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As part of my tinkering with XML, I built a little library for building strings. It's at

I'm looking for feedback of course.

Here's the README.

iostring: a file-flavored interface to strings

Two implementations are included; one in pure Lua, the other as a C
extension.  The interfaces are interchangeable.  If the C
implementation is available, it can be used by adding
"iostring=ciostring" somewhere.

Sometimes you to reuse code designed for the "io" interface to files
as a processing stage.  For example, you may want to read the result
of writing to a file back into a string for further processing.
Creating and deleting a temporary file to hold the data is complicated
(and potentially risky in security-sensitive software.)

iostring provides objects that behave like Lua filehandles but are
implemented on strings.  Currently, only write-only, non-seeking
iostrings are implemented.  (Feel free to implement more, of course.)

iostring.newoutput() returns an object with the following methods:

out = iostring.newoutput()

Returns a write-only, non-seekable iostring object.  Internally, it is
implemented such that repeated appends do not consume time
proportional to the size of the existing contents, using the code from
LTN 9.  Note that this is substantially faster than repeatedly using
the ".." operator to append to a string.

out:write(value1, ...)

Appends each of the arguments to the iostring.  The arguments must be
strings or numbers.


Returns a current copy of the string built by previous calls to


Ensures the string is up to date with all written data.  Note that
out:getstring() implicitly flushes, so for the current output-only
implementation, flush is never necessary.


Closes the iostring object.  It is an error to call anything but
getstring on a closed iostring object.

out:lines(), out:read(...), out:seek(...)

These functions raise an error, as they are not implemented.


function write_squares_table(file)
  for i = 1,10 do
    file:write(i, " ", i*i, "\n")

-- write to file:

diskfile ="squares", "w")

-- write to string:

stringfile = iostring.newoutput()

XML Benchmarks

I wrote a pair of XML serializers, one in the traditional "concat and
return" style, and another that works with output handles, either
files or iostrings.  The serialized document is a shallow table of
squares of the form:


The following were run on a Linux P3/700 box, and are not

iostr is the iostring implementation in pure Lua.
concat is a traditional "return a string" implementation
file uses a temporary file and io.write; then reading in the result
nullio uses a pure Lua iostring implementation that doesn't actually append anything
ciostr uses the iostring interface implemented in C.

1 run
squares	iostr	concat	file	nullio	ciostr
1	0	0	0	0	0
10	0	0	0	0	0
100	0.02	0.01	0	0.01	0.01
1000	0.27	0.35	0.06	0.08	0.06
10000	2.74	49.8	0.66	0.89	0.62

100 runs
squares	iostr	concat	file	nullio	ciostr
1	0.03	0.01	0.02	0.01	0
10	0.23	0.05	0.07	0.07	0.06
100	2.44	0.62	0.62	0.73	0.58
1000	26.6	34.7	6.47	9.3	6.05

10000 runs
squares	iostr	concat	file	nullio	ciostr
1	2.94	0.52	1.73	0.83	0.72
10	23.8	4.64	7	6.98	5.72

Rough analysis:

The "concat as we go" approach is quite efficient for small amounts of
output, but becomes catastrophic once a certain size is reached.  It
may be too risky for a general purpose function.

The pure Lua iostring implementation does not suffer catastrophic
collapse in the face of large output, but has a roughly 4x overhead
for small amounts of output.

The file-based implementation is quite competitive, at least on Linux.

nullio is mysteriously slower than the file and ciostring implementations.

ciostring wins except for the smallest output, where setup costs