FreeBSD Device Drivers

Defining ioctl Commands

To define an ioctl command, you’d call one of the following macros: _IO, _IOR, _IOW, or _IOWR. An explanation of each macro is provided in Table 3-1.

Table 3-1. ioctl Command Macros

Macro	Description
_IO	Creates an ioctl command for an I/O operation that transfers no data—in other words, the data argument in d_ioctl will be unused—for example, ejecting removable media
_IOR	Creates an ioctl command for a read operation; read operations transfer data from the device to user space; for example, retrieving error information
_IOW	Creates an ioctl command for a write operation; write operations transfer data to the device from user space; for example, setting a device parameter
_IOWR	Creates an ioctl command for an I/O operation with bidirectional ...