在第二种情况下，num 不是数组，是指针.sizeof 为您提供指针的大小，在您的平台上似乎是 8 个字节.

In the second case, num is not an array, is a pointer. sizeof is giving you the size of the pointer, which seems to be 8 bytes on your platform.

没有办法知道动态分配的数组的大小，您必须将其保存在其他地方.sizeof 查看类型，但不能从结果中得到完整的数组类型(指定大小的数组类型，如int[5]类型)malloc 以任何方式，并且 sizeof 参数不能应用于不完整的类型，例如 int[].

There is no way to know the size of a dynamically allocated array, you have to save it somewhere else. sizeof looks at the type, but you can't obtain a complete array type (array type with a specified size, like the type int[5]) from the result of malloc in any way, and sizeof argument can't be applied to an incomplete type, like int[].