Understanding Linux Network Internals

Optimized Macro-Based Tagging

The Linux kernel uses a variety of different macros to mark functions and data structures with special properties: for instance, to mark an initialization routine. Most of those macros are defined in include/linux/init.h. Some of those macros tell the linker to place code or data structures with common properties into specific, dedicated memory areas (memory sections) as well. By doing so, it becomes easier for the kernel to take care of an entire class of objects (routines or data structures) with a common property in a simple manner. We will see an example in the section "Memory Optimizations."

Figure 7-3 shows some of the kernel memory sections: on the left side are the names of the pointers that delimit the beginning and the end of each area section (when meaningful).

Figure 7-3. Some of the memory sections used by initialization code

On the right side are the names of the macros used to place data and code into the associated sections. The figure does not include all the memory sections and associated macros; there are too many to list conveniently.

Tables 7-1 and 7-2 list some of the macros used to tag routines and data structures, respectively, along with a brief description. We will not look at all of them for lack of space, but we will spend a few words on the xxx _initcall macros in the section "xxx_initcall Macros" and on _ _init and _ _exit in ...