Not a typewriter

In computing, "Not a typewriter" or ENOTTY[1] is an error code defined in the errno.h found on many Unix systems.

In Version 6 UNIX and earlier, I/O control was limited to serial-connected terminal devices, typically a teletype (abbreviated TTY), through the gtty and stty system calls.

[3][4] The abbreviation TTY, which occurs widely in modern UNIX systems, stands for "Teletypewriter."

One of the first system calls made by the C standard I/O library is in an isatty() call used to determine if the program is being run interactively by a human (in which case isatty() will succeed and the library will write its output a line at a time so the user sees a regular flow of text) or as part of a pipeline (in which case it writes a block at a time for efficiency).

If a library routine fails for some reason unrelated to a system call (for example, because a user name wasn't found in the password file) and a naïve programmer blindly calls the normal error reporting routine perror() on every failure, the leftover ENOTTY will result in an utterly inappropriate "Not a typewriter" (or "Not a teletype", or "Inappropriate ioctl for device") being delivered to the user.