Linux Device Drivers

Assigning an Explicit Size to Data Items

Sometimes kernel code requires data items of a specific size, either to match binary structures^[27] or to align data within structures by inserting ``filler'' fields.

The kernel offers the following data types for this purpose, all declared in <asm/types.h>, which in turn is included by <linux/types.h>:

u8;   /* unsigned byte (8 bits) */
u16;  /* unsigned word (16 bits) */
u32;  /* unsigned 32-bit value */
u64;  /* unsigned 64-bit value */

These data types are accessible only from kernel code (i.e., __KERNEL__ must be defined before including <linux/types.h>). The corresponding signed types exist, but are rarely needed; just replace u with s in the name if you need them.

If a user-space program needs to use these types, it can prefix the names with a double underscore: __u8 and the other types are defined independent of __KERNEL__. If, for example, a driver needs to exchange binary structures with a program running in user space by means of ioctl, the header files should declare 32-bit fields in the structures as __u32.

It’s important to remember that these types are Linux-specific, and using them hinders porting software to other Unix flavors. There are nonetheless situations where explicit data sizing is needed, and the standard header files (the ones you find on every Unix system) don’t declare suitable data types.

You might also note that sometimes the kernel uses conventional types, like unsigned int, for items whose dimension is architecture-independent. ...