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PRINT HINTS

Sending PostScript Files to a LaserWriter

Dave Polaschek

A question that popped up pretty often for me when I worked in Apple's Developer Technical Support group, and that continues to appear in the Mac programming newsgroups on the Internet, is "How do I send PostScript TM files to a LaserWriter?" The simplest answer appears to be to use the PostScriptHandle picture comment. Wrong! In the Print Hints column "The Top 10 Printing Crimes Revisited" in develop Issue 26, I said that this would be a misuse of the PostScriptHandle picture comment, but I didn't give a full explanation of just how to send files to a printer. This column will explain how to send complete PostScript files the correct way.

First, though, I'd like to backtrack a little and explain more thoroughly why you shouldn't use the picture comment to send complete PostScript files. After all, if you do implement code that uses this technique, the files get to the printer, pages come out, and things seem mostly to work. What's the problem?

WHY NOT USE POSTSCRIPTHANDLE?

The problem with using the PostScriptHandle picture comment to send files to a printer is that you're using the Printing Manager to do some of the work for you and then bypassing it unexpectedly. The problems that will show up when you do this may not be obvious, but when a user does notice them it can lead to confusion.

First, when you use the PostScriptHandle picture comment, the LaserWriter driver has already set up the PostScript state for QuickDraw imaging. It has changed the coordinate system and has sent down the md dictionary that it will need to draw pages. At best, this is extra baggage that your PostScript files don't need in the way. At worst, since the driver has changed the coordinate system, your PostScript file may not print correctly. There's also a chance that the extra memory used by the LaserWriter's PostScript dictionary may cause your job not to print at all.

Second, using PostScriptHandle is wasteful. If background printing is enabled, the complete PostScript file, plus the extra things you don't need, will be spooled to the user's hard drive. Since what you're interested in is just getting the file to the printer, this is wasteful. In some cases, the job won't even be able to print, since your user won't have enough free space on the hard drive to hold a second copy of the PostScript file.

Finally, there's an aesthetic problem. The Printing Manager counts the pages that are going to be sent by looking at how many times PrOpenPage is called during your print job. If you've got a multiple-page PostScript file, the page count displayed by the Printing Manager will be out of whack. While messages like "Printing Page 4 of 1" won't actually hurt the user, spreading confusion is bad.

THE RIGHT WAY(S)

There are actually two right ways to send PostScript files to the printer. They're essentially the same in that you open a connection directly to the printer and send it the file you want to print, but the implementations are very different. You can either use classic networking and the PAPWorkStation.o library, or you can use Open Transport, which contains support for the PAP protocol. (For more on PAP, see "PAP? What's That?")

    PAP? WHAT'S THAT?

    The Printer Access Protocol (PAP) is the protocol spoken by all LaserWriters (and third-party Macintosh-compatible PostScript printers). It's designed around a few simple ideas:

    • Only one computer can be talking to the printer at once, so there's a unique connection between the two.
    • When the printer is ready to receive some data, it will ask the computer for that data. Similarly, when the printer wants to send data back to the computer, the computer must have already asked for that data. In other words, a read must always be active.
    • There's a separate status channel for the printer to report things like "Printer On Fire" or "Paper Jam."

    This last point means that there are actually multiple connections open for the one connection to the printer. This is a major source of the complexity of the code in the classic networking case. Open Transport hides this complexity from you.

    You can find out more about PAP in Chapter 10 of Inside AppleTalk.


The classic networking solution has the advantage that it's available in all Macintosh computers, out of the box. But it's more difficult to implement. Since you're working at a lower level, there's more room for you to get things wrong -- but there's also less worry of having library code that doesn't do things the way you want. Note that your code (at least part of it) can't be PowerPC native, because the libraries are 680x0 only.

Open Transport is a much simpler way to implement sending PostScript files to your printer. You talk to the networking library at a higher level, and you get a PowerPC-native implementation of networking. But of course you lose compatibility with older Macintosh models.

We'll look at both these solutions; sample code for each one accompanies this column on this issue's CD and develop's Web site. The choice of which technique to use is up to you. Note also that there may be other, more attractive alternatives in the future. For now, however, these are the two best options.

The sample code for the classic networking solution is the SendPS tool -- an MPW tool rather than a full application, but still useful for demonstrating how things work. The process is also described in the MacTutor article "Laser Print DA for PostScript." If you're interested in the nitty-gritty details, consulting the code and that article is your best bet. Here I'm just going to give an overview of the code. In a few cases, I'll make suggestions for how you might enhance the code to make a real-world application.

The main thing you need to understand about the PAP library that Apple supplies for classic networking is that it reports the connection status to you via a few variables rather than by way of completion functions. It's not truly asynchronous code, so if you want to write a program that will run happily in the background, sending PostScript files to a printer, you've got some extra work to do. It's possible, but a little tricky.

When we're in the process of opening a connection (in the code near the call to PAPOpen in sendps.c), we look at the wstate variable to tell us what's happening. All these state variables have three basic states: a positive value means we're waiting for the printer; negative values are errors; and 0 means the operation we're watching has completed. We also call PAPStatus periodically to get the printer's status so that we can display it to the user.

Once the connection is opened, we issue a PAPRead call. This is necessary because when the printer wants to talk to us, it can't just tell us it's ready to talk, but rather we must have asked it for some data first. (Remember, PAP is a protocol where the parties have to ask for data.) We check the rstate variable periodically to see if the printer has said anything to us. When it does, we need to read the data it has sent us and issue another PAPRead call.

Now we're ready to start sending blocks of data. We issue a PAPWrite call, and for each call, we watch the wstate variable to see when the write is done. While we're waiting for the writes to complete, we need to remember to check the printer's status and display it to the user (our write might be "stuck" because the printer is out of paper and can't print a page, for example; the user should see this so that she can replenish the paper).

Once we've sent all our data, we send the end-of-file to the printer. We do this by sending a packet with no data, but with the eof flag set. If we've got another job to send, there's no need to close the connection and reopen it; we can just start sending it as soon as the printer acknowledges our end-of-file.

That's the quick overview of the SendPS tool. If you need more guidance, see the source code; I've tried to comment it so that everything will make sense.

    You're not allowed to redistribute the PAPWorkStation.o library included with the classic networking sample code without first licensing it from Apple. To license the code, contact sw.license@apple.com for more details. It's really not that expensive, and you don't want to write it all yourself. Trust me.*

THE OPEN TRANSPORT CODE

The main code for the Open Transport solution is considerably easier to follow than the classic networking code, primarily because you don't have to worry about multiple state variables in order to send. You simply create PAP and PAPStatus endpoints (connections), and the main loop calls the Snd and Rcv functions for the PAP endpoint. There's a callback function that gets notified when the state of the endpoint changes and then sets a single state variable to match the state of the endpoint. Exception handling can be localized to this callback, which makes the code easier to read, and the status reporting can be handled in one place as well.

Because we don't need to continually poll the state variables, it's much easier to fit this code into an application, and the application will cooperate better with other applications. Getting good performance is also quite a bit easier with the Open Transport-based code than with the classic networking code, since you don't have to worry about waiting in the wrong place, polling a state variable.

WRAPPING UP

Implementing code to send PostScript files directly to a printer is more difficult than using the PostScriptHandle picture comment, but it offers a number of benefits to your users. The process is more straightforward, no extra print dialogs need to be shown, and no extra storage is required for spool files. You can provide as much or as little status information as you like (for example, you could add progress bars showing how much of the job is completed). And performance should improve in most cases, since you'll be talking directly to the printer, rather than through the layer of the Printing Manager. Convinced? I should hope so.


    RECOMMENDED READING

    • Inside Macintosh: Open Transport and Open Transport Client Developer Note (Apple Computer, Inc., 1996), available on the Open Transport Web site, http://www.devworld.apple.com/dev/opentransport/.
    • Inside Macintosh: Networking by Apple Computer, Inc. (Addison-Wesley, 1994) and Inside AppleTalk, 2nd ed., by Apple Computer, Inc. (Addison-Wesley, 1990).
    • Macintosh Technical Q&A QD 35, "Determining the Selected Printer's Address."
    • "Laser Print DA for PostScript" by Mike Schuster, MacTutor Vol. 2, No. 2 (February 1986). Articles from MacTutor, since renamed MacTech Magazine, can be found at http://web.xplain.com/mactech.com/magazine/features/articlearchives.html.
    • Lost Beauties of the English Language, by Charles Mackay, LL.D. (London: Bibliophile Books, 1987). Originally published by Chatto & Windus in 1874.
    • The UNIX-Haters Handbook by Simson Garfinkel, Daniel Weise, and Steven Strassman (IDG Books, 1994).


DAVE POLASCHEK (davep@best.com, http://www.best.com/~davep/) now works for LaserMaster Corp., where he does "Mac things" that confuse the people who actually build printers, since Dave seems to spend most of his time designing spiffy icons and then dragging them around his screen. In his spare time, Dave scrapes ice off his windshield, jump-starts cars, and de-ices locks. Earlier this year he cut a hole in a lake and spent hours sitting on the ice waiting for fish to find his hook (while he consumed a judicious amount of antifreeze). Who said life in Minnesota isn't exciting?*



Thanks to Rich Kubota, Quinn "The Eskimo!", Steve Simon, and Tony Wingo for reviewing this column.*

 
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