将结构传递给函数

在 C 中，所有参数都按值传递给函数，包括结构。对于小结构，这是一件好事，因为这意味着通过指针访问数据没有任何开销。但是，它也很容易意外地传递一个巨大的结构导致性能不佳，特别是如果程序员习惯于通过引用传递参数的其他语言。

struct coordinates
{
    int x;
    int y;
    int z;
};

// Passing and returning a small struct by value, very fast
struct coordinates move(struct coordinates position, struct coordinates movement)
{
    position.x += movement.x;
    position.y += movement.y;
    position.z += movement.z;
    return position;
}

// A very big struct
struct lotsOfData
{
    int param1;
    char param2[80000];
};

// Passing and returning a large struct by value, very slow!
// Given the large size of the struct this could even cause stack overflow
struct lotsOfData doubleParam1(struct lotsOfData value)
{
    value.param1 *= 2;
    return value;
}

// Passing the large struct by pointer instead, fairly fast
void doubleParam1ByPtr(struct lotsOfData *value)
{
    value->param1 *= 2;
}