17.2 ioctl Function

This function affects an open file referenced by the fd argument.

#include <unistd.h>
int ioctl(int fd, int request, ... /* void *arg */ );
Returns:0 if OK, -1 on error

The third argument is always a pointer, but the type of pointer depends on the request.

4.4BSD defines the second argument to be an unsigned long instead of an int, but that is not a problem since header files define the constants that are used for this argument. As long as the prototype is in scope (i.e., the program using ioctl has included <unistd.h>), the correct type for the system will be used.

Some implementations specify the third argument as a void * pointer instead of the ANSI C ellipsis notation.

There is no standard for the header to include to define the function prototype for ioctl since it is not standardized by POSIX. Many systems define it in <unistd.h>, as we show, but traditional BSD systems define it in <sys/ioctl.h>.

We can divide the requests related to networking into six categories:

• Socket operations

• File operations

• Interface operations

• ARP cache operations

• Routing table operations

• STREAMS system (Chapter 31)

Recall from Figure 7.20 that not only do some of the ioctl operations overlap some of the fcntl operations (e.g., setting a socket to nonblocking), but there are also some operations that can be specified more than one way using ioctl (e.g., setting the process group ownership of a socket).

Figure 17.1 lists the requests, along with the datatype of what the arg address must point to. The following sections describe these requests in more detail.