Re: ioctl

[raja <vnagaraju@xxxxxxxxxxxx>]

yaa,thank you,
I agree with you but,here we are specifying the value for quantum to be
set with.
I thing here instead of _IO we have to use _IOW() and we have to specify
the lat parameter as int?
Am i right?
Wht is wrong with me?


thanks in advance
raja



On Sat, 2005-10-01 at 00:29 -0300, Ribamar Santarosa de Sousa wrote:
> On 9/30/05, raja <vnagaraju@xxxxxxxxxxxx> wrote:
> > Hi,
> >         I am writing a character device driver similar to that in the book LDD3
> > by rubini.While undersatnding the code I have got some doubt regarding
> > the ioctl implemtntation.
> > In scull.h file it is defined SCULL_IOCTQUANTUM as
> > #define SCULL_IOCTQUANTUM _IO(SCULL_IOC_MAGIC,   3)
> > here while defining there is no arguments for this call.
> > But while implementing that in the ioctl call
> >
> > case SCULL_IOCTQUANTUM: /* Tell: arg is the value */
> >                 if (! capable (CAP_SYS_ADMIN))
> >                         return -EPERM;
> >                 scull_quantum = arg;
> >                 break;
> >
> > but here arg is assigned to scull_quantum.is it the correct?I
> 
> Why not? : - )  Using the logic,  do you think it need or doesn't need
> the arg? Of course it need, or it'd have no functionality!
> 
> See: for the line
> #define SCULL_IOCTQUANTUM _IO(SCULL_IOC_MAGIC,   3)
> the macro (do you understand well what is a macro?) _IO (defined in
> <asm/ioctl.h>  ) will operate the parameters SCULL_IOC_MAGIC and 3 to
> create -- in compilation time, not in run time -- a unique ioctl
> number N. This number N is, so, associated to  SCULL_IOCTQUANTUM, as
> it was 'SCULL_IOCTQUANTUM = N'.  Maybe I didn't understand you, but, 
> SCULL_IOCTQUANTUM is not a call (neither a macro); it's only a single
> value that is assinged to a functionality in the ioctl system call for
> the scull module.
> 
> So, when a user-space application ioctls the device:
> 
> #include <scull.h>
> ioctl (dev, SCULL_IOCTQUANTUM, some_number);
> 
> it's done:
> scull_quantum = some_number
> 
> 
> 
> > And wht is the use of  capable (CAP_SYS_ADMIN)?
> 
> It's a better way to manipulate permissions by the linux. Imagine a
> case where users must have read/write access to device, but they
> mustn't change control parameters of a device. Read the section
> "Capabilities and Restricted Operations", page 144 ldd 3rd edition.
> 
> Regards,
> Riba
> 
> --
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