linux内核有main函数吗
linux内核有main函数；main函数是程序的入口，main是应用程序和操作系统之间约定好的一个接口名，所以linux中每个应用程序的第一个函数必须是main。
本教程操作环境：linux5.9.8系统、Dell G3电脑。
linux内核有main函数吗？
linux内核源码之main函数解析
这几天一直在纠结：
main函数是程序的入口，一个程序启动后，经过bootloader的初始化就该经main函数进入C语言的世界，但是linux中每个应用程序的开始都是从main函数开始的。linux下有多个应用程序，岂不是有很多个main。那bootloader会知道跳到哪个main？多个main编译怎么不冲突？
在网上搜索了很久，渐渐的有些明白了：
1、main函数是C语言的入口，这句话没错；但是这句话仅仅是一个约定，而非一个亘古不变的铁律！从程序的更为本质的汇编代码来看，只是大家约定汇编初始化完了后，跳到一个名字叫"main"的标号处；言外之意就是这个标号也是可以改名的，比如linux的C语言入口就是start_kernel（）；从这个标号地址后就是C语言的天下了。用main这个名字仅仅是因为大家的约定而已，不遵守约定能玩的转也行啊，就像苹果充电线啥的都和别人不一样。
2、在编译时是不存多个main函数的！每个应用程序虽说都有一个main函数（从应用程序来看应用程序的入口是main函数哦）；但是应用程序都是独立编译的，不会一起编译，操作系统内核就更不可能和应用程序一起编译了！所以根本不存在多个main冲突的！！可能是统一操作系统与应用程序之间的接口，亦或是侧面影响下main是程序入口的说法，main是应用程序和操作系统之间约定好的一个接口名！所以linux中每个应用程序的第一个函数必须是main。除非你改掉了内核调度的接口地方。
3、linux的应用程序的安装启动也可以类比下我们每天都在用的Windows。Windows应用程序的安装其实也是把一些执行文件拷贝到指定的文件夹里（从绿色软件看），点击就可以运行。linux下也是这样。编译好的bin文件放到指定的文件夹目录下，然后用命令启动执行。
/* * linux/init/main.c * * Copyright (C) 1991, 1992 Linus Torvalds * * GK 2/5/95 - Changed to support mounting root fs via NFS * Added initrd & change_root: Werner Almesberger & Hans Lermen, Feb '96 * Moan early if gcc is old, avoiding bogus kernels - Paul Gortmaker, May '96 * Simplified starting of init: Michael A. Griffith <grif@acm.org> * start_kernel->rest_init->kernel_init创建用户init pid=1 ->kthreadd管理内核线程 pid=x ->pid=0，是idle线程 在rest_init中，会创建kernel_init线程，它负责创建用户init进程,完成工作后，自己 化身为idle线程 */ #include <linux/types.h> #include <linux/module.h> #include <linux/proc_fs.h> #include <linux/kernel.h> #include <linux/syscalls.h> #include <linux/stackprotector.h> #include <linux/string.h> #include <linux/ctype.h> #include <linux/delay.h> #include <linux/ioport.h> #include <linux/init.h> #include <linux/initrd.h> #include <linux/bootmem.h> #include <linux/acpi.h> #include <linux/tty.h> #include <linux/percpu.h> #include <linux/kmod.h> #include <linux/vmalloc.h> #include <linux/kernel_stat.h> #include <linux/start_kernel.h> #include <linux/security.h> #include <linux/smp.h> #include <linux/profile.h> #include <linux/rcupdate.h> #include <linux/moduleparam.h> #include <linux/kallsyms.h> #include <linux/writeback.h> #include <linux/cpu.h> #include <linux/cpuset.h> #include <linux/cgroup.h> #include <linux/efi.h> #include <linux/tick.h> #include <linux/interrupt.h> #include <linux/taskstats_kern.h> #include <linux/delayacct.h> #include <linux/unistd.h> #include <linux/rmap.h> #include <linux/mempolicy.h> #include <linux/key.h> #include <linux/buffer_head.h> #include <linux/page_cgroup.h> #include <linux/debug_locks.h> #include <linux/debugobjects.h> #include <linux/lockdep.h> #include <linux/kmemleak.h> #include <linux/pid_namespace.h> #include <linux/device.h> #include <linux/kthread.h> #include <linux/sched.h> #include <linux/signal.h> #include <linux/idr.h> #include <linux/kgdb.h> #include <linux/ftrace.h> #include <linux/async.h> #include <linux/kmemcheck.h> #include <linux/sfi.h> #include <linux/shmem_fs.h> #include <linux/slab.h> #include <linux/perf_event.h> #include <asm/io.h> #include <asm/bugs.h> #include <asm/setup.h> #include <asm/sections.h> #include <asm/cacheflush.h> #ifdef CONFIG_X86_LOCAL_APIC #include <asm/smp.h> #endif static int kernel_init(void *); extern void init_IRQ(void); extern void fork_init(unsigned long); extern void mca_init(void); extern void sbus_init(void); extern void prio_tree_init(void); extern void radix_tree_init(void); #ifndef CONFIG_DEBUG_RODATA static inline void mark_rodata_ro(void) { } #endif #ifdef CONFIG_TC extern void tc_init(void); #endif /* * Debug helper: via this flag we know that we are in 'early bootup code' * where only the boot processor is running with IRQ disabled. This means * two things - IRQ must not be enabled before the flag is cleared and some * operations which are not allowed with IRQ disabled are allowed while the * flag is set. */ bool early_boot_irqs_disabled __read_mostly; enum system_states system_state __read_mostly; EXPORT_SYMBOL(system_state); /* * Boot command-line arguments */ #define MAX_INIT_ARGS CONFIG_INIT_ENV_ARG_LIMIT #define MAX_INIT_ENVS CONFIG_INIT_ENV_ARG_LIMIT extern void time_init(void); /* Default late time init is NULL. archs can override this later. */ void (*__initdata late_time_init)(void); extern void softirq_init(void); /* Untouched command line saved by arch-specific code. */ char __initdata boot_command_line[COMMAND_LINE_SIZE]; /* Untouched saved command line (eg. for /proc) */ char *saved_command_line; /* Command line for parameter parsing */ static char *static_command_line; static char *execute_command; static char *ramdisk_execute_command; /* * If set, this is an indication to the drivers that reset the underlying * device before going ahead with the initialization otherwise driver might * rely on the BIOS and skip the reset operation. * * This is useful if kernel is booting in an unreliable environment. * For ex. kdump situaiton where previous kernel has crashed, BIOS has been * skipped and devices will be in unknown state. */ unsigned int reset_devices; EXPORT_SYMBOL(reset_devices); static int __init set_reset_devices(char *str) { reset_devices = 1; return 1; } __setup("reset_devices", set_reset_devices); static const char * argv_init[MAX_INIT_ARGS+2] = { "init", NULL, }; const char * envp_init[MAX_INIT_ENVS+2] = { "HOME=/", "TERM=linux", NULL, }; static const char *panic_later, *panic_param; extern const struct obs_kernel_param __setup_start[], __setup_end[]; static int __init obsolete_checksetup(char *line) { const struct obs_kernel_param *p; int had_early_param = 0; p = __setup_start; do { int n = strlen(p->str); if (parameqn(line, p->str, n)) { if (p->early) { /* Already done in parse_early_param? * (Needs exact match on param part). * Keep iterating, as we can have early * params and __setups of same names 8( */ if (line[n] == '\0' || line[n] == '=') had_early_param = 1; } else if (!p->setup_func) { printk(KERN_WARNING "Parameter %s is obsolete," " ignored\n", p->str); return 1; } else if (p->setup_func(line + n)) return 1; } p++; } while (p < __setup_end); return had_early_param; } /* * This should be approx 2 Bo*oMips to start (note initial shift), and will * still work even if initially too large, it will just take slightly longer */ unsigned long loops_per_jiffy = (1<<12); EXPORT_SYMBOL(loops_per_jiffy); static int __init debug_kernel(char *str) { console_loglevel = 10; return 0; } static int __init quiet_kernel(char *str) { console_loglevel = 4; return 0; } early_param("debug", debug_kernel); early_param("quiet", quiet_kernel); static int __init loglevel(char *str) { int newlevel; /* * Only update loglevel value when a correct setting was passed, * to prevent blind crashes (when loglevel being set to 0) that * are quite hard to debug */ if (get_option(&str, &newlevel)) { console_loglevel = newlevel; return 0; } return -EINVAL; } early_param("loglevel", loglevel); /* Change NUL term back to "=", to make "param" the whole string. */ static int __init repair_env_string(char *param, char *val) { if (val) { /* param=val or param="val"? */ if (val == param+strlen(param)+1) val[-1] = '='; else if (val == param+strlen(param)+2) { val[-2] = '='; memmove(val-1, val, strlen(val)+1); val--; } else BUG(); } return 0; } /* * Unknown boot options get handed to init, unless they look like * unused parameters (modprobe will find them in /proc/cmdline). */ static int __init unknown_bootoption(char *param, char *val) { repair_env_string(param, val); /* Handle obsolete-style parameters */ if (obsolete_checksetup(param)) return 0; /* Unused module parameter. */ if (strchr(param, '.') && (!val || strchr(param, '.') < val)) return 0; if (panic_later) return 0; if (val) { /* Environment option */ unsigned int i; for (i = 0; envp_init[i]; i++) { if (i == MAX_INIT_ENVS) { panic_later = "Too many boot env vars at `%s'"; panic_param = param; } if (!strncmp(param, envp_init[i], val - param)) break; } envp_init[i] = param; } else { /* Command line option */ unsigned int i; for (i = 0; argv_init[i]; i++) { if (i == MAX_INIT_ARGS) { panic_later = "Too many boot init vars at `%s'"; panic_param = param; } } argv_init[i] = param; } return 0; } static int __init init_setup(char *str) { unsigned int i; execute_command = str; /* * In case LILO is going to boot us with default command line, * it prepends "auto" before the whole cmdline which makes * the shell think it should execute a script with such name. * So we ignore all arguments entered _before_ init=... [MJ] */ for (i = 1; i < MAX_INIT_ARGS; i++) argv_init[i] = NULL; return 1; } __setup("init=", init_setup); static int __init rdinit_setup(char *str) { unsigned int i; ramdisk_execute_command = str; /* See "auto" comment in init_setup */ for (i = 1; i < MAX_INIT_ARGS; i++) argv_init[i] = NULL; return 1; } __setup("rdinit=", rdinit_setup); #ifndef CONFIG_SMP static const unsigned int setup_max_cpus = NR_CPUS; #ifdef CONFIG_X86_LOCAL_APIC static void __init smp_init(void) { APIC_init_uniprocessor(); } #else #define smp_init() do { } while (0) #endif static inline void setup_nr_cpu_ids(void) { } static inline void smp_prepare_cpus(unsigned int maxcpus) { } #endif /* * We need to store the untouched command line for future reference. * We also need to store the touched command line since the parameter * parsing is performed in place, and we should allow a component to * store reference of name/value for future reference. */ static void __init setup_command_line(char *command_line) { saved_command_line = alloc_bootmem(strlen (boot_command_line)+1); static_command_line = alloc_bootmem(strlen (command_line)+1); strcpy (saved_command_line, boot_command_line); strcpy (static_command_line, command_line); } /* * We need to finalize in a non-__init function or else race conditions * between the root thread and the init thread may cause start_kernel to * be reaped by free_initmem before the root thread has proceeded to * cpu_idle. * * gcc-3.4 accidentally inlines this function, so use noinline. */ static __initdata DECLARE_COMPLETION(kthreadd_done); static noinline void __init_refok rest_init(void) { int pid; rcu_scheduler_starting();//READ-COPY UPDATE启动 /* * We need to spawn init first so that it obtains pid 1, however * the init task will end up wanting to create kthreads, which, if * we schedule it before we create kthreadd, will OOPS. * 创建一个内核线程，它的线程函数是kernel_init,pid=1,内核进程 */ kernel_thread(kernel_init, NULL, CLONE_FS | CLONE_SIGHAND); //numa策略设置 numa_default_policy(); //全局链表kthread_create_list中的kthread内核线程都被运行 //kthreadd线程管理和调度其它内核线程 pid = kernel_thread(kthreadd, NULL, CLONE_FS | CLONE_FILES); rcu_read_lock(); //通过pid,ini_pid_ns取得kthreadd地址 kthreadd_task = find_task_by_pid_ns(pid, &init_pid_ns); rcu_read_unlock(); //通知在kthreadd_done条件的kernel_init线程 complete(&kthreadd_done); /* * The boot idle thread must execute schedule() * at least once to get things moving: * idle 线程初始化 */ init_idle_bootup_task(current); //抢占禁用 schedule_preempt_disabled(); /* Call into cpu_idle with preempt disabled */ cpu_idle(); } /* Check for early params. */ static int __init do_early_param(char *param, char *val) { const struct obs_kernel_param *p; for (p = __setup_start; p < __setup_end; p++) { if ((p->early && parameq(param, p->str)) || (strcmp(param, "console") == 0 && strcmp(p->str, "earlycon") == 0) ) { if (p->setup_func(val) != 0) printk(KERN_WARNING "Malformed early option '%s'\n", param); } } /* We accept everything at this stage. */ return 0; } void __init parse_early_options(char *cmdline) { parse_args("early options", cmdline, NULL, 0, 0, 0, do_early_param); } /* Arch code calls this early on, or if not, just before other parsing. */ void __init parse_early_param(void) { static __initdata int done = 0; static __initdata char tmp_cmdline[COMMAND_LINE_SIZE]; if (done) return; /* All fall through to do_early_param. */ strlcpy(tmp_cmdline, boot_command_line, COMMAND_LINE_SIZE); parse_early_options(tmp_cmdline); done = 1; } /* * Activate the first processor. */ static void __init boot_cpu_init(void) { int cpu = smp_processor_id(); /* Mark the boot cpu "present", "online" etc for SMP and UP case */ set_cpu_online(cpu, true); set_cpu_active(cpu, true); set_cpu_present(cpu, true); set_cpu_possible(cpu, true); } void __init __weak smp_setup_processor_id(void) { } void __init __weak thread_info_cache_init(void) { } /* * Set up kernel memory allocators */ static void __init mm_init(void) { /* * page_cgroup requires contiguous pages, * bigger than MAX_ORDER unless SPARSEMEM. */ page_cgroup_init_flatmem(); mem_init(); kmem_cache_init(); percpu_init_late(); pgtable_cache_init(); vmalloc_init(); } asmlinkage void __init start_kernel(void) { char * command_line; extern const struct kernel_param __start___param[], __stop___param[]; /* * Need to run as early as possible, to initialize the * lockdep hash: */ //初始化2个hash表-Lock Dependency Validator(内核依赖的关系表) lockdep_init(); smp_setup_processor_id(); //空函数 debug_objects_early_init();//初始化内核调试相关 /* * Set up the the initial canary ASAP: */ boot_init_stack_canary();//栈溢出保护初始化 //控制组初始化-cgroup-资源任务分组管理 cgroup_init_early(); local_irq_disable();//关中断 early_boot_irqs_disabled = true; /* * Interrupts are still disabled. Do necessary setups, then * enable them */ tick_init();//时钟初始化 boot_cpu_init();//启动cpu初始化 page_address_init();//页面初始化 printk(KERN_NOTICE "%s", linux_banner); setup_arch(&command_line);//架构相关初始化 mm_init_owner(&init_mm, &init_task);//内存管理初始化 mm_init_cpumask(&init_mm);//内存管理初始化 setup_command_line(command_line);//处理命令行(保存2份) setup_nr_cpu_ids();//cpuid相关 setup_per_cpu_areas();//每cpu变量申请空间(包括gdt) //smp中用来启动的cpu smp_prepare_boot_cpu(); /* arch-specific boot-cpu hooks */ //建立系统内存页区链表 build_all_zonelists(NULL); //内存页相关初始化 page_alloc_init(); printk(KERN_NOTICE "Kernel command line: %s\n", boot_command_line); //命令行boot_command_line parse_early_param(); //解析参数 parse_args("Booting kernel", static_command_line, __start___param, __stop___param - __start___param, -1, -1, &unknown_bootoption); // jump_label_init(); /* * These use large bootmem allocations and must precede * kmem_cache_init() * 内存初始化相关 */ setup_log_buf(0); pidhash_init(); vfs_caches_init_early(); sort_main_extable(); trap_init(); mm_init(); /* * Set up the scheduler prior starting any interrupts (such as the * timer interrupt). Full topology setup happens at smp_init() * time - but meanwhile we still have a functioning scheduler. * 调度初始化 */ sched_init(); /* * Disable preemption - early bootup scheduling is extremely * fragile until we cpu_idle() for the first time. * 抢占禁用 */ preempt_disable(); if (!irqs_disabled()) { printk(KERN_WARNING "start_kernel(): bug: interrupts were " "enabled *very* early, fixing it\n"); local_irq_disable(); } idr_init_cache();//idr perf_event_init();//performance event rcu_init();//read-copy-update 机制 radix_tree_init();//radix树机制 /* init some links before init_ISA_irqs() */ early_irq_init();//中断请求 init_IRQ();//中断请求 prio_tree_init();//优先查找树 init_timers();//时钟 hrtimers_init();//High-resolution kernel timers高精度内核时钟 softirq_init();//软中断 timekeeping_init();//时间相关 time_init();//时间 profile_init();//分配内核性能统计保存的内存 call_function_init();//smp中每cpu的call_single_queue初始化 if (!irqs_disabled()) printk(KERN_CRIT "start_kernel(): bug: interrupts were " "enabled early\n"); early_boot_irqs_disabled = false;//中断请求开 local_irq_enable();//本地中断开 kmem_cache_init_late();//kmem后期初始化 /* * HACK ALERT! This is early. We're enabling the console before * we've done PCI setups etc, and console_init() must be aware of * this. But we do want output early, in case something goes wrong. */ console_init();//初始化系统控制台结构 if (panic_later) panic(panic_later, panic_param); //锁依赖信息 lockdep_info(); /* * Need to run this when irqs are enabled, because it wants * to self-test [hard/soft]-irqs on/off lock inversion bugs * too: */ locking_selftest(); #ifdef CONFIG_BLK_DEV_INITRD if (initrd_start && !initrd_below_start_ok && page_to_pfn(virt_to_page((void *)initrd_start)) < min_low_pfn) { printk(KERN_CRIT "initrd overwritten (0x%08lx < 0x%08lx) - " "disabling it.\n", page_to_pfn(virt_to_page((void *)initrd_start)), min_low_pfn); initrd_start = 0; } #endif page_cgroup_init();//control groups初始化 debug_objects_mem_init();//对象调试 kmemleak_init();//检测内核内存泄漏的功能 setup_per_cpu_pageset();//申请并初始化每cpu页set numa_policy_init();//numa相关 if (late_time_init) late_time_init(); //初始化每cpusched_clock_data=ktime_now sched_clock_init(); calibrate_delay();//计算cpuMIPS百万条指令/s pidmap_init();//pid进程id表初始化 anon_vma_init();//虚拟地址 #ifdef CONFIG_X86 if (efi_enabled)//efi bois efi_enter_virtual_mode(); #endif thread_info_cache_init();//申请thread_info的内存 cred_init();//credential健在分配 //根据物理内存大小，计算可创建进/线程数量 fork_init(totalram_pages); proc_caches_init();//进程内存初始化 buffer_init();//页高速缓存 key_init();//红黑树内存，存keys security_init();//安全相关 dbg_late_init();//调试相关 vfs_caches_init(totalram_pages);//虚拟文件系统初始化 signals_init();//sigqueue申请内存，信号系统 /* rootfs populating might need page-writeback */ page_writeback_init();//页回写 #ifdef CONFIG_PROC_FS proc_root_init();//proc文件系统初始化 #endif cgroup_init();//cgroup相关 cpuset_init();//cpuset相关 taskstats_init_early();//进程计数器 delayacct_init();//进程延时审计 check_bugs();//系统bug相关测试 //acpi总线 acpi_early_init(); /* before LAPIC and SMP init */ sfi_init_late();//Simple Firmware Interface //功能追踪初始化,一种调试工具 ftrace_init(); /* Do the rest non-__init'ed, we're now alive */ rest_init(); } /* Call all constructor functions linked into the kernel. */ static void __init do_ctors(void) { #ifdef CONFIG_CONSTRUCTORS ctor_fn_t *fn = (ctor_fn_t *) __ctors_start; for (; fn < (ctor_fn_t *) __ctors_end; fn++) (*fn)(); #endif } bool initcall_debug; core_param(initcall_debug, initcall_debug, bool, 0644); static char msgbuf[64]; static int __init_or_module do_one_initcall_debug(initcall_t fn) { ktime_t calltime, delta, rettime; unsigned long long duration; int ret; printk(KERN_DEBUG "calling %pF @ %i\n", fn, task_pid_nr(current)); calltime = ktime_get(); ret = fn(); rettime = ktime_get(); delta = ktime_sub(rettime, calltime); duration = (unsigned long long) ktime_to_ns(delta) >> 10; printk(KERN_DEBUG "initcall %pF returned %d after %lld usecs\n", fn, ret, duration); return ret; } int __init_or_module do_one_initcall(initcall_t fn) { int count = preempt_count(); int ret; if (initcall_debug) ret = do_one_initcall_debug(fn); else ret = fn(); msgbuf[0] = 0; if (ret && ret != -ENODEV && initcall_debug) sprintf(msgbuf, "error code %d ", ret); if (preempt_count() != count) { strlcat(msgbuf, "preemption imbalance ", sizeof(msgbuf)); preempt_count() = count; } if (irqs_disabled()) { strlcat(msgbuf, "disabled interrupts ", sizeof(msgbuf)); local_irq_enable(); } if (msgbuf[0]) { printk("initcall %pF returned with %s\n", fn, msgbuf); } return ret; } extern initcall_t __initcall_start[]; extern initcall_t __initcall0_start[]; extern initcall_t __initcall1_start[]; extern initcall_t __initcall2_start[]; extern initcall_t __initcall3_start[]; extern initcall_t __initcall4_start[]; extern initcall_t __initcall5_start[]; extern initcall_t __initcall6_start[]; extern initcall_t __initcall7_start[]; extern initcall_t __initcall_end[]; static initcall_t *initcall_levels[] __initdata = { __initcall0_start, __initcall1_start, __initcall2_start, __initcall3_start, __initcall4_start, __initcall5_start, __initcall6_start, __initcall7_start, __initcall_end, }; static char *initcall_level_names[] __initdata = { "early parameters", "core parameters", "postcore parameters", "arch parameters", "subsys parameters", "fs parameters", "device parameters", "late parameters", }; static void __init do_initcall_level(int level) { extern const struct kernel_param __start___param[], __stop___param[]; initcall_t *fn; strcpy(static_command_line, saved_command_line); parse_args(initcall_level_names[level], static_command_line, __start___param, __stop___param - __start___param, level, level, repair_env_string); for (fn = initcall_levels[level]; fn < initcall_levels[level+1]; fn++) do_one_initcall(*fn); } static void __init do_initcalls(void) { int level; for (level = 0; level < ARRAY_SIZE(initcall_levels) - 1; level++) do_initcall_level(level); } /* * 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) { cpuset_init_smp();//smp cpuset相关 usermodehelper_init();//khelper单线程工作队列 shmem_init();//sheme机制 driver_init();//驱动各子系统 init_irq_proc();//proc中创建irq目录 do_ctors();//内核中所有构造函数,介于.ctors段中的函数 usermodehelper_enable(); //所有编译进内核的驱动模块初始化函数 do_initcalls(); } static void __init do_pre_smp_initcalls(void) { initcall_t *fn; for (fn = __initcall_start; fn < __initcall0_start; fn++) do_one_initcall(*fn); } static void run_init_process(const char *init_filename) { argv_init[0] = init_filename; kernel_execve(init_filename, argv_init, envp_init); } /* This is a non __init function. Force it to be noinline otherwise gcc * makes it inline to init() and it becomes part of init.text section * 这是个非Init函数，防止gcc让它内联到init()，并成为Init.text段的一部分 */ static noinline int init_post(void) { /* need to finish all async __init code before freeing the memory * 在释放init内存前，必须完成所有__init代码执行 */ async_synchronize_full(); free_initmem();//释放init.*段中的内存 //修改页表，保证只读数据段为只读属性read only mark_rodata_ro(); //系统运行状态标志 system_state = SYSTEM_RUNNING; //numa默认策略 numa_default_policy(); //当前进程不能被杀掉，只为它是init current->signal->flags |= SIGNAL_UNKILLABLE; //如果ramdisk_execute_command变量指定了init程序，执行它 if (ramdisk_execute_command) { run_init_process(ramdisk_execute_command); printk(KERN_WARNING "Failed to execute %s\n", ramdisk_execute_command); } /* * 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. * 又一个程序，看能不能执行，如果不能，则执行下面4个之一 */ if (execute_command) { run_init_process(execute_command); printk(KERN_WARNING "Failed to execute %s. Attempting " "defaults...\n", execute_command); } run_init_process("/sbin/init"); run_init_process("/etc/init"); run_init_process("/bin/init"); run_init_process("/bin/sh"); //两个变量和4个init都不能成功执行，报错 panic("No init found. Try passing init= option to kernel. " "See Linux Documentation/init.txt for guidance."); } static int __init kernel_init(void * unused) { /* * Wait until kthreadd is all set-up.等待kthreadd的启动完成 */ wait_for_completion(&kthreadd_done); /* Now the scheduler is fully set up and can do blocking allocations * */ gfp_allowed_mask = __GFP_BITS_MASK; /* * init can allocate pages on any node */ set_mems_allowed(node_states[N_HIGH_MEMORY]); /* * init can run on any cpu. */ set_cpus_allowed_ptr(current, cpu_all_mask); //cad_pid为接收Ctrl-alt-del操作的INT信号的进程ID,设置成了init的pid //说明init可接受这3个键 cad_pid = task_pid(current); //smp系统准备、激活所有cpu smp_prepare_cpus(setup_max_cpus); do_pre_smp_initcalls(); lockup_detector_init(); smp_init(); sched_init_smp(); //初始化设备驱动、内核模块 do_basic_setup(); /* Open the /dev/console on the rootfs, this should never fail * 打开/dev/console设备 */ if (sys_open((const char __user *) "/dev/console", O_RDWR, 0) < 0) printk(KERN_WARNING "Warning: unable to open an initial console.\n"); /* * 复制两次标准输入0，一个是标准输入1，一个是标准错误2 */ (void) sys_dup(0); (void) sys_dup(0); /* * check if there is an early userspace init. If yes, let it do all * the work * 是否有早期用户空间init进程，有的话，让其执行 */ if (!ramdisk_execute_command) ramdisk_execute_command = "/init"; if (sys_access((const char __user *) ramdisk_execute_command, 0) != 0) { ramdisk_execute_command = NULL; prepare_namespace(); } /* * 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.. */ //启动用户空间的init进程 init_post(); return 0; }
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