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>Hello World (part 3): The <TT
CLASS="LITERAL"
>__init</TT
> and <TT
CLASS="LITERAL"
>__exit</TT
> Macros</H1
><A
NAME="AEN285"
></A
><A
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></A
><P
>This demonstrates a feature of kernel 2.2 and later. Notice the change in the definitions of the init and cleanup
functions. The <TT
CLASS="FUNCTION"
>__init</TT
> macro causes the init function to be discarded and its memory freed once the init
function finishes for built-in drivers, but not loadable modules. If you think about when the init function is invoked, this
makes perfect sense.</P
><P
>There is also an <TT
CLASS="FUNCTION"
>__initdata</TT
> which works similarly to <TT
CLASS="FUNCTION"
>__init</TT
> but for init
variables rather than functions.</P
><P
>The <TT
CLASS="FUNCTION"
>__exit</TT
> macro causes the omission of the function when the module is built into the kernel, and
like <TT
CLASS="FUNCTION"
>__exit</TT
>, has no effect for loadable modules. Again, if you consider when the cleanup function runs,
this makes complete sense; built-in drivers don't need a cleanup function, while loadable modules do.</P
><P
>These macros are defined in <TT
CLASS="FILENAME"
>linux/init.h</TT
> and serve to free up kernel memory.
When you boot your kernel and see something like <TT
CLASS="LITERAL"
>Freeing unused kernel memory: 236k freed</TT
>, this is
precisely what the kernel is freeing.</P
><A
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></A
><P
><B
>Example 2-5. hello-3.c</B
></P
><PRE
CLASS="PROGRAMLISTING"
>/* hello-3.c - Illustrating the __init, __initdata and __exit macros.
*/
#include <linux/module.h> /* Needed by all modules */
#include <linux/kernel.h> /* Needed for KERN_ALERT */
#include <linux/init.h> /* Needed for the macros */
static int hello3_data __initdata = 3;
static int __init hello_3_init(void)
{
printk(KERN_ALERT "Hello, world %d\n", hello3_data);
return 0;
}
static void __exit hello_3_exit(void)
{
printk(KERN_ALERT "Goodbye, world 3\n");
}
module_init(hello_3_init);
module_exit(hello_3_exit);</PRE
></DIV
><P
>By the way, you may see the directive "<TT
CLASS="FUNCTION"
>__initfunction()</TT
>" in drivers written for Linux 2.2
kernels:</P
><PRE
CLASS="SCREEN"
> __initfunction(int init_module(void))
{
printk(KERN_ALERT "Hi there.\n");
return 0;
}</PRE
><P
>This macro served the same purpose as <TT
CLASS="FUNCTION"
>__init</TT
>, but is now very deprecated in favor of
<TT
CLASS="FUNCTION"
>__init</TT
>. I only mention it because you might see it modern kernels. As of 2.4.18, there are 38
references to <TT
CLASS="FUNCTION"
>__initfunction()</TT
>, and of 2.4.20, there are 37 references. However, don't use it in your
own code.</P
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