luaposix 35.1 Reference

## lock.lua

    #! /usr/bin/env lua

    local M = require 'posix.fcntl'

    local S = require 'posix.sys.stat'

    local fd = M.open(
       'file.txt',
       M.O_CREAT + M.O_WRONLY + M.O_TRUNC,
       S.S_IRUSR + S.S_IWUSR + S.S_IRGRP + S.S_IROTH
    )

    -- Set lock on file
    local lock = {
       l_type = M.F_WRLCK;     -- Exclusive lock
       l_whence = M.SEEK_SET;  -- Relative to beginning of file
       l_start = 0;            -- Start from 1st byte
       l_len = 0;              -- Lock whole file
    }

    if M.fcntl(fd, M.F_SETLK, lock) == -1 then
       error('file locked by another process')
    end

    -- Do something with file while it's locked
    require 'posix.unistd'.write(fd, 'Lorem ipsum\n')

    -- Release the lock
    lock.l_type = M.F_UNLCK
    M.fcntl(fd, M.F_SETLK, lock)

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