which errno should I use

which errno should I use

[Ralph Blach]

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I am writing a character device driver.  I want this device driver to
return an error if the user request less than say 1024 byte of data.
What errno should I use?  Is it legal to return an error if not enouth
bytes are requested?

Thanks

Chip

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Re: which errno should I use

[Wolfgang Denk]

Hi Ralph,

in message <3A76F163.406CBFDB@raleigh.ibm.com> you wrote:
>
> I am writing a character device driver.  I want this device driver to
> return an error if the user request less than say 1024 byte of data.
> What errno should I use?  Is it legal to return an error if not enouth

EINVAL (Invalid argument) is often used in this situation.

> bytes are requested?

Sure. Many character devices have certain restrictions on the I/O size.

Wolfgang Denk

--
Software Engineering:  Embedded and Realtime Systems,  Embedded Linux
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RE: which errno should I use

[Ian Abbott]

On Tuesday, January 30, 2001 4:53 PM, Ralph Blach
<rcblach@raleigh.ibm.com> wrote:

> I am writing a character device driver.  I want this device driver to
> return an error if the user request less than say 1024 byte of data.
> What errno should I use?  Is it legal to return an error if not enouth
> bytes are requested?

I have a similar driver that wants exactly one long's worth of data to
be read. If the supplied buffer is smaller than a long, I return 0,
otherwise I return sizeof(long) (assuming their are no other errors).
Admittedly this does not let the user level distinguish "buffer too
small" from "no data", but I let the programmer at the user level deal
with that! Just mention it in the documentation for your driver, if
there is any.

Wolfgang Denk suggested EINVAL. I didn't use that as the man page for
read(2) says that means "fd is attached to an object which is unsuitable
for reading." There's no harm in overloading this meaning though. Just
document what your driver does.

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RE: which errno should I use

[Ron Bianco]

Hi Ian,

In your driver's read() fileop implementation, assuming you're using a non-blocking
technique, if the count parameter is not 1024 just return 0 for num byes written to
user's buffer.  The error codes are actually used as -EINVAL for example, any
positive return value or 0 indicates the bytes actually processed.   Also if you're
intending the user program to select() your device then you need to implement the
poll() fileop.

I just finished writing a non-blocking char device driver, so can offer some tips if
need be.

Cheers, Ron

Ron Bianco
Computer Systems Technologist
Level Control Systems
email: ronb@junction.net
phone: 250-549-2558 Ext 8
web:  www.lcsaudio.com


> -----Original Message-----
> From: owner-linuxppc-embedded@lists.linuxppc.org
> [mailto:owner-linuxppc-embedded@lists.linuxppc.org]On Behalf Of Ian
> Abbott
> Sent: Wednesday, January 31, 2001 10:48 AM
> To: Embedded Linux list
> Subject: RE: which errno should I use
>
>
>
> On Tuesday, January 30, 2001 4:53 PM, Ralph Blach
> <rcblach@raleigh.ibm.com> wrote:
>
> > I am writing a character device driver.  I want this device driver to
> > return an error if the user request less than say 1024 byte of data.
> > What errno should I use?  Is it legal to return an error if not enouth
> > bytes are requested?
>
> I have a similar driver that wants exactly one long's worth of data to
> be read. If the supplied buffer is smaller than a long, I return 0,
> otherwise I return sizeof(long) (assuming their are no other errors).
> Admittedly this does not let the user level distinguish "buffer too
> small" from "no data", but I let the programmer at the user level deal
> with that! Just mention it in the documentation for your driver, if
> there is any.
>
> Wolfgang Denk suggested EINVAL. I didn't use that as the man page for
> read(2) says that means "fd is attached to an object which is unsuitable
> for reading." There's no harm in overloading this meaning though. Just
> document what your driver does.
>
>


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