Not a Typewriter
"In computing, 'Not a typewriter' or ENOTTY
is an error code defined in the errno.h
found on many Unix systems. This code is now used to indicate that an invalid ioctl (input/output control) number was specified in an ioctl system call." Reference: https://en.wikipedia.org/wiki/Not_a_typewriter
NuttX
In ioctl() implementations in NuttX, -ENOTTY
is always returned if the ioctl()
command is not recognized. You will often see driver ioctl()
implement ions with a general structure similar to the following:
int driver_ioctl(FAR struct file *filep, int cmd, unsigned long arg)
{
int ret;
switch (cmd)
{
...
default:
ret = -ENOTTY;
break;
}
}
return ret;
}
Note that -ENOTTY
is returned internally in NuttX. This will subsequently be used to set the errno
value to ENOTTY
and to return -1 to indicate the error condition.
ERRORS
These are the return values from a Linux ioctl() call:
EBADF fd is not a valid file descriptor.
EFAULT argp references an inaccessible memory area.
EINVAL request or argp is not valid.ENOTTY fd is not associated with a character special device.
ENOTTY The specified request does not apply to the kind of object
that the file descriptor fd references.
Reference: https://www.man7.org/linux/man-pages/man2/ioctl.2.html
Linux Explains All
On Jun 27 Linus Torvalds wrote:
The correct error code for "I don't understand this ioctl" is ENOTTY. The naming may be odd, but you should think of that error value as a "unrecognized ioctl number, you're feeding me random numbers that I don't understand and I assume for historical reasons that you tried to do some tty operation on me". ... The EINVAL thing goes way back, and is a disaster. It predates Linux itself, as far as I can tell. You'll find lots of man-pages that have this line in it: EINVAL Request or argp is not valid. and it shows up in POSIX etc. And sadly, it generally shows up _before_ the line that says ENOTTY The specified request does not apply to the kind of object that the descriptor d references. so a lot of people get to the EINVAL, and never even notice the ENOTTY. ... At least glibc (and hopefully other C libraries) use a _string_ that makes much more sense: strerror(ENOTTY) is "Inappropriate ioctl for device"
Reference: https://lore.kernel.org/patchwork/patch/258361/
How is this useful?
Knowing that no error occurred but the ioctl() command was not recognized is a useful piece of information. Suppose, for example, I have nfds
open character drivers in an array fd[].
Then I could do something like this:
int do_command(FAR int *fd, int nfds, int cmd)
{
int ret;
int i;
/* Try all file descriptors */
for (i = 0; i < nfds; i++)
{
ret = ioctl(fd[i], cmd, 0ul); /* No argument in this example */ if (ret < 0)
{
int errcode = errno;
/* Try the next file descriptor if this one return ENOTTY */
if (errcode != ENOTTY)
{
/* Other errors, including EINVAL, are fatal */
return -errcode
}
}
else if (ret >= 0)
{ return OK; /* Success! */
}
}
return -ENOENT;
}