把握linux内核设计（一）：系统调用

......
 # system call handler stub
ENTRY(system_call)
 RING0_INT_FRAME # can't unwind into user space anyway
 pushl %eax # save orig_eax
 CFI_ADJUST_CFA_OFFSET 4
 SAVE_ALL
 GET_THREAD_INFO(%ebp)
     # system call tracing in operation / emulation
 testl $_TIF_WORK_SYSCALL_ENTRY,TI_flags(%ebp)
 jnz syscall_trace_entry
 cmpl $(nr_syscalls), %eax
 jae syscall_badsys
syscall_call:
 call *sys_call_table(,%eax,4) //此处执行相应的系统调用
 movl %eax,PT_EAX(%esp) # store the return value
syscall_exit:
 LOCKDEP_SYS_EXIT
 DISABLE_INTERRUPTS(CLBR_ANY) # make sure we don't miss an interrupt
     # setting need_resched or sigpending
     # between sampling and the iret
 TRACE_IRQS_OFF
 movl TI_flags(%ebp), %ecx
 testl $_TIF_ALLWORK_MASK, %ecx # current->work
 jne syscall_exit_work
 ......

$ cat ./arch/x86/include/asm/unistd.h
#ifdef __KERNEL__                                                                                                                      
# ifdef CONFIG_X86_32
#  include "unistd_32.h"
# else
#  include "unistd_64.h"
# endif
#else
# ifdef __i386__
#  include "unistd_32.h"
# else
#  include "unistd_64.h"
# endif
#endif

# cat arch/x86/include/asm/unistd_32.h
#ifndef _ASM_X86_UNISTD_32_H
#define _ASM_X86_UNISTD_32_H

/*
 * This file contains the system call numbers.
 */

#define __NR_restart_syscall      0
#define __NR_exit                 1
#define __NR_fork                 2
#define __NR_read                 3
#define __NR_write                4
#define __NR_open                 5
#define __NR_close                6
#define __NR_waitpid              7
#define __NR_creat                8
#define __NR_link                 9
#define __NR_unlink              10
#define __NR_execve              11
#define __NR_chdir               12
#define __NR_time                13
#define __NR_mknod               14
#define __NR_chmod               15
#define __NR_lchown              16
#define __NR_break               17
#define __NR_oldstat             18
#define __NR_lseek               19
#define __NR_getpid              20
#define __NR_mount               21
......

ENTRY(sys_call_table)
 .long sys_restart_syscall /* 0 - old "setup()" system call, used for restarting */
 .long sys_exit
 .long ptregs_fork
 .long sys_read
 .long sys_write
 .long sys_open /* 5 */
 .long sys_close
 .long sys_waitpid
 .long sys_creat
 .long sys_link
 .long sys_unlink /* 10 */
 .long ptregs_execve
 ......
 .long sys_timerfd_settime /* 325 */
 .long sys_timerfd_gettime
 .long sys_signalfd4
 .long sys_eventfd2
 .long sys_epoll_create1
 .long sys_dup3 /* 330 */
 .long sys_pipe2
 .long sys_inotify_init1
 .long sys_preadv
 .long sys_pwritev
 .long sys_rt_tgsigqueueinfo /* 335 */
 .long sys_perf_event_open

#include <stdio.h>
#include <unistd.h>
#include <sys/types.h>
int main(int argc, const char *argv[])
{
    pid_t   pid;
    asm volatile (
            "mov $0, %%ebx\n\t"
            "mov $20, %%eax\n\t"    //把系统调用号20放入eax寄存器中，20对应于SYS_getpid()系统调用
            "int $0x80\n\t"    //0x80中断
            "mov %%eax, %0\n\t"    //将执行结果存放在pid变量中
            :"=m"(pid)
            );
    printf("int PID: %d\n", pid);
    printf("api PID: %d\n", getpid());
    return 0;
}

#include <stdio.h>
#include <sys/types.h>
#include <unistd.h>
#include <sys/syscall.h>
int main(int argc, const char *argv[])
{
    pid_t     pid, pidt;
    pid = getpid();
    pidt = syscall(SYS_getpid);
    printf("getpid: %d\n", pid);
    printf("SYS_getpid: %d\n", pidt);
    return 0;
}

/**
 * sys_getpid - return the thread group id of the current process
 *
 * Note, despite the name, this returns the tgid not the pid.  The tgid and
 * the pid are identical unless CLONE_THREAD was specified on clone() in
 * which case the tgid is the same in all threads of the same group.
 *
 * This is SMP safe as current->tgid does not change.
 */
SYSCALL_DEFINE0(getpid)
{
 return task_tgid_vnr(current);
}

asmlinkage long sys_getpid(void)

{

return current->tpid;

}

把握linux内核设计（一）：系统调用

原文：http://blog.csdn.net/shallnet/article/details/47113753