## lock.lua

    local p = require "posix"
    local fd = p.creat("file.txt", "rw-r--r--")

    -- Set lock on file
    local lock = {
        l_type = p.F_WRLCK;     -- Exclusive lock
        l_whence = p.SEEK_SET;  -- Relative to beginning of file
        l_start = 0;            -- Start from 1st byte
        l_len = 0;              -- Lock whole file
      }
    local result = p.fcntl(fd, p.F_SETLK, lock)
    if result == -1 then
      error("file locked by another process")
    end

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

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

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