> What is the maximum size of the buffer that
> can be passed to ioctl?

From <sys/ioccom.h>:

/*
 * Ioctl's have the command encoded in the lower word, and the size of
 * any in or out parameters in the upper word.  The high 3 bits of the
 * upper word are used to encode the in/out status of the parameter.
 */
#define IOCPARM_MASK    0x1fff          /* parameter length, at most 13 bits */

Hope this helps,

Martin