Buffer API

The buffer library provides user friendly streams. These are the kind that the io library returns from io.open unlike the raw streams returned by filesystem.open which don't support as many helpful methods. These helper methods on the file handles you get from io.open are defined here, under Instance Methods. Thus, this API documentation is important and helpful even if you aren't building your own buffered streams.

Additionally, this API allows you to create buffered streams. You provide the backend stream read and write, the buffer library provides the formatting and buffering of the data. Generally, users will not need to make their own buffered streams. For reference, the io library uses buffered streams (which includes file io as well as terminal io)

Static Methods

The following methods are called on the buffer library itself.

  • buffer.new([mode: string], stream: table)

    Creates a new buffered stream, wrapping stream with read-write mode. mode can be readonly (r or nil), read-write (rw), or write-only (w). Read about the stream interface methods required on the stream object.

Instance Methods

The following methods can only be called on instances created by buffer.new (note file handles returned by io.open are also buffered streams, created with buffer.new). These methods are instance methods, requiring instance call notation :. In order to help differentiate these instance methods from static methods (e.g. buffer.new), b: will be used to prefix the method names.

  • b:flush()

    If any data is buffered it is immediately written to the stream and released.

  • b:close()

    Flushes the buffer and closes the wrapped stream.

  • b:setvbuf([mode: string], [size: number]) mode, size

    Sets the buffering mode and size and returns the result mode and size. The amount of data buffered is specified by size which defaults to [512, 8192] bytes, depending on available system memory. mode and size can be nil, in which case the previous values are used for either. size is also used in read(n) calls to the stream.
    Modes only affect write, which include:

    • “no” writes are immediately pushed to the stream.
    • “full” writes are buffered up to size bytes. This is the default mode.
    • “line” writes are buffered until newlines are found or size is reached, whichever comes first.
  • b:write([values...])

    Writes each value to the stream, first buffering based on the mode and buffer size (see setvbuf). Note that to write to a file, you have to open it for write.

local file = io.open("/tmp/foo.txt", "w")
file:write("abc", "def", "\n")
-- foo.txt now has "abcdef\n"
  • b:lines([line_formats...]) string array

    Returns a function iterator which reads from the stream until it reaches nil. On each read, the line_formats list of args as passed to stream:read(...). The overwhelmingly typical use is to not define line_formats, i.e. passing no args to lines(). The default behavior (i..e without line_formats) is to read a “line” at a time from the stream.

local file = io.open("/tmp/foobar.txt")
for line in file:lines() do
  • b:read([formats...]) string...

    A fairly advanced reader that support various formats. First of all, if called with no format, i.e an empty param list, it reads the next line from the stream, which is equivalent to read("*l")
    Each format is read from the stream and all returned in a multiple return value list of the results. Note all format strings are prefixed with * and also note that only the first char of the string names of the formats matters, the rest is ignored. These are the supported formats:

    • a number value, e.g. 10

      Read n bytes (in binary mode) or chars (in text mode) from the stream; result is returned as a string. See io.open for more details about how to open files in different modes.

      local chars = b:read(10)

    • “*n” or “*number”

      Read the next series of bytes from the stream that can be interpreted as a number. Note that reading numbers is also affected by the open mode, binary or text. See io.open for more details about how to open files in different modes..

      local number = b:read("*n")

    • “*l” or “*line”

      Read the next line from the stream, chopping off the line ending marker (which may be \n, \r, or \r\n)

      local line = b:read("*l")

    • “*L” or “*Line”

      Read the next line from the stream, like “*line”, but preserves the line ending marker as part of the result

      local whole_line = b:read("*L")

    • “*a” or “*all”

      Reads all remaining data from the stream until nil. There would be no point in having formats following this.

      local the_whole_file = b:read("*a")

  • b:getTimeout() number

    Returns the current timeout (in seconds) set on the buffered stream. math.huge is the default timeout. Read setTimeout for more information about the effects of a buffered stream timeout.

  • b:setTimeout(timeout)

    Sets the time in seconds a buffered stream will try to limit a read operation. Note that this timeout cannot be strictly adhered to. A read operation that completes within a single readChunk (an internal method that invokes the actual read on the stream) does not check the timeout limit. Timeout is only checked between stream reads within a single buffered read (an example follows). Thus, if a read requires multiple chunk reads, and the time between the start of the first read before the start of the last read is greater than or equal to the timeout, then the buffered stream will error. Again note that a timeout is default math.huge.

local file = buffer.new("r", { read = function() os.sleep(5) return "a" end })
file:setvbuf("full", 1) -- set buffer size to 1 char
file:setTimeout(1) -- set buffer timeout to 1 second
-- this will time out before trying to read the 2nd char
local a, b = file:read(1, 1) -- read 1 char, then read 1 char again
  • b:seek([whence:string], [offset:number])

    Moves the stream position by offset bytes from whence, both optional params. whence defaults to “cur”, and offset defaults to 0. Valid whence values:

    • “cur” from the current position.
    • “set” from the start of the stream.
    • “end” from the end of the stream. Returns the result of the seek operation on the stream (which may fail).

Interface Methods

The following methods are expected to be implemented on the buffered streams passed to buffer.new.

  • close() ok, reason

    Close handles, release resources, disconnect – and return success

  • write(arg: string) ok, reason

    Write arg as bytes, assume a string of plain unformatted chars. Return falsey and reason on failure.

  • read(n: number) ok, reason

    Return n bytes, and not n unicode-aware chars. Assume your data is binary data and let the buffer library manage the mode and the unicode string packaging (if applicable). Note that this is exactly how the filesystem library operates.The caller assumes there is more data to read until nil is returned. A empty string or a string shorter than n chars long is a valid return, but the caller may assume there is more data to request until nil is returned.

  • seek([whence: string], [offset: number]) [offset from start] or falsey, reason

    Refer to b:seek() for details. In short, move the stream position to offset from whence, and return the offset from the start of the stream of the position after the seek operation. Note that seek("cur", 0) is a valid request, typical of the caller wanting to determine the current position of the stream. Your stream is not required to support seek, in such case (or in any case of failure) you should return nil, and the reason (as a string) for the failure.