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 -- Kernelnewbies: Help each other learn about the Linux kernel. Archive: http://mail.nl.linux.org/kernelnewbies/ FAQ: http://kernelnewbies.org/faq/
