Every now and then people post a "question" about printing to this mailng list which exposes their confusion. I am putting this email together so that anybody capable of searching through the mailing list can at least have terminology straight before asking for help. Information presented here is in the public domain and I make no claims of posting anything new.
Table of Contents: 1. Print spooling overview: LPD, LPRng, CUPS 2. Common network printing protocols: LPD, IPP, JetDirect 3. Printer driver. 4. Input filters 5. ASCII and page description language PostScript(PS) 6. PostScript Printer Description (PPD) files 7. Printer recommendations 8. Code contribution 1. What is a print spooling? Why is needed? A print spooler is a program/daemon that accepts print jobs from a program or network. It typically consist of two programs: a print spooler daemon that sends jobs to a printer and a command to submit print jobs to the spooler daemon. In general spooler is not needed on an operating system that allows a single user to perform only one task at a time as long as that single user doesn't try to send multiple documents to the printer at the same time. However, UNIX has been designed multitasking, multiuser computer operating systems. Imagine that my wife and I send two documents to a printer at the same time. Her documents gets there first and gets printed. My document losses the race and my job is rejected because the device is busy. I wait a few minutes and I sent my document again but this time my daughter outrace me and her document get printed and not mine. Now imagine the organization with hundreds of users and only a few printers. This is exactly why we need a spooler program/daemon which will listen for the incoming printing requests, stores them in a spool queue, and then sends them to a printer when it becomes available. The original Berkeley spooling system is The Line Printer Daemon protocol/Line Printer Remote protocol (or LPD) and it is available on any default OpenBSD installation. LPD is super simple and writing a lpd daemon should not be a too difficult for an undergraduate CS student. For those of us who are old enough to remember legendary Richard Stevens https://www.oreilly.com/library/view/advanced-programming-in/9780321638014/ch21.html As the computer technology and printing proliferated among common folks like me some system admins felt the need to develop more complex queueing policies. People start hitting limitations of LPD and eventually Dr. Patrick Powell felt compel to rewrite a new spooler program/daemon which will be more capable of complex printing policies and easier to incorporate drivers and input filters (please see below) so the UNIX world got LPRng http://web.mit.edu/ops/services/print/Attic/src/doc/LPRng-HOWTO.html#toc2 as the project grew and never became truly financially viable eventually was replaced with newer and super complex spooling system called CUPS https://www.cups.org/documentation.html Now the true CUPS claim to fame is the support for the new Internet printing protocol (IPP). 2. What are network printing protocols? >From its inception UNIX was designed to a distributed computing environment. A bunch of developers will use dumb terminals to connect to the same computer and do some work. At the same time it became possible for printers to be first class citizens on the LAN. LPD is not just a spooling system it is also a network protocol spoken by the daemon itself but also spoken by any decent quality printer. The major limitation of LPD that is primarily single direction protocol. As printer became more sophisticated and more like a computers than microcontroller boards it became obvious that one could ask the printer about the level of the toner or the state of key mechanical components (drum comes to mind). Thus we got IPP. Actually, we got more than that. Most so called workgroup printers come with a built in CUPS server. That is not it. Manufacturer came up with many different network protocols. I will mention the one I use JetDirect. From wikipedia page: AppSocket, also known as Port 9100, RAW, JetDirect, or Windows TCPmon is a protocol that was developed by Tektronix. It is considered as 'the simplest, fastest, and generally the most reliable network protocol used for printers 3. What are the printer drivers? Do I need them. In "old good times" all printers were capable of printing raw ASCII code. You don't need any drivers to print raw ASCII text on most business grade printers. As printers became more sophisticated users wanted to print more complicated things like pictures as oppose to ASCII art. One of earliest examples of page description language was stack language developed by Adobe called PostScript (to be discussed more later in this document). A high quality (expensive in old times) printers came with built in interpreters for PostScript language. You don't need a driver to print on such printers. Then various manufacturers came with the reduced set of instructions like Printer Command Langauge (PCL). You do need a driver to print to such printer. The good news is that many such drivers are freely available in https://www.ghostscript.com/ but the story doesn't end there. Printer manufacturers came up with a novel business concept for mass market. They decided to give crappy printers essentially for free, offload all the work to OS/consumers' computers and make a kill by selling ink. You DO need proprietary drivers to print on such devices and they are not useful if you are trying to print from your OpenBSD desktop unless you are planning to smack your co-worker with such a device during a friendly office brawl. High quality open source drivers beside vulnerable GhostScript are part of packages like http://gimp-print.sourceforge.net/ or https://developers.hp.com/hp-linux-imaging-and-printing or even for crappy Samsung printers which need firmware http://splix.ap2c.org/ All these drivers are already ported to OpenBSD and maintained single handedly for over 10-15 years by Anthony Jacotot. 4. What are the Input filters? Do I need them? No you don't need them. As long as you buy a decent quality printer which some with the built in PostScript interpreter and you never send anything to that printer except the PostScript file you are golden. For the rest of us which are less perfect various input filters come to the rescue. The most primitive input filter is just a shell script or Perl script which parses your file and decides on the type of the file. Than it calls an auxiliary program which converts the file to the PostScript language. Finally if the printer doesn't speak complete PostScript but some kind reduced version like PCL it calls the driver to convert the PostScript stream into the language spoken by printer (PCL is very common) before sending it to the printer. This is the simplest possible example of such a filter is shamelessly stollen from NetBSD documentation #!/bin/sh # Treat LF as CR+LF printf "\033&k2G" || exit 2 # Print the postscript file /usr/pkg/bin/gs -dSAFER -dBATCH -dQUIET -dNOPAUSE -q -sDEVICE=cdj550 \ -sOutputFile=- -sPAPERSIZE=a4 - && exit 0 exit 2 If you don't know what is your printer capable of maybe you should learn a bit of SNMP protocol. snmpwalk -c public 10.0.1.19 .1.3.6.1.4.1.2699.1.2 where 10.0.1.19 is the IPv4 of your network printer. This is one of my favorite printing posts and Greg Woods really knows what he is talking about https://marc.info/?l=netbsd-users&m=159389498001287&w=2 I wormly recommend searching mailing lists archives for his posts. I would like to finish this section mentioning a few well known input filters which are not exactly one liners. http://ports.su/print/apsfilter or much newer which I use https://wiki.linuxfoundation.org/openprinting/database/foomatic Be careful with the last one foomatic people disabled the support for LPD a few years ago. That is why whe have to have a work around. Make sure you read pkg-readme which comes with cups-filters (yes they even renamed they stupid filter after the CUPS). 5. ASCII and page description language PostScript(PS) By this section we already mentioned that all decent quality printers should be able to print ASCII code without any drivers. As of page description language PostScript(PS) I would just say that is a true stack based programming language (I think it is inspired by my favorite programming language Forth). https://www.adobe.com/jp/print/postscript/pdfs/PLRM.pdf In "old good times" while growing up behind the Iron curtain I as many other people learnt to hack PostScript files by hand to make them printable to the crappy printers (we could not effort to buy good once from the West). It is also an excellent tool for teaching geometry http://www.math.ubc.ca/~cass/courses/m308-7b/ch1.pdf but these days I only teach my own daughters. A "common trick" in old times would be a getting a PostScript module from a junk yard https://www.ebay.com/p/1625921313 and putting it in the cheap printer, instantaneous converting it to the decent quality printer. 6. What the hack are PostScript Printer Description (PPD) files? Those are not drivers? They are simple see below text files describing capabilities of your printer to the Input filter. *%================================================ *% Copyright(C) 2005 Brother Industries, Ltd. *% "Brother HL-5250DN BR-Script3" *%================================================ *%==== General Information Keywords ======================== *FormatVersion: "4.3" *FileVersion: "1.03" *LanguageEncoding: ISOLatin1 *LanguageVersion: English *Manufacturer: "Brother" *PCFileName: "BR5250_2.PPD" *Product: "(Brother HL-5250DN series)" *PSVersion: "(3010.106) 5" *ShortNickName: "Brother HL-5250DN BR-Script3" *ModelName: "Brother HL-5250DN BR-Script3" *NickName: "Brother HL-5250DN BR-Script3" *1284DeviceID: "MFG:Brother;MDL:HL-5250DN series;CMD:PJL,PCL,PCLXL,POSTSCRIPT;" *%==== Basic Device Capabilities ============= *LanguageLevel: "3" *TTRasterizer: Type42 *ColorDevice: False *DefaultColorSpace: Gray *FileSystem: True *?FileSystem:" save /devname (%disk0%) def /ret false def 0 1 7{ devname exch 48 add 5 exch put devname devstatus { 0 ne {/ret true def}if pop pop pop pop pop pop pop }if }for ret {(True)}{(False)} ifelse = flush restore " *End *Throughput: "28" *FreeVM: "6050000" *%==== Emulations and Protocols ========== *Protocols: PJL TBCP *SuggestedJobTimeout: "0" *SuggestedWaitTimeout: "300" *PrintPSErrors: True 7. Could you give me some recommendation for a printer? Sure. I like Brother monochromatic laser printers like this one. https://www.amazon.com/Brother-HL-5250DN-Network-Printer-Duplex/dp/B000BQ535K They speak PostScript or at least PCL6. They come with a built in CUPS server (you will need to use CUPS on your OpenBSD desktop if you want to print that way) but they also support DirectJet Protocol oko# cat /etc/printcap # Remote printer must use jetdirect since foomatic-rip doesn't speak LPD rp|HL-5250DN:\ :lp=9100@192.168.3.15:\ :if=/etc/foomatic-rip/script_brother.sh:\ :sh:sd=/var/spool/output/brother:\ :lf=/var/log/lpd-errs: oko# cat /etc/foomatic-rip/script_brother.sh #!/bin/sh /usr/local/bin/a2ps -BRq --columns=1 -o - | \ /usr/local/bin/foomatic-rip -P HL-5250DN --ppd \ /etc/foomatic-rip/direct/brother-hl-5250dn-postscript-brother.ppd If you want to print Duplex don't forget to embed %! <</Duplex true>>setpagedevice at the beginning of a PostScript file you are trying to print Or for example using a2ps filter from Xpdf. predrag@oko$ head .xpdfrc # Set the default PostScript file or command. psFile "|a2ps -Prp -1 -s2" psPaperSize letter # launch URLs in Firefox urlCommand "firefox-esr '%s'" # launch movie annotations in mplayer movieCommand "/usr/local/bin/mplayer %s" In "old good times" HP priters were the Gold standard. HP LaserJet 4 lasted me for 15 years and was still usable but getting a bit slow for my work flow. 8. Code contribution? You read this post and you feel like you could contribute to UNIX printing. Why not hack on the LPD itself. At some point Eric Faurot was working on the new lpd server for OpenBSD https://undeadly.org/cgi?action=article;sid=20180509184829 Why don't go step further and implement IPP protocol inside LPD? Who need CUPS? Most Kind Regards, Predrag Punosevac
