Question

所以我写了这个程序代表停车场作为一个位置，停车场的每个空间都是一点。我有一个checkSpace函数来检查空间是否被占用，由于某种原因，指向我的停车位集的指针发生了变化，或者在我将其传递给函数后数据发生了变化。为了测试它，我设置了停车场，我检查了一个空间，然后立即再次检查它，并且由于某种原因，返回值正在改变，当它不应该是。任何帮助将不胜感激！

    struct carPark{
        int spaces, levels;
        unsigned char * park;
    };

    struct carPark * emptyCarPark(int levels, int spaces){
        int chars = (spaces*levels)/8;
        if((spaces*levels)%8 != 0){
            chars++;
        }
        unsigned char park[chars];
        for (int i = 0; i < chars; ++i){
            park[i] = 0;
        }
        unsigned char * ptr  = &park[0];
        struct carPark * myPark = malloc(sizeof(struct carPark));
        myPark->park = ptr;
        myPark->spaces = spaces;
        myPark->levels = levels;
        return myPark;
    }

    int checkSpace(int level, int spaceNum, struct carPark * carpark){
        int charPosition = ((level*carpark->spaces) + spaceNum)/8;
        int bitPosition = ((level*carpark->spaces) + spaceNum)%8;
        if(carpark->park[charPosition]&&(1<<bitPosition) != 0){
            return 1;
        }
        return 0;
    }

    int main(int argc, char const *argv[]){
        struct carPark * myPark = emptyCarPark(5,20);
        printf("1st check: %d\n",checkSpace(1,1,myPark));
        printf("Second check: %d\n",checkSpace(1,1,myPark));
        return 0;
    }

所以当我运行程序时，我得到了：

    1st check: 0
    Second check: 1

Answer 1

查看下面的代码 - 在emptyCarPark（）中，您将在堆栈上分配park数组，然后返回指向它的指针。一旦函数返回，就不再分配park数组，并且你有一个悬空指针 - 有关更多信息，请参阅：Cause of dangling pointers (Wikipedia)

    unsigned char park[chars];
    for (int i = 0; i < chars; ++i){
        park[i] = 0;
    }
    // This is the pointer to an object on the stack.
    unsigned char * ptr  = &park[0];

    struct carPark * myPark = malloc(sizeof(struct carPark));
    myPark->park = ptr;

函数调用C后指针正在改变

1 个答案: