11.2. Assigning an Explicit Size to Data Items

Sometimes kernel code requires data items of a specific size, perhaps to match predefined binary structures,^[1] to communicate with user space, or to align data within structures by inserting "padding" fields (but refer to the Section 11.4.4 for information about alignment issues).

^[1] This happens when reading partition tables, when executing a binary file, or when decoding a network packet.

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 */

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 support the C99-standard types, such as uint8_t and uint32_t; if portability is a concern, those types can be used in favor of the Linux-specific variety.

You might also note that sometimes the kernel uses conventional types, such as unsigned int, for items whose dimension is architecture independent. This is usually done for backward compatibility. When u32 and friends were introduced in Version 1.1.67, the developers couldn't change existing data structures to the new types because the compiler issues a warning when there is a type mismatch between the structure field and the value being assigned to it.^[2] Linus didn't expect the operating system (OS) he wrote for his own use to become multiplatform; as a result, old structures are sometimes loosely typed.

^[2] As a matter of fact, the compiler signals type inconsistencies even if the two types are just different names for the same object, such as unsigned long and u32 on the PC.