__init 的用法
[原文:http://blog.csdn.net/eroswang/archive/2008/04/23/2317771.aspx]
在kernel中有很多__init，这个东东到底是何方神圣捏？且听小生我一一道来。
下面是其定义：
file:/include/linux/init.h
43 #define __init __attribute__ ((__section__ (".init.text"))) __cold
44 #define __initdata __attribute__ ((__section__ (".init.data")))
45 #define __exitdata __attribute__ ((__section__(".exit.data")))
46 #define __exit_call __attribute_used__ __attribute__ ((__section__ (".exitcall.exit")))
也许你会问那 __attribute__ ((__section__ (".init.text"))) __cold是什么东东阿?
且看 info gcc C Extensions Attribute Syntax
section ("SECTION-NAME")'
Normally, the compiler places the objects it generates in sections
like `data' and `bss'. Sometimes, however, you need additional
sections, or you need certain particular variables to appear in
special sections, for example to map to special hardware. The
`section' attribute specifies that a variable (or function) lives
in a particular section. For example, this small program uses
several specific section names:
struct duart a __attribute__ ((section ("DUART_A"))) = { 0 };
struct duart b __attribute__ ((section ("DUART_B"))) = { 0 };
char stack[10000] __attribute__ ((section ("STACK"))) = { 0 };
int init_data __attribute__ ((section ("INITDATA"))) = 0;
main()
{
/* Initialize stack pointer */
init_sp (stack + sizeof (stack));
/* Initialize initialized data */
memcpy (&init_data, &data, &edata - &data);
/* Turn on the serial ports */
init_duart (&a);
init_duart (&b);
}
Use the `section' attribute with an _initialized_ definition of a
_global_ variable, as shown in the example. GCC issues a warning
and otherwise ignores the `section' attribute in uninitialized
variable declarations.
You may only use the `section' attribute with a fully initialized
global definition because of the way linkers work. The linker
requires each object be defined once, with the exception that
uninitialized variables tentatively go in the `common' (or `bss')
section and can be multiply "defined". You can force a variable
to be initialized with the `-fno-common' flag or the `nocommon'
attribute.
Some file formats do not support arbitrary sections so the
`section' attribute is not available on all platforms. If you
need to map the entire contents of a module to a particular
section, consider using the facilities of the linker instead.
简单来说是指示gcc把标记的数据或者函数放到指定sector。
linux中把一些启动及初始化时候用的数据用__init标识，然后在适当的时候把它们释放，回收内存。
说到这个__init，就不能不说module_init，subsys_initcall。
在init.h中我们能够找到 #define subsys_initcall(fn) __define_initcall("4",fn,4)
又是一个宏定义，简直是无极中的圆环套圆环之城阿。
file:/include/linux/init.h
100 /* initcalls are now grouped by functionality into separate
101 * subsections. Ordering inside the subsections is determined
102 * by link order.
103 * For backwards compatibility, initcall() puts the call in
104 * the device init subsection.
105 *
106 * The `id' arg to __define_initcall() is needed so that multiple initcalls
107 * can point at the same handler without causing duplicate-symbol build errors.
108 */
109
110 #define __define_initcall(level,fn,id) \
111 static initcall_t __initcall_##fn##id __attribute_used__ \
112 __attribute__((__section__(".initcall" level ".init"))) = fn
subsys_initcall(usb_init)转换后就变成了 static initcall_t __initcall_usbinit4 __attribute_used__ \
__attribute__((__section__(".initcall 4.init"))) = usb_init
就是把usb_init的函数入口指针存放在.initcall4.init中。
file:/include/asm-generic/vmlinux.lds.h
239 #define INITCALLS \
240 *(.initcall0.init) \
241 *(.initcall0s.init) \
242 *(.initcall1.init) \
243 *(.initcall1s.init) \
244 *(.initcall2.init) \
245 *(.initcall2s.init) \
246 *(.initcall3.init) \
247 *(.initcall3s.init) \
248 *(.initcall4.init) \
249 *(.initcall4s.init) \
250 *(.initcall5.init) \
251 *(.initcall5s.init) \
252 *(.initcallrootfs.init) \
253 *(.initcall6.init) \
254 *(.initcall6s.init) \
255 *(.initcall7.init) \
256 *(.initcall7s.init)
file:/arch/kernel/vmlinux_32.lds.S
144 .initcall.init : AT(ADDR(.initcall.init) - LOAD_OFFSET) {
145 __initcall_start = .;
146 INITCALLS
147 __initcall_end = .;
148 }
展开
.initcall.init : AT(ADDR(.initcall.init) - LOAD_OFFSET) {
__initcall_start = .;
*(.initcall0.init) \
*(.initcall0s.init) \
*(.initcall1.init) \
*(.initcall1s.init) \
*(.initcall2.init) \
*(.initcall2s.init) \
*(.initcall3.init) \
*(.initcall3s.init) \
*(.initcall4.init) \
*(.initcall4s.init) \
*(.initcall5.init) \
*(.initcall5s.init) \
*(.initcallrootfs.init) \
*(.initcall6.init) \
*(.initcall6s.init) \
*(.initcall7.init) \
*(.initcall7s.init)
__initcall_end = .;
}
那么系统是如何执行这些函数呢？
此话就长了阿～ 话说盘古开天芙蓉姐姐补天后我们来到了main.c这个linux中举足轻重的文件
进入start_kernel
start_kernel -->rest_init() -->kernel_init() --> do_basic_setup() -->do_initcalls()
这个do_initcalls()就是调用这些函数的地方。
file:/init/main.c
662 static void __init do_initcalls(void)
663 {
664 initcall_t *call;
665 int count = preempt_count();
666
667 for (call = __initcall_start; call < __initcall_end; call++) {
668 ktime_t t0, t1, delta;
669 char *msg = NULL;
670 char msgbuf[40];
671 int result;
672
673 if (initcall_debug) {
674 printk("Calling initcall 0x%p", *call);
675 print_fn_descriptor_symbol(": %s()",
676 (unsigned long) *call);
677 printk("\n");
678 t0 = ktime_get();
679 }
680
681 result = (*call)();
682
683 if (initcall_debug) {
684 t1 = ktime_get();
685 delta = ktime_sub(t1, t0);
686
687 printk("initcall 0x%p", *call);
688 print_fn_descriptor_symbol(": %s()",
689 (unsigned long) *call);
690 printk(" returned %d.\n", result);
691
692 printk("initcall 0x%p ran for %Ld msecs: ",
693 *call, (unsigned long long)delta.tv64 >> 20);
694 print_fn_descriptor_symbol("%s()\n",
695 (unsigned long) *call);
696 }
697
698 if (result && result != -ENODEV && initcall_debug) {
699 sprintf(msgbuf, "error code %d", result);
700 msg = msgbuf;
701 }
702 if (preempt_count() != count) {
703 msg = "preemption imbalance";
704 preempt_count() = count;
705 }
706 if (irqs_disabled()) {
707 msg = "disabled interrupts";
708 local_irq_enable();
709 }
710 if (msg) {
711 printk(KERN_WARNING "initcall at 0x%p", *call);
712 print_fn_descriptor_symbol(": %s()",
713 (unsigned long) *call);
714 printk(": returned with %s\n", msg);
715 }
716 }
717
718 /* Make sure there is no pending stuff from the initcall sequence */
719 flush_scheduled_work();
720 }
