dart:io library Null safety

File, socket, HTTP, and other I/O support for non-web applications.

Important: Browser-based apps can't use this library. Only the following can import and use the dart:io library:

  • Servers
  • Command-line scripts
  • Flutter mobile apps
  • Flutter desktop apps

This library allows you to work with files, directories, sockets, processes, HTTP servers and clients, and more. Many operations related to input and output are asynchronous and are handled using Futures or Streams, both of which are defined in the dart:async library.

To use the dart:io library in your code:

import 'dart:io';

For an introduction to I/O in Dart, see the dart:io library tour.

An instance of File, Directory, or Link represents a file, directory, or link, respectively, in the native file system.

You can manipulate the file system through objects of these types. For example, you can rename a file or directory:

File myFile = new File('myFile.txt');
myFile.rename('yourFile.txt').then((_) => print('file renamed'));

Many methods provided by the File, Directory, and Link classes run asynchronously and return a Future.


File, Directory, and Link all extend FileSystemEntity. In addition to being the superclass for these classes, FileSystemEntity has a number of static methods for working with paths.

To get information about a path, you can use the FileSystemEntity static methods such as 'isDirectory', 'isFile', and 'exists'. Because file system access involves I/O, these methods are asynchronous and return a Future.

FileSystemEntity.isDirectory(myPath).then((isDir) {
  if (isDir) {
    print('$myPath is a directory');
  } else {
    print('$myPath is not a directory');

HttpServer and HttpClient

The classes HttpServer and HttpClient provide HTTP server and HTTP client functionality.

The HttpServer class provides the basic functionality for implementing an HTTP server. For some higher-level building-blocks, we recommend that you try the shelf pub package, which contains a set of high-level classes that, together with the HttpServer class in this library, make it easier to implement HTTP servers.


The Process class provides a way to run a process on the native machine. For example, the following code spawns a process that recursively lists the files under web.

Process.start('ls', ['-R', 'web']).then((process) {

Using start() returns a Future, which completes with a Process object when the process has started. This Process object allows you to interact with the process while it is running. Using run() returns a Future, which completes with a ProcessResult object when the spawned process has terminated. This ProcessResult object collects the output and exit code from the process.

When using start(), you need to read all data coming on the stdout and stderr streams otherwise the system resources will not be freed.


The WebSocket class provides support for the web socket protocol. This allows full-duplex communications between client and server applications.

A web socket server uses a normal HTTP server for accepting web socket connections. The initial handshake is a HTTP request which is then upgraded to a web socket connection. The server upgrades the request using WebSocketTransformer and listens for the data on the returned web socket. For example, here's a mini server that listens for 'ws' data on a WebSocket:

runZoned(() async {
  var server = await HttpServer.bind('', 4040);
  server.listen((HttpRequest req) async {
    if (req.uri.path == '/ws') {
      var socket = await WebSocketTransformer.upgrade(req);
}, onError: (e) => print("An error occurred."));

The client connects to the WebSocket using the connect() method and a URI that uses the Web Socket protocol. The client can write to the WebSocket with the add() method. For example,

var socket = await WebSocket.connect('ws://');
socket.add('Hello, World!');

Check out the websocket_sample app, which uses WebSockets to communicate with a server.

Socket and ServerSocket

Clients and servers use Sockets to communicate using the TCP protocol. Use ServerSocket on the server side and Socket on the client. The server creates a listening socket using the bind() method and then listens for incoming connections on the socket. For example:

ServerSocket.bind('', 4041)
  .then((serverSocket) {
    serverSocket.listen((socket) {

A client connects a Socket using the connect() method, which returns a Future. Using write(), writeln(), or writeAll() are the easiest ways to send data over the socket. For example:

Socket.connect('', 4041).then((socket) {
  socket.write('Hello, World!');

Besides Socket and ServerSocket, the RawSocket and RawServerSocket classes are available for lower-level access to async socket IO.

Standard output, error, and input streams

This library provides the standard output, error, and input streams, named 'stdout', 'stderr', and 'stdin', respectively.

The stdout and stderr streams are both IOSinks and have the same set of methods and properties.

To write a string to 'stdout':

stdout.writeln('Hello, World!');

To write a list of objects to 'stderr':

stderr.writeAll([ 'That ', 'is ', 'an ', 'error.', '\n']);

The standard input stream is a true Stream, so it inherits properties and methods from the Stream class.

To read text synchronously from the command line (the program blocks waiting for user to type information):

 String inputText = stdin.readLineSync();


Builds a list of bytes, allowing bytes and lists of bytes to be added at the end. [...]
Options controlling compression in a WebSocket. [...]
Returned by the startConnect methods on client-side socket types S, ConnectionTask<S> allows cancelling an attempt to connect to a host.
A MIME/IANA media type used as the value of the HttpHeaders.contentTypeHeader header. [...]
Representation of a cookie. For cookies received by the server as Cookie header values only name and value properties will be set. When building a cookie for the 'set-cookie' header in the server and when receiving cookies in the client as 'set-cookie' headers all fields can be used.
A data packet which is received by a RawDatagramSocket.
When detaching a socket from either the HttpServer or the HttpClient due to a HTTP connection upgrade there might be unparsed data already read from the socket. This unparsed data together with the detached socket is returned in an instance of this class.
A reference to a directory (or folder) on the file system. [...]
A reference to a file on the file system. [...]
Type of lock when requesting a lock on a file.
The modes in which a File can be opened.
A FileStat object represents the result of calling the POSIX stat() function on a file system object. It is an immutable object, representing the snapshotted values returned by the stat() call.
File system event for newly created file system objects.
File system event for deletion of file system objects.
The common super class for File, Directory, and Link objects. [...]
The type of an entity on the file system, such as a file, directory, or link. [...]
Base event class emitted by FileSystemEntity.watch.
File system event for modifications of file system objects.
File system event for moving of file system objects.
The GZipCodec encodes raw bytes to GZip compressed bytes and decodes GZip compressed bytes to raw bytes. [...]
Representation of a header value in the form: [...]
A client that receives content, such as web pages, from a server using the HTTP protocol. [...]
Represents credentials for basic authentication.
Represents credentials for digest authentication. Digest authentication is only supported for servers using the MD5 algorithm and quality of protection (qop) of either "none" or "auth".
HTTP request for a client connection. [...]
HTTP response for a client connection. [...]
Information about an HttpRequest, HttpResponse, HttpClientRequest, or HttpClientResponse connection.
Summary statistics about an HttpServers current socket connections.
Utility functions for working with dates with HTTP specific date formats.
Headers for HTTP requests and responses. [...]
This class facilitates overriding HttpClient with a mock implementation. It should be extended by another class in client code with overrides that construct a mock implementation. The implementation in this base class defaults to the actual HttpClient implementation. For example: [...]
A server-side object that contains the content of and information about an HTTP request. [...]
An HTTP response, which returns the headers and data from the server to the client in response to an HTTP request. [...]
A server that delivers content, such as web pages, using the HTTP protocol. [...]
HTTP status codes. Exported in dart:io and dart:html.
An internet address or a Unix domain address. [...]
InternetAddressType is the type an InternetAddress. Currently, IP version 4 (IPv4), IP version 6 (IPv6) and Unix domain address are supported. Unix domain sockets are available only on Linux, MacOS and Android.
This class facilitates overriding various APIs of dart:io with mock implementations. [...]
A combined byte and text output. [...]
Link objects are references to filesystem links.
A NetworkInterface represents an active network interface on the current system. It contains a list of InternetAddresses that are bound to the interface.
Information about the environment in which the current program is running. [...]
The means to execute a program. [...]
ProcessInfo provides methods for retrieving information about the current process.
ProcessResult represents the result of running a non-interactive process started with Process.run or Process.runSync.
On Posix systems, ProcessSignal is used to send a specific signal to a child process, see Process.kill. [...]
Modes for running a new process.
RandomAccessFile provides random access to the data in a file. [...]
A RawDatagramSocket is an unbuffered interface to a UDP socket. [...]
The RawSecureServerSocket is a server socket, providing a stream of low-level RawSecureSockets. [...]
RawSecureSocket provides a secure (SSL or TLS) network connection. Client connections to a server are provided by calling RawSecureSocket.connect. A secure server, created with RawSecureServerSocket, also returns RawSecureSocket objects representing the server end of a secure connection. The certificate provided by the server is checked using the trusted certificates set in the SecurityContext object. The default SecurityContext object contains a built-in set of trusted root certificates for well-known certificate authorities.
A RawServerSocket represents a listening socket, and provides a stream of low-level RawSocket objects, one for each connection made to the listening socket. [...]
A TCP connection. [...]
Events for the RawSocket. [...]
The RawSocketOption is used as a parameter to Socket.setRawOption and RawSocket.setRawOption to set customize the behaviour of the underlying socket. [...]
A low-level class for communicating synchronously over a TCP socket. [...]
The RawZLibFilter class provides a low-level interface to zlib.
Redirect information.
The SecureServerSocket is a server socket, providing a stream of high-level Sockets. [...]
A high-level class for communicating securely over a TCP socket, using TLS and SSL. The SecureSocket exposes both a Stream and an IOSink interface, making it ideal for using together with other Streams.
The object containing the certificates to trust when making a secure client connection, and the certificate chain and private key to serve from a secure server. [...]
A ServerSocket represents a listening socket, and provides a stream of Socket objects, one for each connection made to the listening socket. [...]
A TCP connection between two sockets. [...]
The SocketDirection is used as a parameter to Socket.close and RawSocket.close to close a socket in the specified direction(s).
The SocketOption is used as a parameter to Socket.setOption and RawSocket.setOption to set customize the behaviour of the underlying socket.
Stdin allows both synchronous and asynchronous reads from the standard input stream. [...]
The type of object a standard IO stream is attached to.
Stdout represents the IOSink for either stdout or stderr. [...]
The system encoding is the current code page on Windows and UTF-8 on Linux and Mac.
A two-way HTTP communication object for client or server applications. [...]
WebSocket status codes used when closing a WebSocket connection.
The WebSocketTransformer provides the ability to upgrade a HttpRequest to a WebSocket connection. It supports both upgrading a single HttpRequest and upgrading a stream of HttpRequests. [...]
X509Certificate represents an SSL certificate, with accessors to get the fields of the certificate.
The ZLibCodec encodes raw bytes to ZLib compressed bytes and decodes ZLib compressed bytes to raw bytes.
The ZLibDecoder is used by ZLibCodec and GZipCodec to decompress data.
The ZLibEncoder encoder is used by ZLibCodec and GZipCodec to compress data.
Exposes ZLib options for input parameters. [...]


APPEND → const FileMode
The mode for opening a file for reading and writing to the end of it. The file is created if it does not already exist.
@Deprecated("Use FileMode.append instead")
GZIP → const GZipCodec
@Deprecated("Use gzip instead")
gzip → const GZipCodec
An instance of the default implementation of the GZipCodec.
const GZipCodec._default()
READ → const FileMode
The mode for opening a file only for reading.
@Deprecated("Use FileMode.read instead")
SYSTEM_ENCODING → const SystemEncoding
@Deprecated("Use systemEncoding instead")
const SystemEncoding()
systemEncoding → const SystemEncoding
The current system encoding. [...]
const SystemEncoding()
WRITE → const FileMode
The mode for opening a file for reading and writing. The file is overwritten if it already exists. The file is created if it does not already exist.
@Deprecated("Use FileMode.write instead")
WRITE_ONLY → const FileMode
Mode for opening a file for writing only. The file is overwritten if it already exists. The file is created if it does not already exist.
@Deprecated("Use FileMode.writeOnly instead")
WRITE_ONLY_APPEND → const FileMode
Mode for opening a file for writing only to the end of it. The file is created if it does not already exist.
@Deprecated("Use FileMode.writeOnlyAppend instead")
ZLIB → const ZLibCodec
@Deprecated("Use zlib instead")
zlib → const ZLibCodec
An instance of the default implementation of the ZLibCodec.
const ZLibCodec._default()


exitCode int
Get the global exit code for the Dart VM. [...]
read / write
pid int
Returns the PID of the current process.
stderr Stdout
The standard output stream of errors written by this program. [...]
stdin Stdin
The standard input stream of data read by this program.
stdout Stdout
The standard output stream of data written by this program. [...]


exit(int code) → Never
Exit the Dart VM process immediately with the given exit code. [...]
isInsecureConnectionAllowed(dynamic host) bool
Whether insecure connections to host are allowed. [...]
sleep(Duration duration) → void
Sleep for the duration specified in duration. [...]
stdioType(dynamic object) StdioType
For a stream, returns whether it is attached to a file, pipe, terminal, or something else.


Enum that specifies the compression state of the byte stream of an HttpClientResponse. [...]


BadCertificateCallback(X509Certificate cr, String host, int port) bool

Exceptions / Errors

An exception that happens in the handshake phase of establishing a secure network connection, when looking up or verifying a certificate.
Exception thrown when a file operation fails.
An exception that happens in the handshake phase of establishing a secure network connection.
Base class for all IO related exceptions.
An OSError object holds information about an error from the operating system.
Exception thrown when a socket operation fails.
A secure networking exception caused by a failure in the TLS/SSL protocol.