Networking and Streams

Julia provides a rich interface to deal with streaming I/O objects such as terminals, pipes and TCP sockets. This interface, though asynchronous at the system level, is presented in a synchronous manner to the programmer and it is usually unnecessary to think about the underlying asynchronous operation. This is achieved by making heavy use of Julia cooperative threading (coroutine) functionality.

Basic Stream I/O

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

julia> write(STDOUT,"Hello World");  # suppress return value 11 with ;
Hello World
julia> read(STDIN,Char)

'\n'

Note that write() returns 11, the number of bytes (in "Hello World") written to STDOUT, 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 read() 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\n"

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 eachline():

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 write() 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 write() function and that the returned value is 1 (since 0x61 is one byte).

For text I/O, use the print() or show() methods, depending on your needs (see the standard library reference for a detailed discussion of the difference between the two):

julia> print(STDOUT,0x61)
97

IO Output Contextual Properties

Sometimes IO output can benefit from the ability to pass contextual information into show methods. The IOContext object provides this framework for associating arbitrary metadata with an IO object. For example, showcompact adds a hinting parameter to the IO object that the invoked show method should print a shorter output (if applicable).

Working with Files

Like many other environments, Julia has an open() 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!\n"

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

julia> f = open("hello.txt","w")
IOStream(<file hello.txt>)

julia> write(f,"Hello again.")
12

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 open() 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(readstring(f))
end

You can call:

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

to open hello.txt, call read_and_capitalize 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(readstring(f))
       end
"HELLO AGAIN."

A simple TCP example

Let’s jump right in with a simple example involving TCP sockets. Let’s first create a simple server:

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

julia>

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 listen() 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)
TCPServer(active)

julia> listen(ip"127.0.0.1",2000) # Equivalent to the first
TCPServer(active)

julia> listen(ip"::1",2000) # Listens on localhost:2000 (IPv6)
TCPServer(active)

julia> listen(IPv4(0),2001) # Listens on port 2001 on all IPv4 interfaces
TCPServer(active)

julia> listen(IPv6(0),2001) # Listens on port 2001 on all IPv6 interfaces
TCPServer(active)

julia> listen("testsocket") # Listens on a UNIX domain socket/named pipe
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. The difference is subtle and has to do with the accept() and connect() methods. The accept() 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 connect() 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 connect() 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

As expected we saw “Hello World” printed. So, let’s actually analyze what happened behind the scenes. When we called connect(), we connect to the server we had just created. Meanwhile, the accept function returns a server-side connection to the newly created socket and prints “Hello World” to indicate that the connection was successful.

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 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> @async begin
         server = listen(2001)
         while true
           sock = accept(server)
           @async while isopen(sock)
             write(sock,readline(sock))
           end
         end
       end
Task (runnable) @0x00007fd31dc12e60

julia> clientside=connect(2001)
TCPSocket(open, 0 bytes waiting)

julia> @async while true
          write(STDOUT,readline(clientside))
       end
Task (runnable) @0x00007fd31dc11870

julia> println(clientside,"Hello World from the Echo Server")
Hello World from the Echo Server

As with other streams, use close() to disconnect the socket:

julia> close(clientside)

Resolving IP Addresses

One of the connect() methods that does not follow the listen() 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(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"