I/O and Network

General I/O


Global variable referring to the standard out stream.


Global variable referring to the standard error stream.


Global variable referring to the standard input stream.

open(file_name[, read, write, create, truncate, append]) → IOStream

Open a file in a mode specified by five boolean arguments. The default is to open files for reading only. Returns a stream for accessing the file.

open(file_name[, mode]) → IOStream

Alternate syntax for open, where a string-based mode specifier is used instead of the five booleans. The values of mode correspond to those from fopen(3) or Perl open, and are equivalent to setting the following boolean groups:

r+read, write
wwrite, create, truncate
w+read, write, create, truncate
awrite, create, append
a+read, write, create, append
open(command, mode::AbstractString="r", stdio=DevNull)

Start running command asynchronously, and return a tuple (stream,process). If mode is "r", then stream reads from the process’s standard output and stdio optionally specifies the process’s standard input stream. If mode is "w", then stream writes to the process’s standard input and stdio optionally specifies the process’s standard output stream.

open(f::Function, command, mode::AbstractString="r", stdio=DevNull)

Similar to open(command,mode,stdio), but calls f(stream) on the resulting read or write stream, then closes the stream and waits for the process to complete. Returns the value returned by f.

open(f::Function, args...)

Apply the function f to the result of open(args...) and close the resulting file descriptor upon completion.

Example: open(readall,"file.txt")

IOBuffer() → IOBuffer

Create an in-memory I/O stream.


Create a fixed size IOBuffer. The buffer will not grow dynamically.


Create a read-only IOBuffer on the data underlying the given string

IOBuffer([data][, readable, writable[, maxsize]])

Create an IOBuffer, which may optionally operate on a pre-existing array. If the readable/writable arguments are given, they restrict whether or not the buffer may be read from or written to respectively. By default the buffer is readable but not writable. The last argument optionally specifies a size beyond which the buffer may not be grown.


Obtain the contents of an IOBuffer as an array, without copying. Afterwards, the IOBuffer is reset to its initial state.


Obtain the contents of an IOBuffer as a string, without copying. Afterwards, the IOBuffer is reset to its initial state.

fdio([name::AbstractString, ]fd::Integer[, own::Bool]) → IOStream

Create an IOStream object from an integer file descriptor. If own is true, closing this object will close the underlying descriptor. By default, an IOStream is closed when it is garbage collected. name allows you to associate the descriptor with a named file.


Commit all currently buffered writes to the given stream.


Close an I/O stream. Performs a flush first.

write(stream, x)

Write the canonical binary representation of a value to the given stream. Returns the number of bytes written into the stream.

You can write multiple values with the same :func:write call. i.e. the following are equivalent:

read(stream, type)

Read a value of the given type from a stream, in canonical binary representation.

read(stream, type, dims)

Read a series of values of the given type from a stream, in canonical binary representation. dims is either a tuple or a series of integer arguments specifying the size of Array to return.

read!(stream, array::Array)

Read binary data from a stream, filling in the argument array.

readbytes!(stream, b::Vector{UInt8}, nb=length(b); all=true)

Read at most nb bytes from the stream into b, returning the number of bytes read (increasing the size of b as needed).

See readbytes for a description of the all option.

readbytes(stream, nb=typemax(Int); all=true)

Read at most nb bytes from the stream, returning a Vector{UInt8} of the bytes read.

If all is true (the default), this function will block repeatedly trying to read all requested bytes, until an error or end-of-file occurs. If all is false, at most one read call is performed, and the amount of data returned is device-dependent. Note that not all stream types support the all option.


Get the current position of a stream.

seek(s, pos)

Seek a stream to the given position.


Seek a stream to its beginning.


Seek a stream to its end.

skip(s, offset)

Seek a stream relative to the current position.


Add a mark at the current position of stream s. Returns the marked position.

See also unmark(), reset(), ismarked()


Remove a mark from stream s. Returns true if the stream was marked, false otherwise.

See also mark(), reset(), ismarked()


Reset a stream s to a previously marked position, and remove the mark. Returns the previously marked position. Throws an error if the stream is not marked.

See also mark(), unmark(), ismarked()


Returns true if stream s is marked.

See also mark(), unmark(), reset()

eof(stream) → Bool

Tests whether an I/O stream is at end-of-file. If the stream is not yet exhausted, this function will block to wait for more data if necessary, and then return false. Therefore it is always safe to read one byte after seeing eof return false. eof will return false as long as buffered data is still available, even if the remote end of a connection is closed.

isreadonly(stream) → Bool

Determine whether a stream is read-only.

isopen(object) → Bool

Determine whether an object - such as a stream, timer, or mmap – is not yet closed. Once an object is closed, it will never produce a new event. However, a closed stream may still have data to read in its buffer, use eof to check for the ability to read data. Use poll_fd to be notified when a stream might be writable or readable.

serialize(stream, value)

Write an arbitrary value to a stream in an opaque format, such that it can be read back by deserialize. The read-back value will be as identical as possible to the original. In general, this process will not work if the reading and writing are done by different versions of Julia, or an instance of Julia with a different system image. Ptr values are serialized as all-zero bit patterns (NULL).


Read a value written by serialize.

print_escaped(io, str::AbstractString, esc::AbstractString)

General escaping of traditional C and Unicode escape sequences, plus any characters in esc are also escaped (with a backslash).

print_unescaped(io, s::AbstractString)

General unescaping of traditional C and Unicode escape sequences. Reverse of print_escaped().

print_joined(io, items, delim[, last])

Print elements of items to io with delim between them. If last is specified, it is used as the final delimiter instead of delim.

print_shortest(io, x)

Print the shortest possible representation, with the minimum number of consecutive non-zero digits, of number x, ensuring that it would parse to the exact same number.


Returns the file descriptor backing the stream or file. Note that this function only applies to synchronous File‘s and IOStream‘s not to any of the asynchronous streams.


Create a pipe to which all C and Julia level STDOUT output will be redirected. Returns a tuple (rd,wr) representing the pipe ends. Data written to STDOUT may now be read from the rd end of the pipe. The wr end is given for convenience in case the old STDOUT object was cached by the user and needs to be replaced elsewhere.


Replace STDOUT by stream for all C and Julia level output to STDOUT. Note that stream must be a TTY, a Pipe or a TCPSocket.


Like redirect_stdout, but for STDERR


Like redirect_stdout, but for STDIN. Note that the order of the return tuple is still (rd,wr), i.e. data to be read from STDIN, may be written to wr.


Read the entirety of x as a string but remove trailing newlines. Equivalent to chomp(readall(x)).

truncate(file, n)

Resize the file or buffer given by the first argument to exactly n bytes, filling previously unallocated space with ‘\0’ if the file or buffer is grown

skipchars(stream, predicate; linecomment::Char)

Advance the stream until before the first character for which predicate returns false. For example skipchars(stream,isspace) will skip all whitespace. If keyword argument linecomment is specified, characters from that character through the end of a line will also be skipped.

countlines(io[, eol::Char])

Read io until the end of the stream/file and count the number of lines. To specify a file pass the filename as the first argument. EOL markers other than ‘\n’ are supported by passing them as the second argument.


An IOBuffer that allows reading and performs writes by appending. Seeking and truncating are not supported. See IOBuffer for the available constructors.

PipeBuffer(data::Vector{UInt8}[, maxsize])

Create a PipeBuffer to operate on a data vector, optionally specifying a size beyond which the underlying Array may not be grown.


Read all available data on the stream, blocking the task only if no data is available. The result is a Vector{UInt8,1}.

Text I/O


Write an informative text representation of a value to the current output stream. New types should overload show(io,x) where the first argument is a stream. The representation used by show generally includes Julia-specific formatting and type information.


Show a more compact representation of a value. This is used for printing array elements. If a new type has a different compact representation, it should overload showcompact(io,x) where the first argument is a stream.


Similar to show, except shows all elements of arrays.


Return a string giving a brief description of a value. By default returns string(typeof(x)). For arrays, returns strings like “2x2 Float64 Array”.


Write (to the default output stream) a canonical (un-decorated) text representation of a value if there is one, otherwise call show. The representation used by print includes minimal formatting and tries to avoid Julia-specific details.


Print (using print()) x followed by a newline.

print_with_color(color::Symbol, [io, ]strings...)

Print strings in a color specified as a symbol, for example :red or :blue.


Display an informational message. Argument msg is a string describing the information to be displayed.


Display a warning. Argument msg is a string describing the warning to be displayed.

@printf([io::IOStream, ]"%Fmt", args...)

Print args using C printf() style format specification string. Optionally, an IOStream may be passed as the first argument to redirect output.

@sprintf("%Fmt", args...)

Return @printf formatted output as string.

julia>s=@sprintf"this is a %s%15.1f""test"34.567;julia>println(s)thisisatest34.6
sprint(f::Function, args...)

Call the given function with an I/O stream and the supplied extra arguments. Everything written to this I/O stream is returned as a string.

showerror(io, e)

Show a descriptive representation of an exception object.


Show all user-visible structure of a value.


Show all structure of a value, including all fields of objects.


Read the entire contents of an I/O stream as a string.


Open filename, read the entire contents as a string, then close the file. Equivalent to open(readall,filename).


Read a single line of text, including a trailing newline character (if one is reached before the end of the input), from the given stream (defaults to STDIN),

readuntil(stream, delim)

Read a string, up to and including the given delimiter byte.


Read all lines as an array.


Create an iterable object that will yield each line from a stream.

readdlm(source, delim::Char, T::Type, eol::Char; header=false, skipstart=0, skipblanks=true, use_mmap, ignore_invalid_chars=false, quotes=true, dims, comments=true, comment_char='#')

Read a matrix from the source where each line (separated by eol) gives one row, with elements separated by the given delimeter. The source can be a text file, stream or byte array. Memory mapped files can be used by passing the byte array representation of the mapped segment as source.

If T is a numeric type, the result is an array of that type, with any non-numeric elements as NaN for floating-point types, or zero. Other useful values of T include ASCIIString, AbstractString, and Any.

If header is true, the first row of data will be read as header and the tuple (data_cells,header_cells) is returned instead of only data_cells.

Specifying skipstart will ignore the corresponding number of initial lines from the input.

If skipblanks is true, blank lines in the input will be ignored.

If use_mmap is true, the file specified by source is memory mapped for potential speedups. Default is true except on Windows. On Windows, you may want to specify true if the file is large, and is only read once and not written to.

If ignore_invalid_chars is true, bytes in source with invalid character encoding will be ignored. Otherwise an error is thrown indicating the offending character position.

If quotes is true, column enclosed within double-quote (”) characters are allowed to contain new lines and column delimiters. Double-quote characters within a quoted field must be escaped with another double-quote. Specifying dims as a tuple of the expected rows and columns (including header, if any) may speed up reading of large files. If comments is true, lines beginning with comment_char and text following comment_char in any line are ignored.

readdlm(source, delim::Char, eol::Char; options...)

If all data is numeric, the result will be a numeric array. If some elements cannot be parsed as numbers, a cell array of numbers and strings is returned.

readdlm(source, delim::Char, T::Type; options...)

The end of line delimiter is taken as n.

readdlm(source, delim::Char; options...)

The end of line delimiter is taken as n. If all data is numeric, the result will be a numeric array. If some elements cannot be parsed as numbers, a cell array of numbers and strings is returned.

readdlm(source, T::Type; options...)

The columns are assumed to be separated by one or more whitespaces. The end of line delimiter is taken as n.

readdlm(source; options...)

The columns are assumed to be separated by one or more whitespaces. The end of line delimiter is taken as n. If all data is numeric, the result will be a numeric array. If some elements cannot be parsed as numbers, a cell array of numbers and strings is returned.

writedlm(f, A, delim='\t')

Write A (a vector, matrix or an iterable collection of iterable rows) as text to f (either a filename string or an IO stream) using the given delimeter delim (which defaults to tab, but can be any printable Julia object, typically a Char or AbstractString).

For example, two vectors x and y of the same length can be written as two columns of tab-delimited text to f by either writedlm(f,[xy]) or by writedlm(f,zip(x,y)).

readcsv(source, [T::Type]; options...)

Equivalent to readdlm with delim set to comma.

writecsv(filename, A)

Equivalent to writedlm with delim set to comma.


Returns a new write-only I/O stream, which converts any bytes written to it into base64-encoded ASCII bytes written to ostream. Calling close on the Base64Pipe stream is necessary to complete the encoding (but does not close ostream).


Returns a new read-only I/O stream, which decodes base64-encoded data read from istream.

base64encode(writefunc, args...)

Given a write-like function writefunc, which takes an I/O stream as its first argument, base64(writefunc,args...) calls writefunc to write args... to a base64-encoded string, and returns the string. base64(args...) is equivalent to base64(write,args...): it converts its arguments into bytes using the standard write functions and returns the base64-encoded string.


Decodes the base64-encoded string and returns a Vector{UInt8} of the decoded bytes.

Multimedia I/O

Just as text output is performed by print and user-defined types can indicate their textual representation by overloading show, Julia provides a standardized mechanism for rich multimedia output (such as images, formatted text, or even audio and video), consisting of three parts:

  • A function display(x) to request the richest available multimedia display of a Julia object x (with a plain-text fallback).
  • Overloading writemime allows one to indicate arbitrary multimedia representations (keyed by standard MIME types) of user-defined types.
  • Multimedia-capable display backends may be registered by subclassing a generic Display type and pushing them onto a stack of display backends via pushdisplay.

The base Julia runtime provides only plain-text display, but richer displays may be enabled by loading external modules or by using graphical Julia environments (such as the IPython-based IJulia notebook).

display(d::Display, x)
display(mime, x)
display(d::Display, mime, x)

Display x using the topmost applicable display in the display stack, typically using the richest supported multimedia output for x, with plain-text STDOUT output as a fallback. The display(d,x) variant attempts to display x on the given display d only, throwing a MethodError if d cannot display objects of this type.

There are also two variants with a mime argument (a MIME type string, such as "image/png"), which attempt to display x using the requested MIME type only, throwing a MethodError if this type is not supported by either the display(s) or by x. With these variants, one can also supply the “raw” data in the requested MIME type by passing x::AbstractString (for MIME types with text-based storage, such as text/html or application/postscript) or x::Vector{UInt8} (for binary MIME types).

redisplay(d::Display, x)
redisplay(mime, x)
redisplay(d::Display, mime, x)

By default, the redisplay functions simply call display. However, some display backends may override redisplay to modify an existing display of x (if any). Using redisplay is also a hint to the backend that x may be redisplayed several times, and the backend may choose to defer the display until (for example) the next interactive prompt.

displayable(mime) → Bool
displayable(d::Display, mime) → Bool

Returns a boolean value indicating whether the given mime type (string) is displayable by any of the displays in the current display stack, or specifically by the display d in the second variant.

writemime(stream, mime, x)

The display functions ultimately call writemime in order to write an object x as a given mime type to a given I/O stream (usually a memory buffer), if possible. In order to provide a rich multimedia representation of a user-defined type T, it is only necessary to define a new writemime method for T, via: writemime(stream,::MIME"mime",x::T)=..., where mime is a MIME-type string and the function body calls write (or similar) to write that representation of x to stream. (Note that the MIME"" notation only supports literal strings; to construct MIME types in a more flexible manner use MIME{symbol("")}.)

For example, if you define a MyImage type and know how to write it to a PNG file, you could define a function writemime(stream,::MIME"image/png",x::MyImage)=... to allow your images to be displayed on any PNG-capable Display (such as IJulia). As usual, be sure to importBase.writemime in order to add new methods to the built-in Julia function writemime.

Technically, the MIME"mime" macro defines a singleton type for the given mime string, which allows us to exploit Julia’s dispatch mechanisms in determining how to display objects of any given type.

mimewritable(mime, x)

Returns a boolean value indicating whether or not the object x can be written as the given mime type. (By default, this is determined automatically by the existence of the corresponding writemime function for typeof(x).)

reprmime(mime, x)

Returns an AbstractString or Vector{UInt8} containing the representation of x in the requested mime type, as written by writemime (throwing a MethodError if no appropriate writemime is available). An AbstractString is returned for MIME types with textual representations (such as "text/html" or "application/postscript"), whereas binary data is returned as Vector{UInt8}. (The function istext(mime) returns whether or not Julia treats a given mime type as text.)

As a special case, if x is an AbstractString (for textual MIME types) or a Vector{UInt8} (for binary MIME types), the reprmime function assumes that x is already in the requested mime format and simply returns x.

stringmime(mime, x)

Returns an AbstractString containing the representation of x in the requested mime type. This is similar to reprmime except that binary data is base64-encoded as an ASCII string.

As mentioned above, one can also define new display backends. For example, a module that can display PNG images in a window can register this capability with Julia, so that calling display(x) on types with PNG representations will automatically display the image using the module’s window.

In order to define a new display backend, one should first create a subtype D of the abstract class Display. Then, for each MIME type (mime string) that can be displayed on D, one should define a function display(d::D,::MIME"mime",x)=... that displays x as that MIME type, usually by calling reprmime(mime,x). A MethodError should be thrown if x cannot be displayed as that MIME type; this is automatic if one calls reprmime. Finally, one should define a function display(d::D,x) that queries mimewritable(mime,x) for the mime types supported by D and displays the “best” one; a MethodError should be thrown if no supported MIME types are found for x. Similarly, some subtypes may wish to override redisplay(d::D,...). (Again, one should importBase.display to add new methods to display.) The return values of these functions are up to the implementation (since in some cases it may be useful to return a display “handle” of some type). The display functions for D can then be called directly, but they can also be invoked automatically from display(x) simply by pushing a new display onto the display-backend stack with:


Pushes a new display d on top of the global display-backend stack. Calling display(x) or display(mime,x) will display x on the topmost compatible backend in the stack (i.e., the topmost backend that does not throw a MethodError).


Pop the topmost backend off of the display-backend stack, or the topmost copy of d in the second variant.


Returns a TextDisplay<:Display, which can display any object as the text/plain MIME type (only), writing the text representation to the given I/O stream. (The text representation is the same as the way an object is printed in the Julia REPL.)


Determine whether a MIME type is text data.

Memory-mapped I/O

Mmap.Anonymous(name, readonly, create)

Create an IO-like object for creating zeroed-out mmapped-memory that is not tied to a file for use in Mmap.mmap. Used by SharedArray for creating shared memory arrays.

Mmap.mmap(io::Union{IOStream,AbstractString,Mmap.AnonymousMmap}[, type::Type{Array{T,N}}, dims, offset]; grow::Bool=true, shared::Bool=true)
Mmap.mmap(type::Type{Array{T, N}}, dims)

Create an Array whose values are linked to a file, using memory-mapping. This provides a convenient way of working with data too large to fit in the computer’s memory.

The type is an Array{T,N} with a bits-type element of T and dimension N that determines how the bytes of the array are interpreted. Note that the file must be stored in binary format, and no format conversions are possible (this is a limitation of operating systems, not Julia).

dims is a tuple or single Integer specifying the size or length of the array.

The file is passed via the stream argument, either as an open IOStream or filename string. When you initialize the stream, use "r" for a “read-only” array, and "w+" to create a new array used to write values to disk.

If no type argument is specified, the default is Vector{UInt8}.

Optionally, you can specify an offset (in bytes) if, for example, you want to skip over a header in the file. The default value for the offset is the current stream position for an IOStream.

The grow keyword argument specifies whether the disk file should be grown to accommodate the requested size of array (if the total file size is < requested array size). Write privileges are required to grow the file.

The shared keyword argument specifies whether the resulting Array and changes made to it will be visible to other processes mapping the same file.

For example, the following code:

# Create a file for mmapping# (you could alternatively use mmap to do this step, too)A=rand(1:20,5,30)s=open("/tmp/mmap.bin","w+")# We'll write the dimensions of the array as the first two Ints in the filewrite(s,size(A,1))write(s,size(A,2))# Now write the datawrite(s,A)close(s)# Test by reading it back ins=open("/tmp/mmap.bin")# default is read-onlym=read(s,Int)n=read(s,Int)A2=Mmap.mmap(s,Matrix{Int},(m,n))

creates a m-by-nMatrix{Int}, linked to the file associated with stream s.

A more portable file would need to encode the word size—32 bit or 64 bit—and endianness information in the header. In practice, consider encoding binary data using standard formats like HDF5 (which can be used with memory-mapping).

Mmap.mmap(io, BitArray[, dims, offset])

Create a BitArray whose values are linked to a file, using memory-mapping; it has the same purpose, works in the same way, and has the same arguments, as mmap(), but the byte representation is different.

Example: B=Mmap.mmap(s,BitArray,(25,30000))

This would create a 25-by-30000 BitArray, linked to the file associated with stream s.


Forces synchronization between the in-memory version of a memory-mapped Array or BitArray and the on-disk version.

Network I/O

connect([host, ]port) → TCPSocket

Connect to the host host on port port

connect(path) → PipeEndpoint

Connect to the Named Pipe / Domain Socket at path

listen([addr, ]port) → TCPServer

Listen on port on the address specified by addr. By default this listens on localhost only. To listen on all interfaces pass IPv4(0) or IPv6(0) as appropriate.

listen(path) → PipeServer

Create and listen on a Named Pipe / Domain Socket


Gets the IP address of the host (may have to do a DNS lookup)


Parse a string specifying an IPv4 or IPv6 ip address.

IPv4(host::Integer) → IPv4

Returns IPv4 object from ip address formatted as Integer

IPv6(host::Integer) → IPv6

Returns IPv6 object from ip address formatted as Integer


Returns the number of bytes available for reading before a read from this stream or buffer will block.

accept(server[, client])

Accepts a connection on the given server and returns a connection to the client. An uninitialized client stream may be provided, in which case it will be used instead of creating a new stream.

listenany(port_hint) -> (UInt16, TCPServer)

Create a TCPServer on any port, using hint as a starting point. Returns a tuple of the actual port that the server was created on and the server itself.

poll_fd(fd, timeout_s::Real; readable=false, writable=false)

Monitor a file descriptor fd for changes in the read or write availability, and with a timeout given by timeout_s seconds.

The keyword arguments determine which of read and/or write status should be monitored; at least one of them must be set to true.

The returned value is an object with boolean fields readable, writable, and timedout, giving the result of the polling.

poll_file(path, interval_s::Real, timeout_s::Real) -> (previous::StatStruct, current::StatStruct)

Monitor a file for changes by polling every interval_s seconds until a change occurs or timeout_s seconds have elapsed. The interval_s should be a long period; the default is 5.007 seconds.

Returns a pair of StatStruct objects (previous,current) when a change is detected.

To determine when a file was modified, compare mtime(prev)!=mtime(current) to detect notification of changes. However, using watch_file for this operation is preferred, since it is more reliable and efficient, although in some situations it may not be available.

watch_file(path, timeout_s::Real)

Watch file or directory path for changes until a change occurs or timeout_s seconds have elapsed.

The returned value is an object with boolean fields changed, renamed, and timedout, giving the result of watching the file.

This behavior of this function varies slightly across platforms. See <https://nodejs.org/api/fs.html#fs_caveats> for more detailed information.

bind(socket::Union{UDPSocket, TCPSocket}, host::IPv4, port::Integer)

Bind socket to the given host:port. Note that will listen on all devices.

send(socket::UDPSocket, host::IPv4, port::Integer, msg)

Send msg over socket to host:port.


Read a UDP packet from the specified socket, and return the bytes received. This call blocks.

recvfrom(socket::UDPSocket) -> (address, data)

Read a UDP packet from the specified socket, returning a tuple of (address, data), where address will be either IPv4 or IPv6 as appropriate.

setopt(sock::UDPSocket; multicast_loop = nothing, multicast_ttl=nothing, enable_broadcast=nothing, ttl=nothing)

Set UDP socket options. multicast_loop: loopback for multicast packets (default: true). multicast_ttl: TTL for multicast packets. enable_broadcast: flag must be set to true if socket will be used for broadcast messages, or else the UDP system will return an access error (default: false). ttl: Time-to-live of packets sent on the socket.


Converts the endianness of a value from Network byte order (big-endian) to that used by the Host.


Converts the endianness of a value from that used by the Host to Network byte order (big-endian).


Converts the endianness of a value from Little-endian to that used by the Host.


Converts the endianness of a value from that used by the Host to Little-endian.


The 32-bit byte-order-mark indicates the native byte order of the host machine. Little-endian machines will contain the value 0x04030201. Big-endian machines will contain the value 0x01020304.