系统调用跟我学(1)

#include<linux/unistd.h> /*定义宏_syscall1*/
#include<time.h>     /*定义类型time_t*/
_syscall1(time_t,time,time_t *,tloc)    /*宏，展开后得到time()函数的原型*/
main()
{
        time_t the_time;
        the_time=time((time_t *)0); /*调用time系统调用*/
        printf("The time is %ld\n",the_time);
}
系统调用time返回从格林尼治时间1970年1月1日0:00开始到现在的秒数。
这是最标准的系统调用的形式，宏_syscall1()展开来得到一个函数原型，稍后我会作详细解释。但事实上，如果把程序改成下面的样子，程序也可以运行得同样的结果。
#include<time.h>
main()
{
        time_t the_time;
        the_time=time((time_t *)0); /*调用time系统调用*/
        printf("The time is %ld\n",the_time);
}

_syscall0(type,name)
_syscall1(type,name,type1,arg1)
_syscall2(type,name,type1,arg1,type2,arg2)
_syscall3(type,name,type1,arg1,type2,arg2,type3,arg3)
_syscall4(type,name,type1,arg1,type2,arg2,type3,arg3,type4,arg4)
_syscall5(type,name,type1,arg1,type2,arg2,type3,arg3,type4,arg4,type5,arg5)
_syscall6(type,name,type1,arg1,type2,arg2,type3,arg3,type4,arg4,type5,arg5,type6,arg6)

_syscall1(time_t,time,time_t *,tloc)

time_t   time(time_t *   tloc)
{
    long __res;
    __asm__ volatile("int $0x80" : "=a" (__res) : "0" (13),"b" ((long)(tloc)));
    do {
        if ((unsigned long)(__res) >= (unsigned long)(-125)) {
            errno = -(__res);
            __res  = -1;
        }
        return (time_t) (__res);
    } while (0) ;
}