We have seen that the ioctl system call is implemented for sockets; SIOCSIFADDR and SIOCSIFMAP are examples of “socket ioctls.” Now let’s see how the third argument of the system call is used by networking code.

When the ioctl system call is invoked on a socket, the command number is one of the symbols defined in <linux/sockios.h>, and the function sock_ioctl directly invokes a protocol-specific function (where “protocol” refers to the main network protocol being used, for example, IP or AppleTalk).

Any ioctl command that is not recognized by the protocol layer is passed to the device layer. These device-related ioctl commands accept a third argument from user space, a struct ifreq *. This structure is defined in <linux/if.h>. The SIOCSIFADDR and SIOCSIFMAP commands actually work on the ifreq structure. The extra argument to SIOCSIFMAP, although defined as ifmap, is just a field of ifreq.

In addition to using the standardized calls, each interface can define its own ioctl commands. The plip interface, for example, allows the interface to modify its internal timeout values via ioctl. The ioctl implementation for sockets recognizes 16 commands as private to the interface: SIOCDEVPRIVATE through SIOCDEVPRIVATE+15.

When one of these commands is recognized, dev->do_ioctl is called in the relevant interface driver. The function receives the same struct ifreq * pointer that the general-purpose ioctl function uses:

int (*do_ioctl)(struct net_device ...