File System API

File operations in Emscripten are provided by the FS library. It is used internally for all of Emscripten’s libc and libcxx file I/O.

Note

The API is inspired by the Linux/POSIX File System API, with each presenting a very similar interface.

The underlying behaviour is also similar, except where differences between the native and browser environments make this unreasonable. For example, user and group permissions are defined but ignored in FS.open().

Emscripten predominantly compiles code that uses synchronous file I/O, so the majority of the FS member functions offer a synchronous interface (with errors being reported by raising exceptions of type FS.ErrnoError).

File data in Emscripten is partitioned by mounted file systems. Several file systems are provided. An instance of MEMFS is mounted to / by default. Instances of NODEFS and IDBFS can be mounted to other directories if your application needs to persist data.

The automatic tests in tests/test_core.py (search for test_files) contain many examples of how to use this API. The tutorial also shows how to pre-load a file so that it can be read from compiled C/C++.

A high level overview of the way File Systems work in Emscripten-ported code is provided in the File System Overview.

Including File System Support

Emscripten decides whether to include file system support automatically. Many programs don’t need files, and file system support is not negligible in size, so Emscripten avoids including it when it doesn’t see a reason to. That means that if your C/C++ code does not access files, then the FS object and other file system APIs will not be included in the output. And, on the other hand, if your C/C++ code does use files, then file system support will be automatically included. So normally things will “just work” and you don’t need to think about this at all.

However, if your C/C++ code doesn’t use files, but you want to use them from JavaScript, then you can build with -s FORCE_FILESYSTEM=1, which will make the compiler include file system support even though it doesn’t see it being used.

Persistent data

Applications compiled with Emscripten usually expect synchronous I/O, so Emscripten itself provides file systems with completely synchronous interfaces.

However, due to JavaScript’s event-driven nature, most persistent storage options offer only asynchronous interfaces. Emscripten offers multiple file systems that can be mounted with FS.mount() to help deal with persistence depending on the execution context.

File systems

MEMFS

This is the default file system mounted at / when the runtime is initialized. All files exist strictly in-memory, and any data written to them is lost when the page is reloaded.

NODEFS

Note

This file system is only for use when running inside node.js.

This file system lets a program in node map directories (via a mount operation) on the host filesystem to directories in Emscripten’s virtual filesystem. It uses node’s synchronous FS API to immediately persist any data written to the Emscripten file system to your local disk.

See this test for an example.

IDBFS

Note

This file system is only for use when running code inside a browser.

The IDBFS file system implements the FS.syncfs() interface, which when called will persist any operations to an IndexedDB instance.

This is provided to overcome the limitation that browsers do not offer synchronous APIs for persistent storage, and so (by default) all writes exist only temporarily in-memory.

WORKERFS

Note

This file system is only for use when running code inside a worker.

This file system provides read-only access to File and Blob objects inside a worker without copying the entire data into memory and can potentially be used for huge files.

Devices

Emscripten supports registering arbitrary device drivers composed of a device id and a set of device-specific stream callbacks. Once a driver has been registered with FS.registerDevice(), a device node can be created to reference it (using FS.mkdev()).

The device node acts as an interface between the device and the file system. Any stream referencing the new node will inherit the stream callbacks registered for the device, making all of the high-level FS operations transparently interact with the device.

Note

Every device is different and unique. While common file operations like open, close, read, and write are typically supported (and inherited by file streams to provide a layer of abstraction for the equivalent libc functions to call), each device should implement whatever callbacks it needs based on its unique characteristics.

FS.makedev(ma, mi)

Converts a major and minor number into a single unique integer. This is used as an id to represent the device.

Arguments:
  • ma – Major number.
  • mi – Minor number.
FS.registerDevice(dev, ops)

Registers the specified device driver with a set of callbacks.

Arguments:
  • dev – The specific device driver id, created using makedev().
  • ops (object) – The set of callbacks required by the device. For an example, see the NODEFS default callbacks.

Setting up standard I/O devices

Emscripten standard I/O works by going though the virtual /dev/stdin, /dev/stdout and /dev/stderr devices. You can set them up using your own I/O functions by calling FS.init().

By default:

  • stdin will read from the terminal in command line engines and use window.prompt() in browsers (in both cases, with line buffering).
  • stdout will use a print function if one such is defined, printing to the terminal in command line engines and to the browser console in browsers that have a console (again, line-buffered).
  • stderr will use the same output function as stdout.

Note

All the configuration should be done before the main run() method is executed, typically by implementing Module.preRun. See Interacting with code for more information.

FS.init(input, output, error)

Sets up standard I/O devices for stdin, stdout, and stderr.

The devices are set up using the following (optional) callbacks. If any of the callbacks throw an exception, it will be caught and handled as if the device malfunctioned.

Arguments:
  • input – Input callback. This will be called with no parameters whenever the program attempts to read from stdin. It should return an ASCII character code when data is available, or null when it isn’t.
  • output – Output callback. This will be called with an ASCII character code whenever the program writes to stdout. It may also be called with null to flush the output.
  • error – Error callback. This is similar to output, except it is called when data is written to stderr.

File system API

FS.mount(type, opts, mountpoint)

Mounts the FS object specified by type to the directory specified by mountpoint. The opts object is specific to each file system type.

Arguments:
  • type – The file system type: MEMFS, NODEFS, IDBFS or WORKERFS.
  • opts (object) –

    A generic settings object used by the underlying file system.

    NODFES uses the root parameter to map the Emscripten directory to the physical directory. For example, to mount the current folder as a NODEFS instance:

    FS.mkdir('/working');
    FS.mount(NODEFS, { root: '.' }, '/working');
    

WORKERFS accepts files and blobs parameters to map a provided flat list of files into the mountpoint directory:

var blob = new Blob(['blob data']);
FS.mkdir('/working');
FS.mount(WORKERFS, {
  blobs: [{ name: 'blob.txt', data: blob }],
  files: files, // Array of File objects or FileList
}, '/working');

You can also pass in a package of files, created by tools/file_packager.py with --separate-metadata. You must provide the metadata as a JSON object, and the data as a blob:

// load metadata and blob using XMLHttpRequests, or IndexedDB, or from someplace else
FS.mkdir('/working');
FS.mount(WORKERFS, {
  packages: [{ metadata: meta, blob: blob }]
}, '/working');
Arguments:
  • mountpoint (string) – A path to an existing local Emscripten directory where the file system is to be mounted. It can be either an absolute path, or something relative to the current directory.
FS.unmount(mountpoint)

Unmounts the specified mountpoint.

Arguments:
  • mountpoint (string) – The directory to unmount.
FS.syncfs(populate, callback)

Responsible for iterating and synchronizing all mounted file systems in an asynchronous fashion.

Note

Currently, only the IDBFS file system implements the interfaces needed for synchronization. All other file systems are completely synchronous and don’t require synchronization.

The populate flag is used to control the intended direction of the underlying synchronization between Emscripten`s internal data, and the file system’s persistent data.

For example:

function myAppStartup(callback) {
  FS.mkdir('/data');
  FS.mount(IDBFS, {}, '/data');

  FS.syncfs(true, function (err) {
        // handle callback
  });
}

function myAppShutdown(callback) {
  FS.syncfs(function (err) {
        // handle callback
  });
}

A real example of this functionality can be seen in test_idbfs_sync.c.

Arguments:
  • populate (bool) – true to initialize Emscripten’s file system data with the data from the file system’s persistent source, and false to save Emscripten`s file system data to the file system’s persistent source.
  • callback – A notification callback function that is invoked on completion of the synchronization. If an error occurred, it will be provided as a parameter to this function.
FS.mkdir(path, mode)

Creates a new directory node in the file system. For example:

FS.mkdir('/data');

Note

The underlying implementation does not support user or group permissions. The caller is always treated as the owner of the folder, and only permissions relevant to the owner apply.

Arguments:
FS.mkdev(path, mode, dev)

Creates a new device node in the file system referencing the registered device driver (FS.registerDevice()) for dev. For example:

var id = FS.makedev(64, 0);
FS.registerDevice(id, {});
FS.mkdev('/dummy', id);
Arguments:
  • path (string) – The path name for the new device node.
  • mode (int) –

    File permissions for the new node. The default setting (in octal numeric notation) is 0777.

  • dev (int) – The registered device driver.

Creates a symlink node at newpath linking to oldpath. For example:

FS.writeFile('file', 'foobar');
FS.symlink('file', 'link');
Arguments:
  • oldpath (string) – The path name of the file to link to.
  • newpath (string) – The path to the new symlink node, that points to oldpath.
FS.rename(oldpath, newpath)

Renames the node at oldpath to newpath. For example:

FS.writeFile('file', 'foobar');
FS.rename('file', 'newfile');
Arguments:
  • oldpath (string) – The old path name.
  • newpath (string) – The new path name
FS.rmdir(path)

Removes an empty directory located at path.

Example

FS.mkdir('data');
FS.rmdir('data');
Arguments:
  • path (string) – Path of the directory to be removed.

Unlinks the node at path.

This removes a name from the file system. If that name was the last link to a file (and no processes have the file open) the file is deleted.

For example:

FS.writeFile('/foobar.txt', 'Hello, world');
FS.unlink('/foobar.txt');
Arguments:
  • path (string) – Path of the target node.

Gets the string value stored in the symbolic link at path. For example:

#include <stdio.h>
#include <emscripten.h>

int main() {
  MAIN_THREAD_EM_ASM(
        FS.writeFile('file', 'foobar');
        FS.symlink('file', 'link');
        console.log(FS.readlink('link'));
  );
  return 0;
}

outputs

file
Arguments:
  • path (string) – Path to the target file.
Returns:

The string value stored in the symbolic link at path.

FS.stat(path)

Gets a JavaScript object containing statistics about the node at path. For example:

#include <stdio.h>
#include <emscripten.h>

int main() {
  MAIN_THREAD_EM_ASM(
        FS.writeFile('file', 'foobar');
        console.log(FS.stat('file'));
  );
  return 0;
}

outputs

{
  dev: 1,
  ino: 13,
  mode: 33206,
  nlink: 1,
  uid: 0,
  gid: 0,
  rdev: 0,
  size: 6,
  atime: Mon Nov 25 2013 00:37:27 GMT-0800 (PST),
  mtime: Mon Nov 25 2013 00:37:27 GMT-0800 (PST),
  ctime: Mon Nov 25 2013 00:37:27 GMT-0800 (PST),
  blksize: 4096,
  blocks: 1
}
Arguments:
  • path (string) – Path to the target file.
FS.lstat(path)

Identical to FS.stat(), However, if path is a symbolic link then the returned stats will be for the link itself, not the file that it links to.

Arguments:
  • path (string) – Path to the target file.
FS.chmod(path, mode)

Change the mode flags for path to mode.

Note

The underlying implementation does not support user or group permissions. The caller is always treated as the owner of the folder, and only permissions relevant to the owner apply.

For example:

FS.writeFile('forbidden', 'can\'t touch this');
FS.chmod('forbidden', 0000);
Arguments:
FS.lchmod(path, mode)

Identical to FS.chmod(). However, if path is a symbolic link then the mode will be set on the link itself, not the file that it links to.

Arguments:
FS.fchmod(fd, mode)

Identical to FS.chmod(). However, a raw file descriptor is supplied as fd.

Arguments:
FS.chown(path, uid, gid)

Change the ownership of the specified file to the given user or group id.

Note

This call exists to provide a more “complete” API mapping for ported code. Values set are effectively ignored.

Arguments:
  • path (string) – Path to the target file.
  • uid (int) – The id of the user to take ownership of the file.
  • gid (int) – The id of the group to take ownership of the file.
FS.lchown(path, uid, gid)

Identical to FS.chown(). However, if path is a symbolic link then the properties will be set on the link itself, not the file that it links to.

Note

This call exists to provide a more “complete” API mapping for ported code. Values set are effectively ignored.

Arguments:
  • path (string) – Path to the target file.
  • uid (int) – The id of the user to take ownership of the file.
  • gid (int) – The id of the group to take ownership of the file.
FS.fchown(fd, uid, gid)

Identical to FS.chown(). However, a raw file descriptor is supplied as fd.

Note

This call exists to provide a more “complete” API mapping for ported code. Values set are effectively ignored.

Arguments:
  • fd (int) – Descriptor of target file.
  • uid (int) – The id of the user to take ownership of the file.
  • gid (int) – The id of the group to take ownership of the file.
FS.truncate(path, len)

Truncates a file to the specified length. For example:

#include <stdio.h>
#include <emscripten.h>

int main() {
  MAIN_THREAD_EM_ASM(
        FS.writeFile('file', 'foobar');
        FS.truncate('file', 3);
        console.log(FS.readFile('file', { encoding: 'utf8' }));
  );
  return 0;
}

outputs

foo
Arguments:
  • path (string) – Path of the file to be truncated.
  • len (int) – The truncation length for the file.
FS.ftruncate(fd, len)

Truncates the file identified by the fd to the specified length (len).

Arguments:
  • fd (int) – Descriptor of file to be truncated.
  • len (int) – The truncation length for the file.
FS.utime(path, atime, mtime)

Change the timestamps of the file located at path. The times passed to the arguments are in milliseconds since January 1, 1970 (midnight UTC/GMT).

Note that in the current implementation the stored timestamp is a single value, the maximum of atime and mtime.

Arguments:
  • path (string) – The path of the file to update.
  • atime (int) – The file modify time (milliseconds).
  • mtime (int) – The file access time (milliseconds).
FS.open(path, flags[, mode])

Opens a file with the specified flags. flags can be:

  • r — Open file for reading.
  • r+ — Open file for reading and writing.
  • w — Open file for writing.
  • wx — Like w but fails if path exists.
  • w+ — Open file for reading and writing. The file is created if it does not exist or truncated if it exists.
  • wx+ — Like w+ but fails if path exists.
  • a — Open file for appending. The file is created if it does not exist.
  • ax — Like a but fails if path exists.
  • a+ — Open file for reading and appending. The file is created if it does not exist.
  • ax+ — Like a+ but fails if path exists.

Note

The underlying implementation does not support user or group permissions. The file permissions set in mode are only used if the file is created. The caller is always treated as the owner of the file, and only those permissions apply.

Arguments:
  • path (string) – The path of the file to open.
  • flags (string) – Read and write flags.
  • mode

    File permission flags for the file. The default setting (in octal numeric notation) is 0666.

Returns:

A stream object.

FS.close(stream)

Closes the file stream.

Arguments:
  • stream (object) – The stream to be closed.
FS.llseek(stream, offset, whence)

Repositions the offset of the stream offset bytes relative to the beginning, current position, or end of the file, depending on the whence parameter.

The _llseek() function repositions the offset of the open file associated with the file descriptor fd to (offset_high<<32) | offset_low bytes relative to the beginning of the file, the current position in the file, or the end of the file, depending on whether whence is SEEK_SET, SEEK_CUR, or SEEK_END, respectively. It returns the resulting file position in the argument result.

Arguments:
  • stream (object) – The stream for which the offset is to be repositioned.
  • offset (int) – The offset (in bytes) relative to whence.
  • whence (int) – Point in file (beginning, current point, end) from which to calculate the offset: SEEK_SET (0), SEEK_CUR (1) or SEEK_END (2)
FS.read(stream, buffer, offset, length[, position])

Read length bytes from the stream, storing them into buffer starting at offset.

By default, reading starts from the stream’s current offset, however, a specific offset can be specified with the position argument. For example:

var stream = FS.open('abinaryfile', 'r');
var buf = new Uint8Array(4);
FS.read(stream, buf, 0, 4, 0);
FS.close(stream);
Arguments:
  • stream (object) – The stream to read from.
  • buffer (ArrayBufferView) – The buffer to store the read data.
  • offset (int) – The offset within buffer to store the data.
  • length (int) – The length of data to write in buffer.
  • position (int) – The offset within the stream to read. By default this is the stream’s current offset.
FS.write(stream, buffer, offset, length[, position])

Writes length bytes from buffer, starting at offset.

By default, writing starts from the stream’s current offset, however, a specific offset can be specified with the position argument. For example:

var data = new Uint8Array(32);
var stream = FS.open('dummy', 'w+');
FS.write(stream, data, 0, data.length, 0);
FS.close(stream);
Arguments:
  • stream (object) – The stream to write to.
  • buffer (ArrayBufferView) – The buffer to write.
  • offset (int) – The offset within buffer to write.
  • length (int) – The length of data to write.
  • position (int) – The offset within the stream to write. By default this is the stream’s current offset.
FS.readFile(path, opts)

Reads the entire file at path and returns it as a string (encoding is utf8), or as a new Uint8Array buffer (encoding is binary).

Arguments:
  • path (string) – The file to read.
  • opts (object) –
    • encoding (string)
      Defines the encoding used to return the file contents: binary | utf8 . The default is binary
    • flags (string)
      Read flags, as defined in FS.open(). The default is ‘r’.
Returns:

The file as a string or Uint8Array buffer, depending on the encoding.

FS.writeFile(path, data, opts)

Writes the entire contents of data to the file at path.

The value of opts determines whether data is treated either as a string (encoding = utf8), or as an ArrayBufferView (encoding = binary). For example:

FS.writeFile('file', 'foobar');
var contents = FS.readFile('file', { encoding: ``utf8`` });
Arguments:
  • path (string) – The file to which to write data.
  • data (ArrayBufferView) – The data to write.
  • opts (object) –
    • encoding (string)
      binary | utf8. The default is utf8
    • flags (string)
      Write flags, as defined in FS.open(). The default is ‘w’.
FS.createLazyFile(parent, name, url, canRead, canWrite)

Creates a file that will be loaded lazily on first access from a given URL or local file system path, and returns a reference to it.

Warning

Firefox and Chrome have recently disabled synchronous binary XHRs, which means this cannot work for JavaScript in regular HTML pages (but it works within Web Workers).

Example

FS.createLazyFile('/', 'foo', 'other/page.htm', true, false);
FS.createLazyFile('/', 'bar', '/get_file.php?name=baz', true, true);
Arguments:
  • parent (string/object) – The parent folder, either as a path (e.g. ‘/usr/lib’) or an object previously returned from a FS.createFolder() or FS.createPath() call.
  • name (string) – The name of the new file.
  • url (string) – In the browser, this is the URL whose contents will be returned when this file is accessed. In a command line engine like node.js, this will be the local (real) file system path from where the contents will be loaded. Note that writes to this file are virtual.
  • canRead (bool) – Whether the file should have read permissions set from the program’s point of view.
  • canWrite (bool) – Whether the file should have write permissions set from the program’s point of view.
Returns:

A reference to the new file.

FS.createPreloadedFile(parent, name, url, canRead, canWrite)

Preloads a file asynchronously, and uses preload plugins to prepare its content. You should call this in preRun, run() will be delayed until all preloaded files are ready. This is how the preload-file option works in emcc when --use-preload-plugins has been specified (if you use this method by itself, you will need to build the program with that option).

Arguments:
  • parent (string/object) – The parent folder, either as a path (e.g. ‘/usr/lib’) or an object previously returned from a FS.createFolder() or FS.createPath() call.
  • name (string) – The name of the new file.
  • url (string) – In the browser, this is the URL whose contents will be returned when the file is accessed. In a command line engine, this will be the local (real) file system path the contents will be loaded from. Note that writes to this file are virtual.
  • canRead (bool) – Whether the file should have read permissions set from the program’s point of view.
  • canWrite (bool) – Whether the file should have write permissions set from the program’s point of view.

File types

Emscripten’s file system supports regular files, directories, symlinks, character devices, block devices and sockets. Similarly to most Unix systems, all of these file types can be operated on using the higher-level FS operations like FS.read() and FS.write().

FS.isFile(mode)

Tests if the mode bitmask represents a file.

Arguments:
  • mode – A bitmask of possible file properties.
Returns:

true if the mode bitmask represents a file.

Return type:

bool

FS.isDir(mode)

Tests if the mode bitmask represents a directory.

Returns:true if the mode bitmask represents a directory.
Return type:bool

Tests if the mode bitmask represents a symlink.

Arguments:
  • mode – A bitmask of possible file properties.
Returns:

true if the mode bitmask represents a symlink.

Return type:

bool

FS.isChrdev(mode)

Tests if the mode bitmask represents a character device.

Arguments:
  • mode – A bitmask of possible file properties.
Returns:

true if the mode bitmask represents a character device.

Return type:

bool

FS.isBlkdev(mode)

Tests if the mode bitmask represents a block device.

Arguments:
  • mode – A bitmask of possible file properties.
Returns:

true if the mode bitmask represents a block device.

Return type:

bool

FS.isSocket(mode)

Tests if the mode bitmask represents a socket.

Arguments:
  • mode – A bitmask of possible file properties.
Returns:

true if the mode bitmask represents a socket.

Return type:

bool

Paths

FS.cwd()

Gets the current working directory.

Returns:The current working directory.
FS.lookupPath(path, opts)

Looks up the incoming path and returns an object containing both the resolved path and node.

The options (opts) allow you to specify whether the object, its parent component, a symlink, or the item the symlink points to are returned. For example:

var lookup = FS.lookupPath(path, { parent: true });
Arguments:
  • path (string) – The incoming path.
  • opts (object) –

    Options for the path:

    • parent (bool)
      If true, stop resolving the path once the penultimate component is reached. For example, the path /foo/bar with { parent: true } would return an object representing /foo. The default is false.
    • follow (bool)
      If true, follow the last component if it is a symlink. For example, consider a symlink /foo/symlink that links to /foo/notes.txt. If { follow: true }, an object representing /foo/notes.txt would be returned. If { follow: false }, an object representing the symlink file would be returned. The default is false.
Returns:

an object with the format:

{
  path: resolved_path,
  node: resolved_node
}

FS.getPath(node)

Gets the absolute path to node, accounting for mounts.

Arguments:
  • node – The current node.
Returns:

The absolute path to node.