ioctl(9E) Considerations

Most driver entry points are managed by the 32-bit and 64-bit system caller handlers in the kernel in such a way that a driver does not need to be concerned about whether it is performing an operation on behalf of a 32-bit or a 64-bit application.

However, ioctl(9E) offers a direct connection between applications and the kernel. It enables a driver to implement device-specific operations. That is, it can cause the driver to perform a device-specific command or to pass arbitrary data between the driver and the application.

The third argument to ioctl() is either a simple integral value or a pointer to some other value, typically a data structure. The data structure might be different in size and alignment between a 32-bit and a 64-bit application. Because the form of the interface between the driver and application is generally a private agreement between the driver and the application, the kernel cannot intervene to automatically translate the data structures. It cannot even tell whether the argument is an integer or a pointer.

Therefore, drivers and STREAMS modules need to know how to interpret the data structures passed in from an application.