volatile int volatileInt;
int usualInt;
void function (unsigned x,unsigned y,unsigned z)
{
volatileInt = 0;
usualInt = (x % y) / z;
}
int main()
{
function(rand(),rand(),rand());
}
我用Visual C 10用/ O2编译并得到这个反汇编：
00403940 push ebx 00403941 push esi 276: function(rand(),rand()); 00403942 mov esi,dword ptr [__imp__rand (4050C0h)] 00403948 push edi 00403949 call esi 0040394B mov edi,eax 0040394D call esi 0040394F mov ebx,eax 00403951 call esi 00403953 xor edx,edx 00403955 div eax,ebx <<<< possible UB 00403957 mov dword ptr [volatileInt (4074D0h)],0 00403961 mov eax,edx 00403963 xor edx,edx 00403965 div eax,edi <<<< possible UB 00403967 pop edi 00403968 pop esi 00403969 pop ebx 0040396A mov dword ptr [usualInt (4074CCh)],eax 277: return 0; 0040396F xor eax,eax 00403971 ret
请注意,有两个操作 – “mod”和“div”,如果第二个操作数在运行时为零,则可能产生UB.在发出的代码中,两者都使用div操作码实现,这将触发结构化异常并且程序崩溃,第二个操作数为零.
第一个div是在修改volatile int变量之前,但第二个div是在修改volatile int之后.
因此,如果x为零,程序崩溃而不修改volatile int,但如果x为非零且y为零,则程序修改volatile int然后崩溃.
因此,根据x或y是否为零,程序将表现出不同的可观察行为.
解决方法
A conforming implementation executing a well-formed program shall produce the same observable behavior as one of the possible executions of the corresponding instance of the abstract machine with the same program and the same input. However,if any such execution contains an undefined operation,this International Standard places no requirement on the implementation executing that program with that input (not even with regard to operations preceding the first undefined operation).
