Manual of Lua-MemoryFile

Lua-MemoryFile - Lua 5.1 module for in-memory simulated file I/O


The module allows Lua programs to open strings like files for reading and writing. This can be useful for various reasons:

Loading the module

The MemoryFile module doesn't install itself into any global tables, so you can decide what name you want to use to access it. You will probably want to load it like this:

local MemFile = require "memoryfile"

You can use a variable called something other than MemFile if you'd like, or you could assign the table returned by require to a global variable. In this documentation we'll assume you're using a variable called MemFile.

Simulating a mutable string

This is probably the most useful way of using the MemoryFile module. You want to create a string from lots of individual pieces (perhaps you're adding values from an array, or serializing a tree structure like a set of nested tables or an XML DOM).

Here's an example which turns a table of strings into a single string of Lua source code:

local code =, "w")
local strings = { "foo", "bar", "quux" }

code:write("values = {\n")
for _, value in ipairs(strings) do
    code:write("    ", string.format("%q", value), ",\n")

local code_string = tostring(code)

Of course this would only be useful if the strings table in the example was a lot larger, but in that case it would be much faster to write all the individual strings to a MemoryFile handle than to concatenate them all onto a Lua string.

At the end of the example above we use tostring to get the content of the memory file as a normal Lua string. The example also uses the close method to clear out the data from the file handle, which is a good idea after you're finished with it, because it frees up memory immediately rather than waiting for the Lua garbage collector to get around to doing it for you.

Creating file handles

The table returned by require contains a single function, open. This takes up to two arguments. The first (what would be the filename for is a string used to initialize the file handle. If it is nil or the empty string then the memory file will be empty to start with, otherwise it will contain some data which can be read with the read method.

Note that since Lua strings are immutable, MemoryFile always makes a copy of the string data you initialize it with, so the original string will not be affected by changes to the file's data.

The second argument to open is a mode string. The first character should be r, w, or a. The default is r. The read and write modes are actually the same. Whichever you use, you can read or write with the file handle. In append mode reading and writing will also work the same as normal, except that anything you write on to the file will go at the end of the file, regardless of where you have positioned the file with seek.

Any extra characters in the mode string will be ignored, so the + and b flags will have no affect.

File handle methods

Most of these methods behave basically the same way as the ones provided by Lua's real file handles. See the Lua reference manual for details:


This frees up any resources associated with the file handle, and resets it to contain zero bytes of data. This is identical to calling size(0) on a memoryfile handle. Unlike real file handles, you can continue to use a memoryfile after it has been reset in this way.

Always returns true.


These two functions do nothing, and are merely provided for compatibility with real Lua file handles. They always return true.


Returns an iterator for looping over the lines in the buffer, starting from the current position seeked to. This example counts the lines in the string by iterating over them:

local f ="foo\nbar\nbaz\n")
local count = 0
for line in f:lines() do count = count + 1 end
print("number of lines in string:", count)

Each line will be read as if by read("*l"), so the strings will not include the newline character.


Read some data from the file handle's buffer. Does everything the standard version does. Can be used to read fixed-size pieces of data from the buffer:

local f ="foobarbaz")
local foo, barbaz = f:read(3, 6)

or to get a number value from a string:

local f ="23.25")
local num = f:read("*n")

or to read lines, or the whole of the rest of the file:

local f ="foo\nbar\netc...")
local line1, line2 = f:read("*l", "*l")
local everything_else = f:read("*a")

Same as the version for normal Lua file handles. You give it a string indicating where to start from (set, cur, or end), and a number indicating how many bytes to move relative to that. It moves the position for the next read or write operation to that point, and returns the new position as the number of bytes from the start of the buffer.

Note that the position you seek to will be ignored for writing when the file handle was opened in a (for append) mode. In that case all writes will go at the end of the file, but you can still use seek to read from other places.

The arguments default to cur and zero, so if you call seek without any arguments it will leave the buffer position unchanged. You can use that if you just want to find out where in the buffer you are:

-- Get a byte from a certain position, then restore
-- the original seek position.
function get_byte (f, position)
    local old_position = f:seek()
    f:seek("set", position)
    local byte = f:read(1)
    f:seek("set", old_position)
    return byte

To jump right to the end of a file so that you can write some more data after what already exists in the buffer:


seek returns nil and an error message if you try to seek to a position after the end or before the start of the current buffer.


Returns the number of bytes which are currently stored in the buffer. This is the same as the length of the string which will be returned by passing the file handle to tostring.

Giving size an argument will cause it to set the size as well as return the previous size. It can be used to truncate the file handle's buffer to a certain number of bytes long, or to extend a file without having to write any data into it. Any new data which appears after size makes a buffer bigger will be initialized to have all null (zero) bytes.

Calling size(0) will free up the buffer and reset the handle to the empty string. This can be used when work with a file handle is finished, so that a potentially large memory buffer isn't left on the heap until the garbage collector gets round to freeing it. This is exactly the same as calling the close() method.


Copies the data from its arguments, which must be strings or numbers, into the buffer for a MemoryFile handle. The values are added one after another, with nothing in between.

After calling write the seek position will be moved to the end of the newly written data, except in append mode, when it will be left where it is.

If you use seek, or open a file with some initial data, you can use write to overwrite data already in the buffer. It will still make the buffer bigger if it needs more room, and it will never shrink the buffer. Also, if the MemoryFile handle was opened in append mode then write will ignore the position you seeked to and write all new data onto the end of the buffer.


You can use the Lua tostring function to get a string copy of the current data in a MemoryFile handle. For example, this will produce 512 foos by repeatedly getting the contents of the memory file as a string and then doubling the length of the memoryfile's data by writing that string back onto the end of the buffer:

local f ="foo")
for i = 1, 10 do f:write(tostring(f)) end

Differences from real file handles

The userdata objects returned from MemoryFile's open function are intended to be fairly compatible with the file handles returned by Lua's standard function. There are some differences in how they behave though.


This software and documentation is Copyright © 2007–2012 Geoff Richards <geoff at this domain dot co dot uk>. It is free software; you can redistribute it and/or modify it under the terms of the Lua 5.0 license. The full terms are given in the file COPYRIGHT supplied with the source code package, and are also available here: