将指针的值传递给不修改指针的函数后，为什么会更改它？

Question

main()的指针ptrTop初始化为指向int topDeck = 1 。 每次我运行程序时，（我认为）已取消引用的指针的值都会更改为不同的数字。 我认为问题在于deal()函数。 如果我在main()注释掉对deal()的调用，则指针的值不会被修改。 我没有编写任何更改指针值的语句。 我围绕可能与该问题相关的代码部分编写了注释。

/* 
 * This is a card dealing program. 
 * the fucntion deal() keeps changing the value of 
 * the int pointer ptrTop declared in main(). sometimes, 
 * the value is what it is sopposed to be (1), but when i run 
 * the program again, it is a different value. I used 
 * gcc (GCC) 4.9.2 20150212 (Red Hat 4.9.2-6)
 */

int main(void) {

    int topDeck = 1;
    int *ptrTop = &topDeck;

    //this ptrTop is sopposed to keep track of the last
    //card dealt, but it randomly changes every time I run 

    unsigned int handSuit[5] = {0};
    unsigned int handFace[5] = {0};



    unsigned int deck[SUITS][FACES] = {0};


    deal(deck, face, suit, ptrTop, handSuit, handFace);

    printf("%i\n", *ptrTop);
    // If print the value while I comment out the deal() in line 55, 
    // the value of * ptrTop does not change.
    // this gives me reason to believe that deal() is causing the trouble.

}


void deal(unsigned int wDeck[][FACES], const char *wFace[], const char *wSuit[], 
    int *ptrTop, unsigned int handSuit[], unsigned int handFace[]) 
{
    size_t card;
    size_t row;
    size_t column;
    int top = *ptrTop;
    // i have to use top because if i don't the dea() function will print
    // more than 5 cards (it is sopposed to print a 5 card hand.

    // these for loops loop through the double scripted array wDeck
    for (card = top; card <= top + 4; ++card) {

        for (row = 0; row < SUITS; ++row) {

            for(column = 0; column < FACES; ++column) {

                if(wDeck[row][column] == card) {
                    printf( "%s of %s \n", wFace[ column ], wSuit[ row ]);
                    handFace[card] = column;
                    handSuit[card] = row;
                }
            }
        }
    }
    // *ptrTop = card;
    // the value of *ptrTop consistently becomes six if line above is uncommented.
    // I would think that the value should still be 1 
    // when the program is run in this state.
}

Answer 1

原因是：

 for (card = top; card <= top + 4; ++card)

当您有变量时，卡的索引从1到5

unsigned int handSuit[5] = {0};
unsigned int handFace[5] = {0};

这仅支持索引0到4。您可以越界访问这些数组，并且作为未定义行为的副作用，您将覆盖其他变量。

Answer 2

我不太确定您期望*ptrTop其他值是多少（六以外）； 让我们来看看deal ：

for (card = top; card <= top + 4; ++card)

记住，这是main初始化的top的值：

int topDeck = 1;
int *ptrTop = &topDeck;

因此，在deal ，您循环循环五次，对card递增，然后在最后一次迭代中， for循环将第五次对card进行递增，看得出它是6 ，并且由于6 <= 5为false，它将退出，如果你做*ptrTop = card; ， *ptrTop确实始终等于6。