Linux ENOTTY (errno 25) — Inappropriate ioctl for Device

ENOTTY (errno 25) means the ioctl() system call is not appropriate for the type of file descriptor. This error occurs when you attempt a terminal-related operation on a non-terminal device, such as a pipe, socket, or regular file. It is distinct from ENOTTY (errno 25) being reused for “Inappropriate ioctl” because the original meaning was “Not a typewriter.”

Common Causes

  • Calling tcgetattr() or tcsetattr() on a non-terminal file descriptor
  • Using terminal-specific ioctls on pipes or sockets
  • Redirecting stdin/stdout to a file and then using terminal operations
  • A pseudo-terminal (PTY) is not properly set up

How to Fix ENOTTY

1. Verify File Descriptor is a Terminal

Check if the file descriptor refers to a terminal:

test -t 0 && echo "stdin is a terminal" || echo "stdin is not a terminal"

2. Check if Running in a TTY

Verify the process has an attached terminal:

tty

3. Use isatty() in Code

Check programmatically before calling terminal ioctls:

if (!isatty(fd)) {
    fprintf(stderr, "Error: file descriptor is not a terminal\n");
    return -1;
}

4. Ensure Proper TTY Assignment

If using a script that needs a terminal, allocate one:

script -q /dev/null

Verification

After ensuring the file descriptor is a terminal, retry the operation:

tty && tput cols

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