## rt_sched.lua

    #! /usr/bin/env lua

    --[[
     This example tests sched_setscheduler() / sched_getscheduler()
     The script must be run as privileged process (CAP_SYS_NICE on linux)
    ]]

    local M = require 'posix.sched'

    local mypid = require 'posix.unistd'.getpid()

    -- get sched params from ps(1)
    local function get_ps_sched(pid)
       local fp = io.popen(string.format('ps --no-headers -o "policy,rtprio" %s', pid))
       local res, err = fp:read'*a'
       assert(res, err)
       fp:close()
       local policy, rtprio = string.match(res, '([^%s]+)%s+([^%s]+)')
       assert(policy)
       assert(rtprio)
       return policy, rtprio
    end

    do -- Tests on own process

       -- get scheduler policy for own process
       local res, err = M.sched_getscheduler(0) -- 0 pid: own process
       assert(res == M.SCHED_OTHER)
       local res, err = M.sched_getscheduler() -- default pid: own process
       assert(res == M.SCHED_OTHER)

       local policy, rtprio = get_ps_sched(mypid)
       assert(policy== 'TS')
       assert(rtprio== '-')

       -- set realtime priority on own process : SCHED_FIFO
       local res, err = M.sched_setscheduler(0, p.SCHED_FIFO, 10 )
       assert(res, err)

       local policy, rtprio = get_ps_sched(mypid)
       assert(policy == 'FF')
       assert(rtprio == '10')

       local res, err = M.sched_getscheduler(0)
       assert(res == M.SCHED_FIFO)

       -- set realtime priority on own process SCHED_RR
       local res, err = M.sched_setscheduler(0, M.SCHED_RR, 11 )
       assert(res, err)

       local policy, rtprio = get_ps_sched(mypid)
       assert(policy == 'RR')
       assert(rtprio == '11')

       local res, err = M.sched_getscheduler(0)
       assert(res == M.SCHED_RR)

       -- no parameters: reset own process to normal priority :
       local res, err = M.sched_setscheduler()
       assert(res, err)

       local policy, rtprio = get_ps_sched(mypid)
       assert(policy == 'TS')
       assert(rtprio == '-')

       local res, err = M.sched_getscheduler(0)
       assert(res == M.SCHED_OTHER)
    end

    -- fork a child to check sched_setscheduler on other process
    do
       local U = require 'posix.unistd'

       local r, w = U.pipe()
       local childpid = U.fork()
       if childpid == 0 then
          -- child: block on pipe until parent is finshed
          U.close(w)  -- close unused write end
          local b = U.read(r,1)
          U._exit(0)
       end

       -- parent:
       U.close(r)

       do -- do tests
          -- get scheduler policy for child process
          local res, err = M.sched_getscheduler(childpid)
          assert(res == M.SCHED_OTHER)

          local policy, rtprio = get_ps_sched(childpid)
          assert(policy == 'TS')
          assert(rtprio == '-')

          -- set realtime priority on child process : SCHED_FIFO
          local res, err = M.sched_setscheduler(childpid, M.SCHED_FIFO, 10 )
          assert(res, err)

          local policy, rtprio = get_ps_sched(childpid)
          assert(policy == 'FF')
          assert(rtprio == '10')

          local res, err = M.sched_getscheduler(childpid)
          assert(res == M.SCHED_FIFO)

          -- set realtime priority on child process SCHED_RR
          local res, err = M.sched_setscheduler(childpid, M.SCHED_RR, 11 )
          assert(res, err)

          local policy, rtprio = get_ps_sched(childpid)
          assert(policy == 'RR')
          assert(rtprio == '11')

          local res, err = M.sched_getscheduler(childpid)
          assert(res == M.SCHED_RR)

          -- no parameters after pid: reset child process to normal priority :
          local res, err = M.sched_setscheduler(childpid)
          assert(res, err)

          local policy, rtprio = get_ps_sched(childpid)
          assert(policy == 'TS')
          assert(rtprio == '-')

          local res, err = M.sched_getscheduler(childpid)
          assert(res == M.SCHED_OTHER)
       end

       -- stop child
       U.write(w, 'stop')
       U.close(w)
       require 'posix.sys.wait'.wait(childpid)
    end

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