Re: what about _IOC_SIZE ??

Brian Gerst (
Thu, 25 Mar 1999 16:50:28 -0500

Fabien Klein wrote:
> Hello
> Can someone tell me the meaning of the _IOC_SIZE(cmd) value?

The ioctl numbers encode 4 values:
direction (read/write)
type (usually related to the major number)
size (size of argument)

Having the size of the parameter of an ioctl makes it easy to use
verify_{read,write} without needing to use a switch statement for each

For example, from <asm-i386/ioctls.h>:
#define TIOCGPTN _IOR('T',0x30, unsigned int)
/* Get Pty Number (of pty-mux device) */

This means that the parameter passed via ioctl is a pointer to an
unsigned int, and thus will have a size of 4 (on an i386).

An even better example, using a variable size for a string buffer, from
#define JSIOCGNAME(len) _IOC(_IOC_READ, 'j', 0x13, len)

This allows you to pass the length of your string buffer to the ioctl,
instead of having to use something a parameter block.


Brian Gerst

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