To do PIO, you create a file descriptor by opening one of the special devices in /dev/vme. These files correspond to VME devices. For details on the naming of these files, see the uservme(7) reference page.
The name of the device that you open and pass as the file descriptor determines the bus address space (A16, A24, or A32). The values you specify in off and len must agree with accessible locations in that VME bus space. A read or write to a location in the mapped segment causes a call to the read or write entry of the kernel device driver for VME PIO. An attempt to read or write an invalid location in the bus address space causes a SIGBUS exception to all processes that have mapped the device.
Note: On the CHALLENGE® and Onyx® hardware, PIO reads and writes are asynchronous. Following an invalid read or write, as much as 10 milliseconds can elapse before the SIGBUS signal is raised. For a detailed discussion of VME PIO, see the IRIX Device Driver Programmer's Guide.
Note: Mapping of devices through mmap() is an IRIX feature that is not defined by POSIX standard. Do not use the POSIX shm_open() function with device special files.