Sleep Function

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A common need is to pause (sleep) a program for a certain number of seconds, preferably without busy waiting.

This function to do this without busy waiting does not exist in ANSI C, so it does not exist in stock Lua. However, there are extension libraries and calls to external programs that can do this.

Solution: Busy Wait

local clock = os.clock
function sleep(n)  -- seconds
  local t0 = clock()
  while clock() - t0 <= n do end
end
-- warning: clock can eventually wrap around for sufficiently large n
-- (whose value is platform dependent).  Even for n == 1, clock() - t0
-- might become negative on the second that clock wraps.

Solution: C extension

There is a sleep function in ExtensionProposal. This may call Win32 Sleep or POSIX usleep. Here's a [usleep/sleep C wrapper] example.

The LuaApr binding has an [apr.sleep()] function that works on Windows & UNIX and supports sub-second resolution.

The lalarm library[1] can set an alarm on POSIX.

winapi (Windows only) has a [sleep] function. [github]

If an FFI interface (Alien or c/invoke -- BindingCodeToLua) is available, you can call whichever OS function you have.

Solution: sleep command

function sleep(n)
  os.execute("sleep " .. tonumber(n))
end

Windows does not have such a built-in command. However, there's a sleep in the Windows Server Resource Kit. There is also sleep in Cygwin and MinGW. Also, there is "timeout" utility available in Windows 7

  os.execute("timeout " .. tonumber(n)) -- specific to win7 (and probably higher)

Solution: ping or other programs

function sleep(n)
  if n > 0 then os.execute("ping -n " .. tonumber(n+1) .. " localhost > NUL") end
end
-- version 20100715 - fixed off-by-one second

This is mainly for Windows in the absence of a sleep command. Other variations exist, e.g. "perl -e 'sleep(" .. tonumber(n) .. ")'" or "php -r sleep(" .. tonumber(n) .. ");".

Solution: I/O wait

io.stdin:read'*l'

This is not a sleep but may be useful in similar cases. It waits for the user to press the Enter key.

Solution: Using WScript (Windows)

function sleep(n)
  local vb = "test.vbs"
  local f = assert(io.open(vb,"w"))
  f:write("WScript.Sleep(" .. (tonumber(n) * 1000) .. ")\n")
  f:close()
  os.execute(vb)
end

See [2].

Solution: sleep()

The POSIX sleep() call provides integer second sleeps.

require "posix"

posix.sleep(3)

Solution: socket.sleep()

The LuaSocket? module provides a sleep function.

socket = require("socket")

function sleep(sec)
    socket.sleep(sec)
end

sleep(0.2)

Solution: ngx.sleep()

Nginx Lua module provides a sleep function. One can specify time resolution up to 0.001 seconds (i.e., one milliseconds). Behind the scene, this method makes use of the Nginx timers.

ngx.sleep(sec)

Solution: lsocket.select()

The select() timeout provides a fairly portable sub-second sleep, if you can tolerate the socket library dependency.

local lsocket  = require("lsocket")

function sleep(sec)
    lsocket.select(sec)
end

sleep(2)

Solution: LuaJIT FFI/LuaFFI

local ffi = require "ffi"
ffi.cdef "unsigned int sleep(unsigned int seconds);"
ffi.C.sleep(2)

Solution: os.time()

function sleep(s)
  local ntime = os.time() + s
  repeat until os.time() > ntime
end

Solution: os.clock()

. .

Using the os.clock() method instead of os.time(), you can get precision down to one 100th of a second while os.time() only allows intervals based on the timestamp, which at execution can be at anything from 0.1 to 1 second. The os.time() method is great for longer periods over 2 seconds where precision isn't that much of a deal.

function sleep(s)
  local ntime = os.clock() + s/10
  repeat until os.clock() > ntime
end