[英]Why does the value of a pointer change after passing it to a function that does not modify the pointer?
main()
的指針ptrTop
初始化為指向int topDeck = 1
。 每次我運行程序時,(我認為)已取消引用的指針的值都會更改為不同的數字。 我認為問題在於deal()
函數。 如果我在main()
注釋掉對deal()
的調用,則指針的值不會被修改。 我沒有編寫任何更改指針值的語句。 我圍繞可能與該問題相關的代碼部分編寫了注釋。
/*
* 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.
}
原因是:
for (card = top; card <= top + 4; ++card)
當您有變量時,卡的索引從1到5
unsigned int handSuit[5] = {0};
unsigned int handFace[5] = {0};
這僅支持索引0到4。您可以越界訪問這些數組,並且作為未定義行為的副作用,您將覆蓋其他變量。
我不太確定您期望*ptrTop
其他值是多少(六以外); 讓我們來看看deal
:
for (card = top; card <= top + 4; ++card)
記住,這是main
初始化的top
的值:
int topDeck = 1;
int *ptrTop = &topDeck;
因此,在deal
,您循環循環五次,對card
遞增,然后在最后一次迭代中, for
循環將第五次對card
進行遞增,看得出它是6
,並且由於6 <= 5
為false,它將退出,如果你做*ptrTop = card;
, *ptrTop
確實始終等於6。
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