work. O_CREATE
requests the file to be created if it does not exist. However, if O_EXCL
is also set, then the file must not exist.

Implement and test this behavior.

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vfs for golang Build Status GoDoc Coverage Status Join the chat at

vfs is library to support virtual filesystems. It provides basic abstractions of filesystems and implementations, like OS accessing the file system of the underlying OS and memfs a full filesystem in-memory.


$ go get

Note: Always vendor your dependencies or fix on a specific version tag.

// Create a vfs accessing the filesystem of the underlying OS
var osfs vfs.Filesystem = vfs.OS()
osfs.Mkdir("/tmp", 0777)

// Make the filesystem read-only:
osfs = vfs.ReadOnly(osfs) // Simply wrap filesystems to change its behaviour

// os.O_CREATE will fail and return vfs.ErrReadOnly
// os.O_RDWR is supported but Write(..) on the file is disabled
f, _ := osfs.OpenFile("/tmp/example.txt", os.O_RDWR, 0)

// Return vfs.ErrReadOnly
_, err := f.Write([]byte("Write on readonly fs?"))
if err != nil {
    fmt.Errorf("Filesystem is read only!\n")

// Create a fully writable filesystem in memory
mfs := memfs.Create()
mfs.Mkdir("/root", 0777)

// Create a vfs supporting mounts
// The root fs is accessing the filesystem of the underlying OS
fs := mountfs.Create(osfs)

// Mount a memfs inside /memfs
// /memfs may not exist
fs.Mount(mfs, "/memfs")

// This will create /testdir inside the memfs
fs.Mkdir("/memfs/testdir", 0777)

// This would create /tmp/testdir inside your OS fs
// But the rootfs `osfs` is read-only
fs.Mkdir("/tmp/testdir", 0777)

Check detailed examples below. Also check the GoDocs.

Why should I use this lib?

  • Only Stdlib
  • (Nearly) Fully tested (Coverage >90%)
  • Easy to create your own filesystem
  • Mock a full filesystem for testing (or use included memfs)
  • Compose/Wrap Filesystems ReadOnly(OS()) and write simple Wrappers
  • Many features, see GoDocs and examples below

Features and Examples

Current state: ALPHA

While the functionality is quite stable and heavily tested, interfaces are subject to change.

You need more/less abstraction? Let me know by creating a Issue, thank you.


I simply couldn't find any lib supporting this wide range of variation and adaptability.


Feel free to make a pull request. For bigger changes create a issue first to discuss about it.


See LICENSE file.