以阅读源代码的方式研究 linux的启动过程，是我早已有之的心愿。今天总算是开工了。由于理解系统初始化过程要有汇编的基础，所以我只好先从init开始。
init的源代码在/usr/src/linux-2.4.19-9mdk/init目录下，在这个目录下共有三个文件do_mounts.c、main.c和version.c。其中main.c就是init进程的源代码。这段代码并不长，只有640行。
首先用ctags -x main.c 生成一个tags文件，用vi 打开后，可以看到各个函数的索引：
LPS_PREC macro 183 main.c #define LPS_PREC 8
MAX_INIT_ARGS macro 125 main.c #define MAX_INIT_ARGS 8
MAX_INIT_ENVS macro 126 main.c #define MAX_INIT_ENVS 8
__KERNEL_SYSCALLS__ macro 12 main.c #define __KERNEL_SYSCALLS__
argv_init variable 135 main.c static char *
argv_init[MAX_INIT_ARGS+2] = { "init", NULL, };
calibrate_delay function 185 main.c void __init
calibrate_delay(void)
checksetup function 160 main.c static int __init
checksetup(char *line)
child_reaper variable 498 main.c struct task_struct
*child_reaper = &init_task;
cols variable 131 main.c int rows, cols;
debug_kernel function 226 main.c static int __init
debug_kernel(char *str)
do_basic_setup function 521 main.c static void __init
do_basic_setup(void)
do_initcalls function 500 main.c static void __init
do_initcalls(void)
envp_init variable 136 main.c char *
envp_init[MAX_INIT_ENVS+2] = { "HOME=/", "TERM=linux", NULL, };
execute_command variable 133 main.c char *execute_command;
gr_setup function 148 main.c static int __init
gr_setup(char *str)
init function 603 main.c static int init(void
*unused)
loops_per_jiffy variable 178 main.c unsigned long
loops_per_jiffy = (1<<12);
parse_options function 254 main.c static void __init
parse_options(char *line)
profile_setup function 138 main.c static int __init
profile_setup(char *str)
quiet_kernel function 234 main.c static int __init
quiet_kernel(char *str)
rest_init function 389 main.c static void rest_init(void)
rows variable 131 main.c int rows, cols;
smp_init function 349 main.c static void __init
smp_init(void)
smp_init function 361 main.c static void __init
smp_init(void)
smp_init macro 354 main.c #define smp_init() do { }
while (0)
start_kernel function 401 main.c asmlinkage void __init
start_kernel(void)
wait_init_idle variable 344 main.c unsigned long
wait_init_idle;
有了这个索引后，查找函数就方便了。再用vi 打开main.c，找到init函数，如下：
源码:-------------------------------------------------------
static int init(void * unused)
{
lock_kernel();
do_basic_setup();
prepare_namespace();
#ifdef CONFIG_GRKERNSEC
grsecurity_init();
#endif
/*
* Ok, we have completed the initial bootup, and
* we're essentially up and running. Get rid of the
* initmem segments and start the user-mode stuff..
*/
free_initmem();
unlock_kernel();
if (open("/dev/console", O_RDWR, 0) < 0)
printk("Warning: unable to open an initial console.\n");
(void) dup(0);
(void) dup(0);
/*
* We try each of these until one succeeds.
*
* The Bourne shell can be used instead of init if we are
* trying to recover a really broken machine.
*/
if (execute_command)
execve(execute_command,argv_init,envp_init);
execve("/sbin/init",argv_init,envp_init);
execve("/etc/init",argv_init,envp_init);
execve("/bin/init",argv_init,envp_init);
execve("/bin/sh",argv_init,envp_init);
panic("No init found. Try passing init= option to kernel.");
-----------------------------------------------------------
在源代码中，可以看到很多如同#ifdef
CONFIG_GRKERNSEC的宏定义，这些宏定义可以在/usr/src/linux-2.4.19-9mdk/目录下的.config文件中找到。用vi
查看.config文件中的宏定义，发现"# CONFIG_GRKERNSEC is not
set"，也就是没有定义，因此，这个宏定义可以不管它。先来看执行流程。
一、do_basic_setup()函数
init进程第一个执行的函数是lock_kernel()，这个函数在很多内核的源代码中都有，但我没有找到它的函数定义，只好放弃。
第二个执行的函数就是do_basic_setup()，这个函数的内容如下：
源码:-------------------------------------------------------
/*
* Ok, the machine is now initialized. None of the devices
* have been touched yet, but the CPU subsystem is up and
* running, and memory and process management works.
*
* Now we can finally start doing some real work..
*/
static void __init do_basic_setup(void)
{
/*
* Tell the world that we're going to be the grim
* reaper of innocent orphaned children.
*
* We don't want people to have to make incorrect
* assumptions about where in the task array this
* can be found.
*/
child_reaper = current;
#if defined(CONFIG_MTRR) /* Do this after SMP initialization */
/*
* We should probably create some architecture-dependent "fixup after
* everything is up" style function where this would belong better
* than in init/main.c..
*/
mtrr_init();
#endif /*mtrr(Memory Type Range Register)是Inter P6系列处理器用来控制处理器读写内存范围的。*/
#ifdef CONFIG_SYSCTL
sysctl_init();
#endif /* 对/proc文件系统和sysctl()系统调用相关部分进行初始化*/
/*
* Ok, at this point all CPU's should be initialized, so
* we can start looking into devices..
*/
#if defined(CONFIG_ARCH_S390)
s390_init_machine_check();
#endif
#ifdef CONFIG_PCI
pci_init();
#endif /* 初始化PCI总线 */
#ifdef CONFIG_SBUS
sbus_init();
#endif
#if defined(CONFIG_PPC)
ppc_init();
#endif
#ifdef CONFIG_MCA
mca_init();
#endif
#ifdef CONFIG_ARCH_ACORN
ecard_init();
#endif
#ifdef CONFIG_ZORRO
zorro_init();
#endif
#ifdef CONFIG_DIO
dio_init();
#endif
#ifdef CONFIG_NUBUS
nubus_init();
#endif
#ifdef CONFIG_ISAPNP
isapnp_init();
#endif /* 对ISA总线即插即用初始化 */
#ifdef CONFIG_TC
tc_init();
#endif
/* Networking initialization needs a process context */
sock_init(); /* 初始化网络协议栈 */
start_context_thread();
do_initcalls();
#ifdef CONFIG_IRDA
irda_proto_init();
irda_device_init(); /* Must be done after protocol initialization */
#endif
#ifdef CONFIG_PCMCIA
init_pcmcia_ds(); /* Do this last */
#endif
}
------------------------------------------------------------
很明显，这段代码是用来进行对系统初始化的。开头的一段注释告诉我们，系统硬件此时只有cpu子系统在运转，内存管理和进程管理也开始工作了。接下来，就是对硬件的初始化。
这一部分与硬件密切相关，在编译核心时，将根据配置文件.config来编译相应的部分。用vi查看.config文件，发现定义的项目如下：
CONFIG_MTRR=y
CONFIG_SYSCTL=y
CONFIG_PCI=y
# CONFIG_PCI_GOBIOS is not set
# CONFIG_PCI_GODIRECT is not set
CONFIG_PCI_GOANY=y
CONFIG_PCI_BIOS=y
CONFIG_PCI_DIRECT=y
CONFIG_PCI_NAMES=y
CONFIG_PCI_HERMES=m
# CONFIG_SBUS is not set
# CONFIG_MCA is not set
CONFIG_ISAPNP=y
CONFIG_TCIC=y
CONFIG_TC35815=m
CONFIG_IRDA=m
CONFIG_IRDA_ULTRA=y
CONFIG_IRDA_CACHE_LAST_LSAP=y
CONFIG_IRDA_FAST_RR=y
# CONFIG_IRDA_DEBUG is not set
CONFIG_PCMCIA=m
CONFIG_PCMCIA_AHA152X=m
CONFIG_PCMCIA_FDOMAIN=m
C
