Linux系统调用流程分析
一直很好奇系统调用的详细过程，只简单的了解系统调用是用户与内核交互的借口，看了几天的内容，不知道从何下手开始写，今天略有头绪，做以简单总结。
1， 什么是系统调用。
系统调用：系统调用是操作系统为用户提供的一系列API；系统调用将用户的请求发给内核，内核执行完以后，将结果返回给用户；
2， 一个系统调用的流程。
如上图，是系统调用write()函数的调用过程，不难看出，在用户开始调用系统调用时，触发了软中断Int0X80。到这肯定有人疑问，为什么中断号是0X80，因为代码中把系统调用跟0X80绑定在一起。
中断向量：一个内存单元，存放中断处理程序入口地址和程序运行时所需处理机状态字。
这里不再深入讲解向量表的生成，只需要知道每个中断都有对应的中断号并对应处理程序。
linux的中断向量表：
a.0X80中断号：
/*
* 我们需要下面这些内嵌语句- 从内核空间创建进程(forking)将导致没有写时复
* 制（COPY ON WRITE）!!!直到一个执行execve 调用。这对堆栈可能带来问题。处
* 理的方法是在fork()调用之后不让main()使用任何堆栈。因此就不能有函数调
* 用- 这意味着fork 也要使用内嵌的代码，否则我们在从fork()退出时就要使用堆栈了。
*
* 实际上只有pause 和fork 需要使用内嵌方式，以保证从main()中不会弄乱堆栈，
* 但是我们同时还定义了其它一些函数。
*/
/*
_syscall0 是unistd.h 中的内嵌宏代码。以嵌入汇编的形式调用Linux 的系统调用中断0x80。
Linux 的系统调用中断0x80，该中断是所有系统调用的入口。
syscall0 名称中最后的0 表示无参数，1 表示1 个参数。
*/
static _inline _syscall0(int,fork)
static _inline _syscall0(int,pause) // int pause()系统调用：暂停进程的执行，直到收到一个信号。
static _inline _syscall1(int,setup,void *,BIOS) // int setup(void * BIOS)系统调用，仅用于linux 初始化（仅在这个程序中被调用）。
static _inline _syscall0(int,sync) // int sync()系统调用：更新文件系统。
#include <linux/tty.h> // tty 头文件，定义了有关tty_io，串行通信方面的
// 参数、常数。
#include <linux/sched.h> // 调度程序头文件，定义了任务结构task_struct、第1 个
// 初始任务的数据。还有一些以宏的形式定义的有关描述符
// 参数设置和获取的嵌入式汇编函数程序。
#include <linux/head.h> // head 头文件，定义了段描述符的简单结构，
// 和几个选择符常量。
#include <asm/system.h> // 系统头文件。以宏的形式定义了许多有关设置或修改
// 描述符/中断门等的嵌入式汇编子程序。
#include <asm/io.h> // io 头文件。以宏的嵌入汇编程序形式定义对io 端
// 口操作的函数。
#include <stddef.h> // 标准定义头文件。定义了NULL, offsetof(TYPE, MEMBER)。
#include <stdarg.h> // 标准参数头文件。以宏的形式定义变量参数列表。主要说
// 明了-个类型(va_list)和三个宏(va_start, va_arg 和
// va_end)，vsprintf、vprintf、vfprintf。
#include <fcntl.h> // 文件控制头文件。用于文件及其描述符的操作控制常数
// 符号的定义。
#include <sys/types.h>// 类型头文件。定义了基本的系统数据类型。
#include <linux/fs.h>// 文件系统头文件。定义文件表结构
//（file,buffer_head,m_inode 等）。
static char printbuf[1024];
extern int vsprintf(); // 送格式化输出到一字符串中（在kernel/vsprintf.c）。
extern void init(void); // 函数原形，初始化（在后面）。
extern void blk_dev_init(void); // 块设备初始化子程序（kernel/blk_drv/ll_rw_blk.c）
extern void chr_dev_init(void); // 字符设备初始化（kernel/chr_drv/tty_io.c）
extern void hd_init(void); // 硬盘初始化程序（kernel/blk_drv/hd.c）
extern void floppy_init(void); // 软驱初始化程序（kernel/blk_drv/floppy.c）
extern void mem_init(long start, long end); // 内存管理初始化（mm/memory.c）
extern long rd_init(long mem_start, int length); //虚拟盘初始化(kernel/blk_drv/ramdisk.c)
extern long kernel_mktime(struct tm * tm); // 建立内核时间（秒）。
extern long startup_time; // 内核启动时间（开机时间）（秒）。
/*
* 以下这些数据是由setup.s 程序在引导时间设置的。
*/
#define EXT_MEM_K (*(unsigned short *)0x90002) // 1M 以后的扩展内存大小（KB）。
#define DRIVE_INFO (*(struct drive_info *)0x90080) // 硬盘参数表基址。
#define ORIG_ROOT_DEV (*(unsigned short *)0x901FC) // 根文件系统所在设备号。
/*
* 是啊，是啊，下面这段程序很差劲，但我不知道如何正确地实现，而且好象
* 它还能运行。如果有关于实时时钟更多的资料，那我很感兴趣。这些都是试
* 探出来的，以及看了一些bios 程序，呵！
*/
// 这段宏读取CMOS 实时时钟信息。
// 0x70 是写端口号，0x80|addr 是要读取的CMOS 内存地址。
// 0x71 是读端口号。
/*
#define CMOS_READ(addr) ({ \
outb_p(0x80|addr,0x70); \
inb_p(0x71); \
})*/
_inline unsigned char CMOS_READ(unsigned char addr)
{
outb_p(addr,0x70);
return inb_p(0x71);
}
下面是_system_call的汇编代码(asm.S)：
_system_call:
cmp eax,nr_system_calls-1 ;// 调用号如果超出范围的话就在eax 中置-1 并退出。
ja bad_sys_call
push ds ;// 保存原段寄存器值。
push es
push fs
push edx ;// ebx,ecx,edx 中放着系统调用相应的C 语言函数的调用参数。
push ecx ;// push %ebx,%ecx,%edx as parameters
push ebx ;// to the system call
mov edx,10h ;// set up ds,es to kernel space
mov ds,dx ;// ds,es 指向内核数据段(全局描述符表中数据段描述符)。
mov es,dx
mov edx,17h ;// fs points to local data space
mov fs,dx ;// fs 指向局部数据段(局部描述符表中数据段描述符)。
;// 下面这句操作数的含义是：调用地址 = _sys_call_table + %eax * 4。参见列表后的说明。
;// 对应的C 程序中的sys_call_table 在include/linux/sys.h 中，其中定义了一个包括72 个
;// 系统调用C 处理函数的地址数组表。
call [_sys_call_table+eax*4]
push eax ;// 把系统调用号入栈。
mov eax,_current ;// 取当前任务（进程）数据结构地址??eax。
;// 下面97-100 行查看当前任务的运行状态。如果不在就绪状态(state 不等于0)就去执行调度程序。
;// 如果该任务在就绪状态但counter[??]值等于0，则也去执行调度程序。
cmp dword ptr [state+eax],0 ;// state
jne reschedule
cmp dword ptr [counter+eax],0 ;// counter
je reschedule
b.现场保护与恢复
因为用户态和内核态不共用一个堆栈，所以用户态调用系统调用时，得先保存用户态的信息到寄存器，然后切入到内核态，将寄存器中的信息拷贝到内核栈开始执行，执行结束后，还原之前用户态的状态让用户态继续往下执行。
用户态程序通过ebx,ecx,edx等寄存器，传递函数参数给系统调用，ebx,ecx,edx等寄存器通过SAVE_ALL宏压到内核栈。
系统调用结束之后，RESTORE_ALL宏把SAVE_ALL宏压入的寄存器反向弹出，以用于恢复现场。
c.调用系统调用表
syscall_handler_t *sys_call_table[] = {
[ __NR_restart_syscall ] = (syscall_handler_t *) sys_restart_syscall,
[ __NR_exit ] = (syscall_handler_t *) sys_exit,
[ __NR_fork ] = (syscall_handler_t *) sys_fork,
[ __NR_read ] = (syscall_handler_t *) sys_read,
[ __NR_write ] = (syscall_handler_t *) sys_write,
/* These three are declared differently in asm/unistd.h */
[ __NR_open ] = (syscall_handler_t *) sys_open,
[ __NR_close ] = (syscall_handler_t *) sys_close,
[ __NR_creat ] = (syscall_handler_t *) sys_creat,
[ __NR_link ] = (syscall_handler_t *) sys_link,
[ __NR_unlink ] = (syscall_handler_t *) sys_unlink,
[ __NR_execve ] = (syscall_handler_t *) sys_execve,
/* declared differently in kern_util.h */
[ __NR_chdir ] = (syscall_handler_t *) sys_chdir,
[ __NR_time ] = um_time,
[ __NR_mknod ] = (syscall_handler_t *) sys_mknod,
[ __NR_chmod ] = (syscall_handler_t *) sys_chmod,
[ __NR_lchown ] = (syscall_handler_t *) sys_lchown16,
[ __NR_lseek ] = (syscall_handler_t *) sys_lseek,
[ __NR_getpid ] = (syscall_handler_t *) sys_getpid,
[ __NR_mount ] = um_mount,
[ __NR_setuid ] = (syscall_handler_t *) sys_setuid16,
[ __NR_getuid ] = (syscall_handler_t *) sys_getuid16,
[ __NR_ptrace ] = (syscall_handler_t *) sys_ptrace,
[ __NR_alarm ] = (syscall_handler_t *) sys_alarm,
[ __NR_pause ] = (syscall_handler_t *) sys_pause,
[ __NR_utime ] = (syscall_handler_t *) sys_utime,
[ __NR_access ] = (syscall_handler_t *) sys_access,
[ __NR_sync ] = (syscall_handler_t *) sys_sync,
[ __NR_kill ] = (syscall_handler_t *) sys_kill,
[ __NR_rename ] = (syscall_handler_t *) sys_rename,
[ __NR_mkdir ] = (syscall_handler_t *) sys_mkdir,
[ __NR_rmdir ] = (syscall_handler_t *) sys_rmdir,
/* Declared differently in asm/unistd.h */
[ __NR_dup ] = (syscall_handler_t *) sys_dup,
[ __NR_pipe ] = (syscall_handler_t *) sys_pipe,
[ __NR_times ] = (syscall_handler_t *) sys_times,
[ __NR_brk ] = (syscall_handler_t *) sys_brk,
[ __NR_setgid ] = (syscall_handler_t *) sys_setgid16,
[ __NR_getgid ] = (syscall_handler_t *) sys_getgid16,
[ __NR_geteuid ] = (syscall_handler_t *) sys_geteuid16,
[ __NR_getegid ] = (syscall_handler_t *) sys_getegid16,
[ __NR_acct ] = (syscall_handler_t *) sys_acct,
[ __NR_umount2 ] = (syscall_handler_t *) sys_umount,
[ __NR_ioctl ] = (syscall_handler_t *) sys_ioctl,
[ __NR_fcntl ] = (syscall_handler_t *) sys_fcntl,
[ __NR_setpgid ] = (syscall_handler_t *) sys_setpgid,
[ __NR_umask ] = (syscall_handler_t *) sys_umask,
[ __NR_chroot ] = (syscall_handler_t *) sys_chroot,
[ __NR_ustat ] = (syscall_handler_t *) sys_ustat,
[ __NR_dup2 ] = (syscall_handler_t *) sys_dup2,
[ __NR_getppid ] = (syscall_handler_t *) sys_getppid,
[ __NR_getpgrp ] = (syscall_handler_t *) sys_getpgrp,
[ __NR_setsid ] = (syscall_handler_t *) sys_setsid,
[ __NR_setreuid ] = (syscall_handler_t *) sys_setreuid16,
[ __NR_setregid ] = (syscall_handler_t *) sys_setregid16,
[ __NR_sethostname ] = (syscall_handler_t *) sys_sethostname,
[ __NR_setrlimit ] = (syscall_handler_t *) sys_setrlimit,
[ __NR_getrlimit ] = (syscall_handler_t *) sys_old_getrlimit,
[ __NR_getrusage ] = (syscall_handler_t *) sys_getrusage,
[ __NR_gettimeofday ] = (syscall_handler_t *) sys_gettimeofday,
[ __NR_settimeofday ] = (syscall_handler_t *) sys_settimeofday,
[ __NR_getgroups ] = (syscall_handler_t *) sys_getgroups16,
[ __NR_setgroups ] = (syscall_handler_t *) sys_setgroups16,
[ __NR_symlink ] = (syscall_handler_t *) sys_symlink,
[ __NR_readlink ] = (syscall_handler_t *) sys_readlink,
[ __NR_uselib ] = (syscall_handler_t *) sys_uselib,
[ __NR_swapon ] = (syscall_handler_t *) sys_swapon,
[ __NR_reboot ] = (syscall_handler_t *) sys_reboot,
[ __NR_munmap ] = (syscall_handler_t *) sys_munmap,
[ __NR_truncate ] = (syscall_handler_t *) sys_truncate,
[ __NR_ftruncate ] = (syscall_handler_t *) sys_ftruncate,
[ __NR_fchmod ] = (syscall_handler_t *) sys_fchmod,
[ __NR_fchown ] = (syscall_handler_t *) sys_fchown16,
[ __NR_getpriority ] = (syscall_handler_t *) sys_getpriority,
[ __NR_setpriority ] = (syscall_handler_t *) sys_setpriority,
[ __NR_statfs ] = (syscall_handler_t *) sys_statfs,
[ __NR_fstatfs ] = (syscall_handler_t *) sys_fstatfs,
[ __NR_ioperm ] = (syscall_handler_t *) sys_ni_syscall,
[ __NR_syslog ] = (syscall_handler_t *) sys_syslog,
[ __NR_setitimer ] = (syscall_handler_t *) sys_setitimer,
[ __NR_getitimer ] = (syscall_handler_t *) sys_getitimer,
[ __NR_stat ] = (syscall_handler_t *) sys_newstat,
[ __NR_lstat ] = (syscall_handler_t *) sys_newlstat,
[ __NR_fstat ] = (syscall_handler_t *) sys_newfstat,
[ __NR_vhangup ] = (syscall_handler_t *) sys_vhangup,
[ __NR_wait4 ] = (syscall_handler_t *) sys_wait4,
[ __NR_swapoff ] = (syscall_handler_t *) sys_swapoff,
[ __NR_sysinfo ] = (syscall_handler_t *) sys_sysinfo,
[ __NR_fsync ] = (syscall_handler_t *) sys_fsync,
[ __NR_clone ] = (syscall_handler_t *) sys_clone,
[ __NR_setdomainname ] = (syscall_handler_t *) sys_setdomainname,
[ __NR_uname ] = (syscall_handler_t *) sys_newuname,
[ __NR_adjtimex ] = (syscall_handler_t *) sys_adjtimex,
[ __NR_mprotect ] = (syscall_handler_t *) sys_mprotect,
[ __NR_create_module ] = (syscall_handler_t *) sys_ni_syscall,
[ __NR_init_module ] = (syscall_handler_t *) sys_init_module,
[ __NR_delete_module ] = (syscall_handler_t *) sys_delete_module,
[ __NR_get_kernel_syms ] = (syscall_handler_t *) sys_ni_syscall,
[ __NR_quotactl ] = (syscall_handler_t *) sys_quotactl,
[ __NR_getpgid ] = (syscall_handler_t *) sys_getpgid,
[ __NR_fchdir ] = (syscall_handler_t *) sys_fchdir,
[ __NR_sysfs ] = (syscall_handler_t *) sys_sysfs,
[ __NR_personality ] = (syscall_handler_t *) sys_personality,
[ __NR_afs_syscall ] = (syscall_handler_t *) sys_ni_syscall,
[ __NR_setfsuid ] = (syscall_handler_t *) sys_setfsuid16,
[ __NR_setfsgid ] = (syscall_handler_t *) sys_setfsgid16,
[ __NR_getdents ] = (syscall_handler_t *) sys_getdents,
[ __NR_flock ] = (syscall_handler_t *) sys_flock,
[ __NR_msync ] = (syscall_handler_t *) sys_msync,
[ __NR_readv ] = (syscall_handler_t *) sys_readv,
[ __NR_writev ] = (syscall_handler_t *) sys_writev,
[ __NR_getsid ] = (syscall_handler_t *) sys_getsid,
[ __NR_fdatasync ] = (syscall_handler_t *) sys_fdatasync,
[ __NR__sysctl ] = (syscall_handler_t *) sys_sysctl,
[ __NR_mlock ] = (syscall_handler_t *) sys_mlock,
[ __NR_munlock ] = (syscall_handler_t *) sys_munlock,
[ __NR_mlockall ] = (syscall_handler_t *) sys_mlockall,
[ __NR_munlockall ] = (syscall_handler_t *) sys_munlockall,
[ __NR_sched_setparam ] = (syscall_handler_t *) sys_sched_setparam,
[ __NR_sched_getparam ] = (syscall_handler_t *) sys_sched_getparam,
[ __NR_sched_setscheduler ] = (syscall_handler_t *) sys_sched_setscheduler,
[ __NR_sched_getscheduler ] = (syscall_handler_t *) sys_sched_getscheduler,
[ __NR_sched_yield ] = (syscall_handler_t *) yield,
[ __NR_sched_get_priority_max ] = (syscall_handler_t *) sys_sched_get_priority_max,
[ __NR_sched_get_priority_min ] = (syscall_handler_t *) sys_sched_get_priority_min,
[ __NR_sched_rr_get_interval ] = (syscall_handler_t *) sys_sched_rr_get_interval,
[ __NR_nanosleep ] = (syscall_handler_t *) sys_nanosleep,
[ __NR_mremap ] = (syscall_handler_t *) sys_mremap,
[ __NR_setresuid ] = (syscall_handler_t *) sys_setresuid16,
[ __NR_getresuid ] = (syscall_handler_t *) sys_getresuid16,
[ __NR_query_module ] = (syscall_handler_t *) sys_ni_syscall,
[ __NR_poll ] = (syscall_handler_t *) sys_poll,
[ __NR_nfsservctl ] = (syscall_handler_t *) NFSSERVCTL,
[ __NR_setresgid ] = (syscall_handler_t *) sys_setresgid16,
[ __NR_getresgid ] = (syscall_handler_t *) sys_getresgid16,
[ __NR_prctl ] = (syscall_handler_t *) sys_prctl,
[ __NR_rt_sigreturn ] = (syscall_handler_t *) sys_rt_sigreturn,
[ __NR_rt_sigaction ] = (syscall_handler_t *) sys_rt_sigaction,
[ __NR_rt_sigprocmask ] = (syscall_handler_t *) sys_rt_sigprocmask,
[ __NR_rt_sigpending ] = (syscall_handler_t *) sys_rt_sigpending,
[ __NR_rt_sigtimedwait ] = (syscall_handler_t *) sys_rt_sigtimedwait,
[ __NR_rt_sigqueueinfo ] = (syscall_handler_t *) sys_rt_sigqueueinfo,
[ __NR_rt_sigsuspend ] = (syscall_handler_t *) sys_rt_sigsuspend,
[ __NR_pread64 ] = (syscall_handler_t *) sys_pread64,
[ __NR_pwrite64 ] = (syscall_handler_t *) sys_pwrite64,
[ __NR_chown ] = (syscall_handler_t *) sys_chown16,
[ __NR_getcwd ] = (syscall_handler_t *) sys_getcwd,
[ __NR_capget ] = (syscall_handler_t *) sys_capget,
[ __NR_capset ] = (syscall_handler_t *) sys_capset,
[ __NR_sigaltstack ] = (syscall_handler_t *) sys_sigaltstack,
[ __NR_sendfile ] = (syscall_handler_t *) sys_sendfile,
[ __NR_getpmsg ] = (syscall_handler_t *) sys_ni_syscall,
[ __NR_putpmsg ] = (syscall_handler_t *) sys_ni_syscall,
[ __NR_vfork ] = (syscall_handler_t *) sys_vfork,
[ __NR_getdents64 ] = (syscall_handler_t *) sys_getdents64,
[ __NR_gettid ] = (syscall_handler_t *) sys_gettid,
[ __NR_readahead ] = (syscall_handler_t *) sys_readahead,
[ __NR_setxattr ] = (syscall_handler_t *) sys_setxattr,
[ __NR_lsetxattr ] = (syscall_handler_t *) sys_lsetxattr,
[ __NR_fsetxattr ] = (syscall_handler_t *) sys_fsetxattr,
[ __NR_getxattr ] = (syscall_handler_t *) sys_getxattr,
[ __NR_lgetxattr ] = (syscall_handler_t *) sys_lgetxattr,
[ __NR_fgetxattr ] = (syscall_handler_t *) sys_fgetxattr,
[ __NR_listxattr ] = (syscall_handler_t *) sys_listxattr,
[ __NR_llistxattr ] = (syscall_handler_t *) sys_llistxattr,
[ __NR_flistxattr ] = (syscall_handler_t *) sys_flistxattr,
[ __NR_removexattr ] = (syscall_handler_t *) sys_removexattr,
[ __NR_lremovexattr ] = (syscall_handler_t *) sys_lremovexattr,
[ __NR_fremovexattr ] = (syscall_handler_t *) sys_fremovexattr,
[ __NR_tkill ] = (syscall_handler_t *) sys_tkill,
[ __NR_futex ] = (syscall_handler_t *) sys_futex,
[ __NR_sched_setaffinity ] = (syscall_handler_t *) sys_sched_setaffinity,
[ __NR_sched_getaffinity ] = (syscall_handler_t *) sys_sched_getaffinity,
[ __NR_io_setup ] = (syscall_handler_t *) sys_io_setup,
[ __NR_io_destroy ] = (syscall_handler_t *) sys_io_destroy,
[ __NR_io_getevents ] = (syscall_handler_t *) sys_io_getevents,
[ __NR_io_submit ] = (syscall_handler_t *) sys_io_submit,
[ __NR_io_cancel ] = (syscall_handler_t *) sys_io_cancel,
[ __NR_exit_group ] = (syscall_handler_t *) sys_exit_group,
[ __NR_lookup_dcookie ] = (syscall_handler_t *) sys_lookup_dcookie,
[ __NR_epoll_create ] = (syscall_handler_t *) sys_epoll_create,
[ __NR_epoll_ctl ] = (syscall_handler_t *) sys_epoll_ctl,
[ __NR_epoll_wait ] = (syscall_handler_t *) sys_epoll_wait,
[ __NR_set_tid_address ] = (syscall_handler_t *) sys_set_tid_address,
[ __NR_timer_create ] = (syscall_handler_t *) sys_timer_create,
[ __NR_timer_settime ] = (syscall_handler_t *) sys_timer_settime,
[ __NR_timer_gettime ] = (syscall_handler_t *) sys_timer_gettime,
[ __NR_timer_getoverrun ] = (syscall_handler_t *) sys_timer_getoverrun,
[ __NR_timer_delete ] = (syscall_handler_t *) sys_timer_delete,
[ __NR_clock_settime ] = (syscall_handler_t *) sys_clock_settime,
[ __NR_clock_gettime ] = (syscall_handler_t *) sys_clock_gettime,
[ __NR_clock_getres ] = (syscall_handler_t *) sys_clock_getres,
[ __NR_clock_nanosleep ] = (syscall_handler_t *) sys_clock_nanosleep,
[ __NR_statfs64 ] = (syscall_handler_t *) sys_statfs64,
[ __NR_fstatfs64 ] = (syscall_handler_t *) sys_fstatfs64,
[ __NR_tgkill ] = (syscall_handler_t *) sys_tgkill,
[ __NR_utimes ] = (syscall_handler_t *) sys_utimes,
[ __NR_fadvise64_64 ] = (syscall_handler_t *) sys_fadvise64_64,
[ __NR_vserver ] = (syscall_handler_t *) sys_vserver,
[ __NR_mbind ] = (syscall_handler_t *) sys_mbind,
[ __NR_get_mempolicy ] = (syscall_handler_t *) sys_get_mempolicy,
[ __NR_set_mempolicy ] = (syscall_handler_t *) sys_set_mempolicy,
[ __NR_mq_open ] = (syscall_handler_t *) sys_mq_open,
[ __NR_mq_unlink ] = (syscall_handler_t *) sys_mq_unlink,
[ __NR_mq_timedsend ] = (syscall_handler_t *) sys_mq_timedsend,
[ __NR_mq_timedreceive ] = (syscall_handler_t *) sys_mq_timedreceive,
[ __NR_mq_notify ] = (syscall_handler_t *) sys_mq_notify,
[ __NR_mq_getsetattr ] = (syscall_handler_t *) sys_mq_getsetattr,
[ __NR_sys_kexec_load ] = (syscall_handler_t *) sys_ni_syscall,
[ __NR_waitid ] = (syscall_handler_t *) sys_waitid,
[ 285 ] = (syscall_handler_t *) sys_ni_syscall,
[ __NR_add_key ] = (syscall_handler_t *) sys_add_key,
[ __NR_request_key ] = (syscall_handler_t *) sys_request_key,
[ __NR_keyctl ] = (syscall_handler_t *) sys_keyctl,
ARCH_SYSCALLS
[ LAST_SYSCALL + 1 ... NR_syscalls ] =
(syscall_handler_t *) sys_ni_syscall
};
从这里会跳转到具体的系统调用。
每个系统调用对应一个调用号（unistd.h）
#ifndef _ALPHA_UNISTD_H
#define _ALPHA_UNISTD_H
#define __NR_osf_syscall 0 /* not implemented */
#define __NR_exit 1
#define __NR_fork 2
#define __NR_read 3
#define __NR_write 4
#define __NR_osf_old_open 5 /* not implemented */
#define __NR_close 6
#define __NR_osf_wait4 7
#define __NR_osf_old_creat 8 /* not implemented */
#define __NR_link 9
#define __NR_unlink 10
#define __NR_osf_execve 11 /* not implemented */
#define __NR_chdir 12
#define __NR_fchdir 13
#define __NR_mknod 14
#define __NR_chmod 15
#define __NR_chown 16
#define __NR_brk 17
#define __NR_osf_getfsstat 18 /* not implemented */
#define __NR_lseek 19
#define __NR_getxpid 20
#define __NR_osf_mount 21
#define __NR_umount 22
#define __NR_setuid 23
#define __NR_getxuid 24
#define __NR_exec_with_loader 25 /* not implemented */
#define __NR_ptrace 26
#define __NR_osf_nrecvmsg 27 /* not implemented */
#define __NR_osf_nsendmsg 28 /* not implemented */
#define __NR_osf_nrecvfrom 29 /* not implemented */
#define __NR_osf_naccept 30 /* not implemented */
#define __NR_osf_ngetpeername 31 /* not implemented */
#define __NR_osf_ngetsockname 32 /* not implemented */
#define __NR_access 33
#define __NR_osf_chflags 34 /* not implemented */
#define __NR_osf_fchflags 35 /* not implemented */
#define __NR_sync 36
#define __NR_kill 37
#define __NR_osf_old_stat 38 /* not implemented */
#define __NR_setpgid 39
#define __NR_osf_old_lstat 40 /* not implemented */
#define __NR_dup 41
#define __NR_pipe 42
#define __NR_osf_set_program_attributes 43
#define __NR_osf_profil 44 /* not implemented */
#define __NR_open 45
#define __NR_osf_old_sigaction 46 /* not implemented */
#define __NR_getxgid 47
#define __NR_osf_sigprocmask 48
#define __NR_osf_getlogin 49 /* not implemented */
#define __NR_osf_setlogin 50 /* not implemented */
#define __NR_acct 51
#define __NR_sigpending 52
#define __NR_ioctl 54
#define __NR_osf_reboot 55 /* not implemented */
#define __NR_osf_revoke 56 /* not implemented */
#define __NR_symlink 57
#define __NR_readlink 58
#define __NR_execve 59
#define __NR_umask 60
#define __NR_chroot 61
#define __NR_osf_old_fstat 62 /* not implemented */
#define __NR_getpgrp 63
#define __NR_getpagesize 64
#define __NR_osf_mremap 65 /* not implemented */
#define __NR_vfork 66
#define __NR_stat 67
#define __NR_lstat 68
#define __NR_osf_sbrk 69 /* not implemented */
#define __NR_osf_sstk 70 /* not implemented */
#define __NR_mmap 71 /* OSF/1 mmap is superset of Linux */
#define __NR_osf_old_vadvise 72 /* not implemented */
#define __NR_munmap 73
#define __NR_mprotect 74
#define __NR_madvise 75
#define __NR_vhangup 76
#define __NR_osf_kmodcall 77 /* not implemented */
#define __NR_osf_mincore 78 /* not implemented */
#define __NR_getgroups 79
#define __NR_setgroups 80
#define __NR_osf_old_getpgrp 81 /* not implemented */
#define __NR_setpgrp 82 /* BSD alias for setpgid */
#define __NR_osf_setitimer 83
#define __NR_osf_old_wait 84 /* not implemented */
#define __NR_osf_table 85 /* not implemented */
#define __NR_osf_getitimer 86
#define __NR_gethostname 87
#define __NR_sethostname 88
#define __NR_getdtablesize 89
#define __NR_dup2 90
#define __NR_fstat 91
#define __NR_fcntl 92
#define __NR_osf_select 93
#define __NR_poll 94
#define __NR_fsync 95
#define __NR_setpriority 96
#define __NR_socket 97
#define __NR_connect 98
#define __NR_accept 99
#define __NR_getpriority 100..........