在解释完linux/include/linux/compiler.h后，回到本来的初衷linux/include/linux/init.h。在此贴上代码：
1 #ifndef _LINUX_INIT_H 2 #define _LINUX_INIT_H 3 4 #include 5 6 /* These macros are used to mark some functions or 7 * initialized data (doesn't apply to uninitialized data) 8 * as `initialization' functions. The kernel can take this 9 * as hint that the function is used only during the initialization 10 * phase and free up used memory resources after 11 * 12 * Usage: 13 * For functions: 14 * 15 * You should add __init immediately before the function name, like: 16 * 17 * static void __init initme(int x, int y) 18 * { 19 * extern int z; z = x * y; 20 * } 21 * 22 * If the function has a prototype somewhere, you can also add 23 * __init between closing brace of the prototype and semicolon: 24 * 25 * extern int initialize_foobar_device(int, int, int) __init; 26 * 27 * For initialized data: 28 * You should insert __initdata between the variable name and equal 29 * sign followed by value, e.g.: 30 * 31 * static int init_variable __initdata = 0; 32 * static const char linux_logo[] __initconst = { 0x32, 0x36, ... }; 33 * 34 * Don't forget to initialize data not at file scope, i.e. within a function, 35 * as gcc otherwise puts the data into the bss section and not into the init 36 * section. 37 * 38 * Also note, that this data cannot be "const". 39 */ 40 41 /* These are for everybody (although not all archs will actually 42 discard it in modules) */ 43 #define __init __section(.init.text) __cold notrace 44 #define __initdata __section(.init.data) 45 #define __initconst __section(.init.rodata) 46 #define __exitdata __section(.exit.data) 47 #define __exit_call __used __section(.exitcall.exit) 48 49 /* modpost check for section mismatches during the kernel build. 50 * A section mismatch happens when there are references from a 51 * code or data section to an init section (both code or data). 52 * The init sections are (for most archs) discarded by the kernel 53 * when early init has completed so all such references are potential bugs. 54 * For exit sections the same issue exists. 55 * The following markers are used for the cases where the reference to 56 * the *init / *exit section (code or data) is valid and will teach 57 * modpost not to issue a warning. 58 * The markers follow same syntax rules as __init / __initdata. */ 59 #define __ref __section(.ref.text) noinline 60 #define __refdata __section(.ref.data) 61 #define __refconst __section(.ref.rodata) 62 63 /* compatibility defines */ 64 #define __init_refok __ref 65 #define __initdata_refok __refdata 66 #define __exit_refok __ref 67 68 69 #ifdef MODULE 70 #define __exitused 71 #else 72 #define __exitused __used 73 #endif 74 75 #define __exit __section(.exit.text) __exitused __cold 76 77 /* Used for HOTPLUG */ 78 #define __devinit __section(.devinit.text) __cold 79 #define __devinitdata __section(.devinit.data) 80 #define __devinitconst __section(.devinit.rodata) 81 #define __devexit __section(.devexit.text) __exitused __cold 82 #define __devexitdata __section(.devexit.data) 83 #define __devexitconst __section(.devexit.rodata) 84 85 /* Used for HOTPLUG_CPU */ 86 #define __cpuinit __section(.cpuinit.text) __cold 87 #define __cpuinitdata __section(.cpuinit.data) 88 #define __cpuinitconst __section(.cpuinit.rodata) 89 #define __cpuexit __section(.cpuexit.text) __exitused __cold 90 #define __cpuexitdata __section(.cpuexit.data) 91 #define __cpuexitconst __section(.cpuexit.rodata) 92 93 /* Used for MEMORY_HOTPLUG */ 94 #define __meminit __section(.meminit.text) __cold 95 #define __meminitdata __section(.meminit.data) 96 #define __meminitconst __section(.meminit.rodata) 97 #define __memexit __section(.memexit.text) __exitused __cold 98 #define __memexitdata __section(.memexit.data) 99 #define __memexitconst __section(.memexit.rodata) 100 101 /* For assembly routines */ 102 #define __HEAD .section ".head.text","ax" 103 #define __INIT .section ".init.text","ax" 104 #define __FINIT .previous 105 106 #define __INITDATA .section ".init.data","aw",%progbits 107 #define __INITRODATA .section ".init.rodata","a",%progbits 108 #define __FINITDATA .previous 109 110 #define __DEVINIT .section ".devinit.text", "ax" 111 #define __DEVINITDATA .section ".devinit.data", "aw" 112 #define __DEVINITRODATA .section ".devinit.rodata", "a" 113 114 #define __CPUINIT .section ".cpuinit.text", "ax" 115 #define __CPUINITDATA .section ".cpuinit.data", "aw" 116 #define __CPUINITRODATA .section ".cpuinit.rodata", "a" 117 118 #define __MEMINIT .section ".meminit.text", "ax" 119 #define __MEMINITDATA .section ".meminit.data", "aw" 120 #define __MEMINITRODATA .section ".meminit.rodata", "a" 121 122 /* silence warnings when references are OK */ 123 #define __REF .section ".ref.text", "ax" 124 #define __REFDATA .section ".ref.data", "aw" 125 #define __REFCONST .section ".ref.rodata", "a" 126 127 #ifndef __ASSEMBLY__ 128 /* 129 * Used for initialization calls.. 130 */ 131 typedef int (*initcall_t)(void); 132 typedef void (*exitcall_t)(void); 133 134 extern initcall_t __con_initcall_start[], __con_initcall_end[]; 135 extern initcall_t __security_initcall_start[], __security_initcall_end[]; 136 137 /* Used for contructor calls. */ 138 typedef void (*ctor_fn_t)(void); 139 140 /* Defined in init/main.c */ 141 extern int do_one_initcall(initcall_t fn); 142 extern char __initdata boot_command_line[]; 143 extern char *saved_command_line; 144 extern unsigned int reset_devices; 145 146 /* used by init/main.c */ 147 void setup_arch(char **); 148 void prepare_namespace(void); 149 150 extern void (*late_time_init)(void); 151 152 extern int initcall_debug; 153 154 #endif 155 156 #ifndef MODULE 157 158 #ifndef __ASSEMBLY__ 159 160 /* initcalls are now grouped by functionality into separate 161 * subsections. Ordering inside the subsections is determined 162 * by link order. 163 * For backwards compatibility, initcall() puts the call in 164 * the device init subsection. 165 * 166 * The `id' arg to __define_initcall() is needed so that multiple initcalls 167 * can point at the same handler without causing duplicate-symbol build errors. 168 */ 169 170 #define __define_initcall(level,fn,id) / 171 static initcall_t __initcall_##fn##id __used / 172 __attribute__((__section__(".initcall" level ".init"))) = fn 173 174 /* 175 * Early initcalls run before initializing SMP. 176 * 177 * Only for built-in code, not modules. 178 */ 179 #define early_initcall(fn) __define_initcall("early",fn,early) 180 181 /* 182 * A "pure" initcall has no dependencies on anything else, and purely 183 * initializes variables that couldn't be statically initialized. 184 * 185 * This only exists for built-in code, not for modules. 186 */ 187 #define pure_initcall(fn) __define_initcall("",fn,0) 188 189 #define core_initcall(fn) __define_initcall("1",fn,1) 190 #define core_initcall_sync(fn) __define_initcall("1s",fn,1s) 191 #define postcore_initcall(fn) __define_initcall("2",fn,2) 192 #define postcore_initcall_sync(fn) __define_initcall("2s",fn,2s) 193 #define arch_initcall(fn) __define_initcall("3",fn,3) 194 #define arch_initcall_sync(fn) __define_initcall("3s",fn,3s) 195 #define subsys_initcall(fn) __define_initcall("4",fn,4) 196 #define subsys_initcall_sync(fn) __define_initcall("4s",fn,4s) 197 #define fs_initcall(fn) __define_initcall("5",fn,5) 198 #define fs_initcall_sync(fn) __define_initcall("5s",fn,5s) 199 #define rootfs_initcall(fn) __define_initcall("rootfs",fn,rootfs) 200 #define device_initcall(fn) __define_initcall("6",fn,6) 201 #define device_initcall_sync(fn) __define_initcall("6s",fn,6s) 202 #define late_initcall(fn) __define_initcall("7",fn,7) 203 #define late_initcall_sync(fn) __define_initcall("7s",fn,7s) 204 205 #define __initcall(fn) device_initcall(fn) 206 207 #define __exitcall(fn) / 208 static exitcall_t __exitcall_##fn __exit_call = fn 209 210 #define console_initcall(fn) / 211 static initcall_t __initcall_##fn / 212 __used __section(.con_initcall.init) = fn 213 214 #define security_initcall(fn) / 215 static initcall_t __initcall_##fn / 216 __used __section(.security_initcall.init) = fn 217 218 struct obs_kernel_param { 219 const char *str; 220 int (*setup_func)(char *); 221 int early; 222 }; 223 224 /* 225 * Only for really core code. See moduleparam.h for the normal way. 226 * 227 * Force the alignment so the compiler doesn't space elements of the 228 * obs_kernel_param "array" too far apart in .init.setup. 229 */ 230 #define __setup_param(str, unique_id, fn, early) / 231 static const char __setup_str_##unique_id[] __initconst / 232 __aligned(1) = str; / 233 static struct obs_kernel_param __setup_##unique_id / 234 __used __section(.init.setup) / 235 __attribute__((aligned((sizeof(long))))) / 236 = { __setup_str_##unique_id, fn, early } 237 238 #define __setup(str, fn) / 239 __setup_param(str, fn, fn, 0) 240 241 /* NOTE: fn is as per module_param, not __setup! Emits warning if fn 242 * returns non-zero. */ 243 #define early_param(str, fn) / 244 __setup_param(str, fn, fn, 1) 245 246 /* Relies on boot_command_line being set */ 247 void __init parse_early_param(void); 248 void __init parse_early_options(char *cmdline); 249 #endif /* __ASSEMBLY__ */ 250 251 /** 252 * module_init() - driver initialization entry point 253 * @x: function to be run at kernel boot time or module insertion 254 * 255 * module_init() will either be called during do_initcalls() (if 256 * builtin) or at module insertion time (if a module). There can only 257 * be one per module. 258 */ 259 #define module_init(x) __initcall(x); 260 261 /** 262 * module_exit() - driver exit entry point 263 * @x: function to be run when driver is removed 264 * 265 * module_exit() will wrap the driver clean-up code 266 * with cleanup_module() when used with rmmod when 267 * the driver is a module. If the driver is statically 268 * compiled into the kernel, module_exit() has no effect. 269 * There can only be one per module. 270 */ 271 #define module_exit(x) __exitcall(x); 272 273 #else /* MODULE */ 274 275 /* Don't use these in modules, but some people do... */ 276 #define early_initcall(fn) module_init(fn) 277 #define core_initcall(fn) module_init(fn) 278 #define postcore_initcall(fn) module_init(fn) 279 #define arch_initcall(fn) module_init(fn) 280 #define subsys_initcall(fn) module_init(fn) 281 #define fs_initcall(fn) module_init(fn) 282 #define device_initcall(fn) module_init(fn) 283 #define late_initcall(fn) module_init(fn) 284 285 #define security_initcall(fn) module_init(fn) 286 287 /* Each module must use one module_init(). */ 288 #define module_init(initfn) / 289 static inline initcall_t __inittest(void) / 290 { return initfn; } / 291 int init_module(void) __attribute__((alias(#initfn))); 292 293 /* This is only required if you want to be unloadable. */ 294 #define module_exit(exitfn) / 295 static inline exitcall_t __exittest(void) / 296 { return exitfn; } / 297 void cleanup_module(void) __attribute__((alias(#exitfn))); 298 299 #define __setup_param(str, unique_id, fn) /* nothing */ 300 #define __setup(str, func) /* nothing */ 301 #endif 302 303 /* Data marked not to be saved by software suspend */ 304 #define __nosavedata __section(.data.nosave) 305 306 /* This means "can be init if no module support, otherwise module load 307 may call it." */ 308 #ifdef CONFIG_MODULES 309 #define __init_or_module 310 #define __initdata_or_module 311 #define __initconst_or_module 312 #define __INIT_OR_MODULE .text 313 #define __INITDATA_OR_MODULE .data 314 #define __INITRODATA_OR_MODULE .section ".rodata","a",%progbits 315 #else 316 #define __init_or_module __init 317 #define __initdata_or_module __initdata 318 #define __initconst_or_module __initconst 319 #define __INIT_OR_MODULE __INIT 320 #define __INITDATA_OR_MODULE __INITDATA 321 #define __INITRODATA_OR_MODULE __INITRODATA 322 #endif /*CONFIG_MODULES*/ 323 324 /* Functions marked as __devexit may be discarded at kernel link time, depending 325 on config options. Newer versions of binutils detect references from 326 retained sections to discarded sections and flag an error. Pointers to 327 __devexit functions must use __devexit_p(function_name), the wrapper will 328 insert either the function_name or NULL, depending on the config options. 329 */ 330 #if defined(MODULE) || defined(CONFIG_HOTPLUG) 331 #define __devexit_p(x) x 332 #else 333 #define __devexit_p(x) NULL 334 #endif 335 336 #ifdef MODULE 337 #define __exit_p(x) x 338 #else 339 #define __exit_p(x) NULL 340 #endif 341 342 #endif /* _LINUX_INIT_H */ 343
43 #define __init __section(.init.text) __cold notrace
44 #define __initdata __section(.init.data)
45 #define __initconst __section(.init.rodata)
46 #define __exitdata __section(.exit.data)
47 #define __exit_call __used __section(.exitcall.exit)
这一类的宏很是幸运，代码中给出了详尽的注释。这里用到的__section，__cold， notrace都来自compiler.h。
__init 宏最常用的地方是驱动模块初始化函数的定义处，其目的是将驱动模块的初始化函数放入名叫.init.text的输入段。对于__initdata来说，用于数据定义，目的是将数据放入名叫.init.data的输入段。其它几个宏也类似。
131 typedef int (*initcall_t)(void);
132 typedef void (*exitcall_t)(void);
133
134 extern initcall_t __con_initcall_start[], __con_initcall_end[];
135 extern initcall_t __security_initcall_start[], __security_initcall_end[];
这几行看不懂，就是C语言基础差了。
170 #define __define_initcall(level,fn,id) /
171 static initcall_t __initcall_##fn##id __used /
172 __attribute__((__section__(".initcall" level ".init"))) = fn
这是一个可扩展的宏。这条宏带有3个参数：level,fn, id，分析该宏可以看出：
１.其用来定义类型为initcall_t的static函数指针，函数指针的名称由参数fn和id决定：__initcall_##fn##id，这就是函数指针的名称，它其实是一个变量名称。从该名称的定义方法我们其学到了宏定义的一种高级用法，即利用宏的参数产生名称，这要借助于"##"这一符号组合的作用。
２. 这一函数指针变量放入什么输入段呢，请看__attribute__ ((__section__ (".initcall" levle ".init")))，输入段的名称由level决定，如果level="1"，则输入段是.initcall1.init，如果level="3s"，则输入段是.initcall3s.init。这一函数指针变量就是放在用这种方法决定的输入段中的。
３. 这一定义的函数指针变量的初始值是什么，其实就是宏参数fn，实际使用中，fn其实就是真实定义好的函数。
该宏定义并不直接使用，请看接下来的这些宏定义：
#define pure_initcall(fn) __define_initcall("0",fn,0)
#define core_initcall(fn) __define_initcall("1",fn,1)
#define core_initcall_sync(fn) __define_initcall("1s",fn,1s)
#define postcore_initcall(fn) __define_initcall("2",fn,2)
#define postcore_initcall_sync(fn) __define_initcall("2s",fn,2s)
#define arch_initcall(fn) __define_initcall("3",fn,3)
#define arch_initcall_sync(fn) __define_initcall("3s",fn,3s)
#define subsys_initcall(fn) __define_initcall("4",fn,4)
#define subsys_initcall_sync(fn) __define_initcall("4s",fn,4s)
#define fs_initcall(fn) __define_initcall("5",fn,5)
#define fs_initcall_sync(fn) __define_initcall("5s",fn,5s)
#define rootfs_initcall(fn) __define_initcall("rootfs",fn,rootfs)
#define device_initcall(fn) __define_initcall("6",fn,6)
#define device_initcall_sync(fn) __define_initcall("6s",fn,6s)
#define late_initcall(fn) __define_initcall("7",fn,7)
#define late_initcall_sync(fn) __define_initcall("7s",fn,7s)
这些宏定义出来是为了方便的使用__define_initcall宏定义的，上面每条宏第一次使用时都会产生一个新的输入段。
接下来还有一条
#define __initcall(fn) device_initcall(fn)
这一条其实只是定义了另一个别名，即平常使用的__initcall其实就是这儿的device_initcall，用它定义的函数指定位于段.initcall6.init中。
#define __setup_param(str, unique_id, fn, early) /
static char __setup_str_##unique_id[] __initdata __aligned(1) = str; /
static struct obs_kernel_param __setup_##unique_id /
__used __section(.init.setup) /
__attribute__((aligned((sizeof(long))))) /
= { __setup_str_##unique_id, fn, early }
#define __setup(str, fn) /
__setup_param(str, fn, fn, 0)
__setup这条宏在Linux Kernel中使用最多的地方就是定义处理Kernel启动参数的函数及数据结构。
使用Kernel中的例子分析一下这两条定义：
__setup("root=",root_dev_setup);
这条语句出现在init/do_mounts.c中，其作用是处理Kernel启动时的像root=/dev/mtdblock3之类的参数的。
分解一下这条语句，首先变为：
__setup_param("root=",root_dev_setup,root_dev_setup,0);
继续分解，将得到下面这段代吗：
static char __setup_str_root_dev_setup_id[] __initdata __aligned(1) = "root=";
static struct obs_kernel_param __setup_root_dev_setup_id
__used __section(.init.setup)
__attribute__((aligned((sizeof(long)))))
= { __setup_str_root_dev_setup_id, root_dev_setup, 0 };
这段代码定义了两个变量：字符数组变量__setup_str_root_dev_setup_id，其初始化内容为"root="，由于该变量用__initdata修饰，它将被放入.init.data输入段；另一变量是结构变量__setup_root_dev_setup_id，其类型为struct obs_kernel_param， 该变理被放入输入段.init.setup中。结构struct struct obs_kernel_param也在该文件中定义如下：
struct obs_kernel_param {
const char *str;
int (*setup_func)(char *);
int early;
};
变量__setup_root_dev_setup_id的三个成员分别被初始化为：
__setup_str_root_dev_setup_id －－> 前面定义的字符数组变量，初始内容为"root="。
root_dev_setup －－> 通过宏传过来的处理函数。
0 －－>常量0，该成员的作用以后分析。
现在不难想像内核启动时怎么处理启动参数的了：通过__setup宏定义obs_kernel_param结构变量都被放入.init.setup段中，这样一来实际是使.init.setup段变成一张表，Kernel在处理每一个启动参数时，都会来查找这张表，与每一个数据项中的成员str进行比较，如果完全相同，就会调用该数据项的函数指针成员setup_func所指向的函数（该函数是在使用__setup宏定义该变量时传入的函数参数），并将启动参数如root=后面的内容传给该处理函数。
