Linux Device Drivers, Second Edition

Interface-Specific Types

Most of the commonly used data types in the kernel have their own typedef statements, thus preventing any portability problems. For example, a process identifier (pid) is usually pid_t instead of int. Using pid_t masks any possible difference in the actual data typing. We use the expression interface-specific to refer to a type defined by a library in order to provide an interface to a specific data structure.

Even when no interface-specific type is defined, it’s always important to use the proper data type in a way consistent with the rest of the kernel. A jiffy count, for instance, is always unsigned long, independent of its actual size, so the unsigned long type should always be used when working with jiffies. In this section we concentrate on use of “_t” types.

The complete list of _t types appears in <linux/types.h>, but the list is rarely useful. When you need a specific type, you’ll find it in the prototype of the functions you need to call or in the data structures you use.

Whenever your driver uses functions that require such “custom” types and you don’t follow the convention, the compiler issues a warning; if you use the -Wall compiler flag and are careful to remove all the warnings, you can feel confident that your code is portable.

The main problem with _t data items is that when you need to print them, it’s not always easy to choose the right printk or printf format, and warnings you resolve on one architecture reappear on another. ...