Linux Device Drivers, Second Edition

Assigning an Explicit Size to Data Items

Sometimes kernel code requires data items of a specific size, either to match predefined binary structures^[39] or to align data within structures by inserting “filler” fields (but please refer to Section 10.4.4 later in this chapter for information about alignment issues).

The kernel offers the following data types to use whenever you need to know the size of your data. All the types are 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. Systems with recent compilers will support the C99-standard types, such as uint8_t and uint32_t; when possible, those types should ...