Linux Device Drivers

Interface-Specific Types

Most commonly used data types in the kernel have their own typedef statement, 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. I use the expression ``interface-specific'' to refer to the programming interface to specific data items.

Other data items that belong to a specific ``standard'' type can be considered interface-specific as well. A jiffy count, for instance, is always unsigned long, independent of its actual size--would you like using jiffy_t every so often? I’m concentrating here on the first class of interface-specific types, the ones ending in _t.

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. For example, how would you print a size_t which is unsigned long on some platforms ...