如果能解释一下系统调用宏就更好了 如果能解释一下系统调用宏就更好了
|
系统调用宏有点难以理解。 我花了一段时间才弄清楚宏 syscall1(int,setuid,uid_t,uid) 如何扩展到显示的汇编代码。 如果能展示宏，并稍微解释一下它是如何扩展的，那就更好了。 这是 _syscall1 宏的源代码
#define _syscall1(type,name,type1,arg1) \
type name(type1 arg1) \
{ \
long __res; \
__asm__ volatile ("int $0x80" \
: "=a" (__res) \
: "0" (__NR_##name),"b" ((long)(arg1))); \
if (__res >= 0) \
return (type) __res; \
errno = -__res; \
return -1; \
}
When expanded, this become the code
int setuid(uid_t uid)
{
long __res;
__asm__ volatile ("int $0x80" \
: "=a" (__res) \
: "0" (__NR_setuid), "b" ((long)(uid)));
if (__res >= 0 )
return (int) __res;
errno = -__res;
return -1;
}
It's pretty easy to see how the cleanup code converts into assembly, but the setup code eluded me until I figured out the following: "=a" (__res) means the result comes back in %eax "0" (__NR_setuid) means put the system call number into %eax on entry "b" ((long)(uid) means put the first argument into %ebx on entry 使用额外参数的 syscallX 宏使用 %ecx、%edx、%esi 和 %edi 来保存通过调用传递的额外值。 |