lp0 on fire

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lp0 on fire (aka Printer on Fire) is a semi-obsolete error message still generated on some Unix/Linux operating systems in response to certain types of printer errors. lp0 is the Unix device handle for the first line printer, but the error can be displayed for any printer attached to a Unix/Linux system.

The origin of the "on fire" message was in the 1970's when line printers were large mechanical affairs with a high speed drum rotating at 1200 to 2400 RPM and impact printing heads. Misaligned operating components could cause the paper to come into direct contact with the high speed rotating parts, generating quite a bit of paper dust and increasing the likelihood of a paper jam. If a jam were not detected soon enough, the accumulated paper dust, ink dust and paper could generate enough friction along the rotating drum to start a fire. Furthermore, the cleaning solutions used in the printers were usually alcohol based, the fumes of which also presented a fire hazard. However, it must be noted that there have never been any actual reports of printers which had friction related fires.

The line printer employed a series of status codes, specifically ready, online, and check. If the online status were set to "off" and the check status were set to "on," the operating system would interpret this as the printer running out of paper. However, if the online code were set to "on" along with the check code also set to "on" this would mean that the printer still has paper, but is suffering an error, and is still running. Due to the hazardous conditions which could arise in early line printers, Unix displayed the message "on fire", which would serve to motivate any system operator viewing the message to go and check on the line printer immediately.^[1]

While modern inkjet and laser printers are nowhere near as flammable as their mainframe ancestors, the lp device code in all sorts of POSIX compliant operating systems retained this legacy message. As printer drivers and support improved this error condition could be trapped and handled appropriately, and as a result, most Unix/Linux users today have never encountered the "on fire" message. However, the message is still present in many kernels, especially with legacy parallel port device support, and continues to baffle users to this day.^[2]

The "on fire" message is also present in other components, often to a humorous effect. For example, in some kernels' CPU code, a CPU thermal failure could result in the message "CPU#0: Possible thermal failure (CPU on fire ?)." ^[3]