I was working on an embedded log API for an ARM system in Lua. The problem definition required limiting the amount of data that’s being written on the disk and a log file was eating all of that space to an extent that the machine was unusable. This happens when you hire inexperienced consultants who don’t understand the basics of embedded system programming. Anyway, I had to replace his code with a log API that only holds the latest log messages and here it is: the result of 3 days of my work.

The code has a lot of inline documentation so it should be obvious how it works but basically it makes for example 5 log files and the latest one is always the one with a bigger number (log-5.log is newer than log-4.log). The log message itself is also interesting. It tries to find the source of the log message and date/time.

--There can be several files. Their names are like:
-- LOG_FILE_PREFIX..N..LOG_FILE_POSTFIX
-- N will be variable between 1 to LOG_FILE_MAX_NUM
-- When file size reaches LOG_MAX_SIZE, an older log
-- file is removed and a new one is created

--{{{{{{{{{{{{{{{{{{{{{{{{{{{{{{{{{{{{{Options
--- LOG_FILE_PREFIX is the prefix that will be used to create the log file name.
--  it also indicates the absolute path for the value. Default="/tmp/log"
LOG_FILE_PREFIX ="./logfile-"
---Number of log files. The files will have a number in their name that range from [1-N] inclusive
LOG_FILE_MAX_NUM =5
---The postfix to the log file. Default=".log"
LOG_FILE_POSTFIX=".log"
---Maximum length of a log file before trying to make a new one
LOG_MAX_SIZE    =100*1024      --bytes
--}}}}}}}}}}}}}}}}}}}}}}}}}}}}}}}}}}}}}Options

require "lfs"

local currFile,n

local function writeToFile(str)
	local function fname(n) return LOG_FILE_PREFIX..n..LOG_FILE_POSTFIX end

	if not currFile then
		--trying to find the latest log file (from its number)
		for i=LOG_FILE_MAX_NUM,1,-1 do
			local fn=fname(i)
			local f=io.open(fn)
			if f then
				f:close()
				currFile,n=io.open(fn,"a+"),i
				break
			end
		end
		--if still couldn't find the latest file, make the file#1
		if not currFile then currFile,n=io.open(fname(1),"w+"),1 end
	end

	str=str or ""
	--if there's not enough space
	print("currsize="..currFile:seek("end")+str:len())
	if (currFile:seek("end")+str:len())>LOG_MAX_SIZE then
		currFile:close()
		currFile=nil
		--do we need to create a brand new file?
		for i=n+1,LOG_FILE_MAX_NUM do
			local fn=fname(i)
			local f=io.open(fn)
			if not f then
				currFile,n=io.open(fn,"a+"),i
				break
			end
			f:close()
		end
		--did the above loop succeed to find a new file?
		if currFile then
			--remove all the files with a bigger number
			for i=n+1,LOG_FILE_MAX_NUM do os.remove(fname(i)) end
		else
			--ok, then we need to use a currently existing file
			os.rename(fname(1),fname(LOG_FILE_MAX_NUM+1))
			--shift file names
			for i=1,LOG_FILE_MAX_NUM do os.rename(fname(i+1),fname(i)) end
			currFile,n=io.open(fname(LOG_FILE_MAX_NUM),"w+"),LOG_FILE_MAX_NUM
		end
	end

	currFile:write(str)
	currFile:write("\n")
	currFile:flush(str)
end

---Write something as a log to the screen (or maybe later to a file)
-- Note: this function adds the current date and time to the log report.
-- @msg the message to be logged
function log(msg)
	local debuginfo=debug.getinfo(2,"lSn")
	if debuginfo then
		if      debuginfo.what=="main" then sourcename="MAIN"
		elseif debuginfo.what=="C"    then sourcename="C "..debuginfo.name
		elseif debuginfo.what=="Lua"  then sourcename="function "..debuginfo.name.."()"
		end
		sourcefile=debuginfo.source:match("@?(.*)")
		linenumber=debuginfo.currentline
		datetime=os.date("%H:%M:%S %d/%m/%Y")
		return writeToFile(datetime.." "..sourcefile..":"..linenumber..":["..sourcename.."]: "..msg)
	else
		return writeToFile("--"..msg)
	end
end

for i=1,3000 do
	log("hi, this is a log "..i)
end