2.6. Preliminaries

We are getting closer to looking at some actual module code. But first, we need to look at some other things that need to appear in your module source files. The kernel is a unique environment, and it imposes its own requirements on code that would interface with it.

Most kernel code ends up including a fairly large number of header files to get definitions of functions, data types, and variables. We'll examine these files as we come to them, but there are a few that are specific to modules, and must appear in every loadable module. Thus, just about all module code has the following:

#include <linux/module.h>
#include <linux/init.h>

module.h contains a great many definitions of symbols and functions needed by loadable modules. You need init.h to specify your initialization and cleanup functions, as we saw in the "hello world" example above, and which we revisit in the next section. Most modules also include moduleparam.h to enable the passing of parameters to the module at load time; we will get to that shortly.

It is not strictly necessary, but your module really should specify which license applies to its code. Doing so is just a matter of including a MODULE_LICENSE line:

MODULE_LICENSE("GPL");

The specific licenses recognized by the kernel are "GPL" (for any version of the GNU General Public License), "GPL v2" (for GPL version two only), "GPL and additional rights," "Dual BSD/GPL," "Dual MPL/GPL," and "Proprietary." Unless your module is explicitly marked as being under a free license recognized by the kernel, it is assumed to be proprietary, and the kernel is "tainted" when the module is loaded. As we mentioned in Section 1.6, kernel developers tend to be unenthusiastic about helping users who experience problems after loading proprietary modules.

Other descriptive definitions that can be contained within a module include MODULE_AUTHOR (stating who wrote the module), MODULE_DESCRIPTION (a human-readable statement of what the module does), MODULE_VERSION (for a code revision number; see the comments in <linux/module.h> for the conventions to use in creating version strings), MODULE_ALIAS (another name by which this module can be known), and MODULE_DEVICE_TABLE (to tell user space about which devices the module supports).

The various MODULE_ declarations can appear anywhere within your source file outside of a function. A relatively recent convention in kernel code, however, is to put these declarations at the end of the file.