在 C 中，将指向结构的指针值发送到函数时返回浮点数会更改结构的值

Question

我正在尝试在 C 语言中使用结构和指向结构的指针进行一些练习。我遇到了一个困扰我的问题。我已经设置了一个函数，它从一个指向结构的指针中获取 3 个值并返回一个浮点数。但是，当调用此函数时，它会修改原始结构的值，最终导致访问冲突和一些令人讨厌的字符反刍。单步执行代码后，值正常，直到我返回函数的值。

这是我的结构：

struct product
{
    int ID;
    char name[MAXNAME];
    char department[MAXDEPT];
    float price;
    int taxable;
    int free_shipping_eligible;
};
typedef struct product Product;

函数原型：

Product * getInput(void);
float transactionTotal(const float price, const int taxable,
const int free_shipping);
void displayTransaction(const Product * inventory,
const float total_price);

主要：

int main(void)
{
    // Declare a pointer to a struct.
    Product * invPoint = NULL;
    float grand_total = 0.0;

    // Get the value of the address of a struct.
    invPoint = getInput();

    // Use that address to get the total of a transaction.
    grand_total = transactionTotal(invPoint->price, invPoint->taxable,
        invPoint->free_shipping_eligible);

    // Display product information and the transaction's total.
    displayTransaction(invPoint, grand_total);
    return EXIT_SUCCESS;
}

这是填充结构成员值的函数：

Product * getInput(void)
{
    // Declare our struct and struct pointer variables.
    Product inventory;
    Product * fillInventory = NULL;

    // Get user input.
    printf("Please enter the product's ID.\n\t");
    scanf("%d", &inventory.ID);
    printf("Please enter the product's name.\n\t");
    scanf("%s", &inventory.name);
    printf("Please enter the product's price.\n\t");
    scanf("%f", &inventory.price);
    printf("Which department is the product located in?\n\t");
    scanf("%s", &inventory.department);
    printf("Is it taxable?  0 for no, 1 for yes.\n\t");
    scanf("%d", &inventory.taxable);
    printf("Is it free shipping eligible?  1 for no, 0 for yes.\n\t");
    scanf("%d", &inventory.free_shipping_eligible);
    fillInventory = &inventory;

    // Return a pointer to our struct.
    return fillInventory;
}

transactionTotal 函数似乎是罪魁祸首。这是它的代码：

float transactionTotal(const float price, const int taxable,
const int free_shipping)
{
    float total_price = 0.00;
    float tax = (float)(taxable * TAX_RATE);
    float shipping = (float)(free_shipping * SHIPPING_RATE);
    total_price = price + price * tax + price * shipping;
    return total_price;
}

在启动上述函数之前，我输入了以下值：

ID：232

名称：“咖啡”

部门：“食品”

价格：8.99

应税：0

free_shipping_eligible: 1

transactionTotal函数后：

ID：-858993460

姓名：（奇怪的字符）

部门：（奇怪的字符）

价格：9.88899994

应税：-858993460

free_shipping_eligible: 14679252

Answer 1

    // Return a pointer to our struct.
    return fillInventory;
}

您的结构对象在您的函数中声明，并且其生命周期在您的函数返回后结束。当您的函数返回并尝试访问它之后调用未定义的行为时，结构对象将被丢弃。将您的结构对象作为参数传递给函数或使用malloc 分配其存储空间以解决您的问题。

Answer 2

问题出在getInput:

Product inventory;
Product * fillInventory = NULL;
....
fillInventory = &inventory;

// Return a pointer to our struct.
return fillInventory;

您正在返回一个局部变量的地址。当函数返回时，该变量超出范围，导致undefined behavior。

您需要为此结构动态分配空间。

Product * getInput(void)
{
    Product * fillInventory = malloc(sizeof Product);
    ...

那么调用者需要free那个结构体：

int main(void)
{
    // Declare a pointer to a struct.
    Product * invPoint = NULL;
    float grand_total = 0.0;

    // Get the value of the address of a struct.
    invPoint = getInput();

    // Use that address to get the total of a transaction.
    grand_total = transactionTotal(invPoint->price, invPoint->taxable,
        invPoint->free_shipping_eligible);

    // Display product information and the transaction's total.
    displayTransaction(invPoint, grand_total);
    free(invPoint);         // free the memory
    return EXIT_SUCCESS;
}