在c中使用带浮点数的offsetof

Question

代码适用于int,但是当我想使用float时它会失败,除非我将结构转换为字符指针.这是它的样子:

struct test
{
    float a;
    float b;
};

void stuff(int offset, test* location);

int main()
{
    test *t;
    t = (test*)malloc(sizeof(test));

    char choice = '\0';

    //Find the byte offset of 'a' within the structure
    int offset;
    printf("Edit a or b?");
    scanf("%c", &choice);
    switch (toupper(choice))
    {

    case 'A':
        offset = offsetof(test, a);
        stuff(offset, t);
        break;
    case 'B':
        offset = offsetof(test, b);
        stuff(offset, t);
        break;
    }
    printf("%f %f\n", t->a, t->b);
    return 0;
}

void stuff(int offset, test* location)
{
    float imput;
    printf("What would you like to put in it? ");
    scanf("%f", &imput);
    *(float *)((char *)location + offset) = imput;
    //*(float *)(location + offset) = imput   Will Not Work
}

*(float *)(location + offset)= imput 不适用于浮点数而是投射位置和偏移量作为int指针.

我试过在线看,但我找不到太多问题.

Answer 1

这是因为指针具有'单位',即它们指向的对象的大小.

让我们假设您有一个指向p地址1000 的指针.

如果你有

int* p = 1000;
p += 10;

p将指向104032位机器,因为int它的大小为4个字节.

但如果你有

char* p = 1000;
p += 10;

p会指出1010.

这就是为什么

*(float *)((char *)location + offset) = imput;

有效,但是

*(float *)(location + offset) = imput   Will Not Work

没有.