Filenames of "PRN" (or "PrN" etc) on Win32.

Cautionary note for Win32 users. This isn't restricted to Perl use, but I fell foul of it through Perl, so I'll note it here.

I'm generating filenames of the form "path/to/xyz.ext" programmically. The ".ext" part is constant, and the "xyz" are alpha-numeric. My program would run along fine for a while and then just hang.

After much debugging, the realisation seeped into my conciousness that each time the program hung, the current filename was "path/to/PRN.ext" or "path/to/PrN.ext" or some other combination of "(P|p)(R|r)(N|n)" as the filename.

A long forgotten memory recalled that under DOS, the name "PRN" was a pseudo-handle for the (parallel port?) print device, much like "CON" is a pseudo-handle for the console. I also recalled that these pseudo-handles were defined to be "considered to exists at all levels in the directory structure".

What this piece of legacy crap design means is that any attempt to use "PRN" or "prn" (etc.) as a filename (for output), regardless of it being prefixed with a path, and/or postfixed with an extension will result in a successful open. BUT attempting to write to it will attempt to write to the printer. In my case, where there is no printer attached, the write just hangs. I guess it may eventually time out, but if it does, it takes longer than I have the patience to wait.

As a workaround, pre- or post-fixing the name part with any characters, or switching to "ext.PRN" avoids the problem.

I've long enjoyed the shortness of prog >nul relative to prog >/dev/null, but it has it's downside!

Examine what is said, not who speaks.

"Efficiency is intelligent laziness." -David Dunham
"Think for yourself!" - Abigail
"Memory, processor, disk in that order on the hardware side. Algorithm, algorithm, algorithm on the code side." - tachyon

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Re: Filenames of "PRN" (or "PrN" etc) on Win32. by theorbtwo (Prior) on Aug 15, 2004 at 20:56 UTC
If you're on a copy of windows that has a mem.exe (most seem to, in $windir%\system32 on NTish systems, and %windir%\command on 95ish systems), `mem /d` will, IIRC, list known device names. I don't, however, have a running windows system to test this on. Warning: Unless otherwise stated, code is untested. Do not use without understanding. Code is posted in the hopes it is useful, but without warranty. All copyrights are relinquished into the public domain unless otherwise stated. I am not an angel. I am capable of error, and err on a fairly regular basis. If I made a mistake, please let me know (such as by replying to this node).	[reply] [d/l]
Re^2: Filenames of "PRN" (or "PrN" etc) on Win32. by BUU (Prior) on Aug 15, 2004 at 21:44 UTC
If anyone cares, this the output for me on win2k: `IO CON AUX PRN CLOCK$ COM1 LPT1 LPT2 LPT3 COM2 COM3 COM4 MSDOS IO KBD HIMEM XMSXXXX0 COMMAND MSDOS COMMAND DOSX MEM MEM MSDOS SYSTEM MSDOS MSCDEXNT DOSX VWIPXSPX MSDOS SYSTEM IO MOUSE REDIR NW16 MSDOS` [download]	[reply] [d/l]
Re^3: Filenames of "PRN" (or "PrN" etc) on Win32. by ikegami (Patriarch) on Aug 16, 2004 at 05:11 UTC
Only those under the first IO are relevant: `CON AUX PRN CLOCK\$ LPT\d+ COM\d+` [download] There's also `NUL`, not listed.	[reply] [d/l] [select]
Re^3: Filenames of "PRN" (or "PrN" etc) on Win32. by PhilHibbs (Hermit) on Aug 16, 2004 at 09:54 UTC
I get this on Win2K SP4: Address Name Size Type ------- -------- ------ ------ 000000 000400 Interrupt Vector 000400 000100 ROM Communication Area 000500 000200 DOS Communication Area 000700 IO 000370 System Data CON System Device Driver AUX System Device Driver PRN System Device Driver CLOCK$ System Device Driver COM1 System Device Driver LPT1 System Device Driver LPT2 System Device Driver LPT3 System Device Driver COM2 System Device Driver COM3 System Device Driver COM4 System Device Driver 000A70 MSDOS 001670 System Data 0020E0 IO 003120 System Data KBD 000CD0 System Program ANSI 001060 DEVICE= CON Installed Device Driver HIMEM 0004E0 DEVICE= XMSXXXX0 Installed Device Driver 000490 FILES= 000090 FCBS= 0001B0 LASTDRIVE= 0007D0 STACKS= 005210 COMMAND 000A20 Program 005C40 MSDOS 000070 -- Free -- 005CC0 COMMAND 000BA0 Environment 006870 DOSX 0087A0 Program 00F020 MEM 000AB0 Environment 00FAE0 KB16 0017D0 Program 0112C0 MEM 017550 Program 028820 MSDOS 0777C0 -- Free -- 09FFF0 SYSTEM 02F000 System Program 0CF000 IO 003100 System Data MOUSE 0030F0 System Program 0D2110 MSDOS 000AC0 -- Free -- 0D2BE0 MSCDEXNT 0001D0 Program 0D2DC0 REDIR 000A70 Program 0D3840 DOSX 000080 Data 0D38D0 MSDOS 00C720 -- Free -- 655360 bytes total conventional memory 655360 bytes available to MS-DOS 584992 largest executable program size 1048576 bytes total contiguous extended memory 0 bytes available contiguous extended memory 941056 bytes available XMS memory MS-DOS resident in High Memory Area Oddly, if I don't redirect the output it clears the screen halfway through. Must be generating a spurious character somewhere, which doesn't occur if you > the output.	[reply]
Re^2: Filenames of "PRN" (or "PrN" etc) on Win32. by BrowserUk (Patriarch) on Aug 15, 2004 at 21:05 UTC
Indeed it does (on XP). Thanks, I was unaware of that one. Examine what is said, not who speaks. "Efficiency is intelligent laziness." -David Dunham "Think for yourself!" - Abigail "Memory, processor, disk in that order on the hardware side. Algorithm, algorithm, algorithm on the code side." - tachyon	[reply]
Re: Filenames of "PRN" (or "PrN" etc) on Win32. by gaal (Parson) on Aug 15, 2004 at 17:14 UTC
Oh, my. This reminds me of the BBS days where the 133t kiddies crashed remote systems by uploading junk with the filename `CLOCK$`. This was nasty -- it actually overwrote the CMOS!	[reply] [d/l]