Networking and Streams

All Julia streams expose at least a read and a method, taking the stream as their first argument, e.g.:

Note that write returns 11, the number of bytes (in ) written to , but this return value is suppressed with the ;.

Here Enter was pressed again so that Julia would read the newline. Now, as you can see from this example, write takes the data to write as its second argument, while takes the type of the data to be read as the second argument.

For example, to read a simple byte array, we could do:

julia> x = zeros(UInt8, 4)
4-element Array{UInt8,1}:
 0x00
 0x00
 0x00
 0x00
julia> read!(stdin, x)
abcd
4-element Array{UInt8,1}:
 0x61
 0x62
 0x63
 0x64

However, since this is slightly cumbersome, there are several convenience methods provided. For example, we could have written the above as:

julia> read(stdin, 4)
abcd
4-element Array{UInt8,1}:
 0x61
 0x62
 0x63
 0x64

or if we had wanted to read the entire line instead:

julia> readline(stdin)
abcd
"abcd"

Note that depending on your terminal settings, your TTY may be line buffered and might thus require an additional enter before the data is sent to Julia.

To read every line from stdin you can use :

for line in eachline(stdin)
    print("Found $line")
end

or read if you wanted to read by character instead:

while !eof(stdin)
    x = read(stdin, Char)
    println("Found: $x")
end

Text I/O

Note that the method mentioned above operates on binary streams. In particular, values do not get converted to any canonical text representation but are written out as is:

julia> write(stdout, 0x61);  # suppress return value 1 with ;
a

Note that a is written to stdout by the function and that the returned value is 1 (since 0x61 is one byte).

For text I/O, use the print or methods, depending on your needs (see the documentation for these two methods for a detailed discussion of the difference between them):

julia> print(stdout, 0x61)
97

Sometimes IO output can benefit from the ability to pass contextual information into show methods. The object provides this framework for associating arbitrary metadata with an IO object. For example, :compact => true adds a hinting parameter to the IO object that the invoked show method should print a shorter output (if applicable). See the IOContext documentation for a list of common properties.

Working with Files

Like many other environments, Julia has an function, which takes a filename and returns an IOStream object that you can use to read and write things from the file. For example, if we have a file, hello.txt, whose contents are Hello, World!:

julia> f = open("hello.txt")
IOStream(<file hello.txt>)
julia> readlines(f)
1-element Array{String,1}:
 "Hello, World!"

If you want to write to a file, you can open it with the write ("w") flag:

If you examine the contents of hello.txt at this point, you will notice that it is empty; nothing has actually been written to disk yet. This is because the IOStream must be closed before the write is actually flushed to disk:

julia> close(f)

Examining hello.txt again will show its contents have been changed.

Opening a file, doing something to its contents, and closing it again is a very common pattern. To make this easier, there exists another invocation of which takes a function as its first argument and filename as its second, opens the file, calls the function with the file as an argument, and then closes it again. For example, given a function:

function read_and_capitalize(f::IOStream)
    return uppercase(read(f, String))

You can call:

julia> open(read_and_capitalize, "hello.txt")
"HELLO AGAIN."

to open hello.txt, call on it, close hello.txt and return the capitalized contents.

To avoid even having to define a named function, you can use the do syntax, which creates an anonymous function on the fly:

julia> open("hello.txt") do f
           uppercase(read(f, String))
       end
"HELLO AGAIN."

Let’s jump right in with a simple example involving TCP sockets. This functionality is in a standard library package called Sockets. Let’s first create a simple server:

julia> using Sockets
julia> errormonitor(@async begin
           server = listen(2000)
           while true
               sock = accept(server)
               println("Hello World\n")
           end
       end)
Task (runnable) @0x00007fd31dc11ae0

To those familiar with the Unix socket API, the method names will feel familiar, though their usage is somewhat simpler than the raw Unix socket API. The first call to will create a server waiting for incoming connections on the specified port (2000) in this case. The same function may also be used to create various other kinds of servers:

julia> listen(2000) # Listens on localhost:2000 (IPv4)
Sockets.TCPServer(active)
julia> listen(ip"127.0.0.1",2000) # Equivalent to the first
Sockets.TCPServer(active)
julia> listen(ip"::1",2000) # Listens on localhost:2000 (IPv6)
Sockets.TCPServer(active)
julia> listen(IPv4(0),2001) # Listens on port 2001 on all IPv4 interfaces
Sockets.TCPServer(active)
julia> listen(IPv6(0),2001) # Listens on port 2001 on all IPv6 interfaces
Sockets.TCPServer(active)
julia> listen("testsocket") # Listens on a UNIX domain socket
Sockets.PipeServer(active)
julia> listen("\\\\.\\pipe\\testsocket") # Listens on a Windows named pipe
Sockets.PipeServer(active)

Note that the return type of the last invocation is different. This is because this server does not listen on TCP, but rather on a named pipe (Windows) or UNIX domain socket. Also note that Windows named pipe format has to be a specific pattern such that the name prefix (\\.\pipe\) uniquely identifies the file type. The difference between TCP and named pipes or UNIX domain sockets is subtle and has to do with the and connect methods. The method retrieves a connection to the client that is connecting on the server we just created, while the connect function connects to a server using the specified method. The function takes the same arguments as listen, so, assuming the environment (i.e. host, cwd, etc.) is the same you should be able to pass the same arguments to as you did to listen to establish the connection. So let’s try that out (after having created the server above):

julia> connect(2000)
TCPSocket(open, 0 bytes waiting)
julia> Hello World

A great strength of Julia is that since the API is exposed synchronously even though the I/O is actually happening asynchronously, we didn’t have to worry about callbacks or even making sure that the server gets to run. When we called connect the current task waited for the connection to be established and only continued executing after that was done. In this pause, the server task resumed execution (because a connection request was now available), accepted the connection, printed the message and waited for the next client. Reading and writing works in the same way. To see this, consider the following simple echo server:

julia> errormonitor(@async begin
           server = listen(2001)
           while true
               sock = accept(server)
               @async while isopen(sock)
                   write(sock, readline(sock, keep=true))
               end
           end
       end)
Task (runnable) @0x00007fd31dc12e60
julia> clientside = connect(2001)
TCPSocket(RawFD(28) open, 0 bytes waiting)
julia> errormonitor(@async while isopen(clientside)
       end)
julia> println(clientside,"Hello World from the Echo Server")
Hello World from the Echo Server

As with other streams, use to disconnect the socket:

One of the connect methods that does not follow the methods is connect(host::String,port), which will attempt to connect to the host given by the host parameter on the port given by the port parameter. It allows you to do things like:

julia> connect("google.com", 80)
TCPSocket(RawFD(30) open, 0 bytes waiting)

At the base of this functionality is getaddrinfo, which will do the appropriate address resolution:

julia> getaddrinfo("google.com")
ip"74.125.226.225"

All I/O operations exposed by Base.read and can be performed asynchronously through the use of coroutines. You can create a new coroutine to read from or write to a stream using the macro:

julia> task = @async open("foo.txt", "w") do io
           write(io, "Hello, World!")
       end;
julia> wait(task)
julia> readlines("foo.txt")
1-element Array{String,1}:
 "Hello, World!"

It’s common to run into situations where you want to perform multiple asynchronous operations concurrently and wait until they’ve all completed. You can use the @sync macro to cause your program to block until all of the coroutines it wraps around have exited:

julia> using Sockets
julia> @sync for hostname in ("google.com", "github.com", "julialang.org")
           @async begin
               conn = connect(hostname, 80)
               write(conn, "GET / HTTP/1.1\r\nHost:$(hostname)\r\n\r\n")
               readline(conn, keep=true)
               println("Finished connection to $(hostname)")
           end
       end
Finished connection to google.com
Finished connection to julialang.org
Finished connection to github.com

Multicast

Julia supports over IPv4 and IPv6 using the User Datagram Protocol (UDP) as transport.

Unlike the Transmission Control Protocol (), UDP makes almost no assumptions about the needs of the application. TCP provides flow control (it accelerates and decelerates to maximize throughput), reliability (lost or corrupt packets are automatically retransmitted), sequencing (packets are ordered by the operating system before they are given to the application), segment size, and session setup and teardown. UDP provides no such features.

A common use for UDP is in multicast applications. TCP is a stateful protocol for communication between exactly two devices. UDP can use special multicast addresses to allow simultaneous communication between many devices.

To transmit data over UDP multicast, simply recv on the socket, and the first packet received will be returned. Note that it may not be the first packet that you sent however!

using Sockets
group = ip"228.5.6.7"
socket = Sockets.UDPSocket()
bind(socket, ip"0.0.0.0", 6789)
join_multicast_group(socket, group)
println(String(recv(socket)))
leave_multicast_group(socket, group)
close(socket)

To transmit data over UDP multicast, simply send to the socket. Notice that it is not necessary for a sender to join the multicast group.

using Sockets
group = ip"228.5.6.7"
socket = Sockets.UDPSocket()
send(socket, group, 6789, "Hello over IPv4")
close(socket)

This example gives the same functionality as the previous program, but uses IPv6 as the network-layer protocol.

using Sockets
group = Sockets.IPv6("ff05::5:6:7")
socket = Sockets.UDPSocket()
bind(socket, Sockets.IPv6("::"), 6789)
join_multicast_group(socket, group)
println(String(recv(socket)))
leave_multicast_group(socket, group)
close(socket)

Sender:

using Sockets
group = Sockets.IPv6("ff05::5:6:7")
socket = Sockets.UDPSocket()