可以解釋一下這個C輸出嗎？

Question

可能重複：
Could anyone explain these undefined behaviors (i = i++ + ++i , i = i++, etc…)

#include<stdio.h> 
void call(int,int,int); 

int main(){ 

int a=10; 
call(a,a++,++a); 
return 0; 
} 

void call(int x,int y,int z){ 
printf("x=%d y=%d z=%d\n",x,y,z); 
} 

時運行該代碼是給我的12 11 12輸出。有人可以解釋這是怎麼發生的？

Answer 1

你的代碼的行爲是undefined因爲你sequence points之間改變a兩次：

call(a,a++,++a); 

Answer 2

的behaviour is undefined因爲改變一個變量兩個控制順序點之間兩次。

c99 standard : 5.1.2.3 Program execution 

"Accessing a volatile object, modifying an object, modifying a file, or calling a function 
that does any of those operations are all `side effects` which are changes in the state of 
the `execution environment`. Evaluation of an expression may produce side effects. At 
certain specified points in the execution sequence called `sequence points`, all side effects 
of previous evaluations shall be complete and no side effects of subsequent evaluations 
shall have taken place." 

這裏要修改可變a兩者之間兩次序列點。

擴展編輯：如果你知道這些概念已經和思維有關,逗號操作符是一個序列點，所以應該作爲一個well defined工作program.Then你錯在函數調用這裏使用,是comma separator不comma operator