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.

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

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.

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) .. ")'".

Solution: I/O wait

io.stdin:read'*l'

This is not a sleep but may be useful in similar cases. It waits for the use 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: select()

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

require "socket"

function sleep(sec)
    socket.select(nil, nil, sec)
end

sleep(0.2)