不了解空指针/如何施放？ （C 编程）

Question

Suppose I want to write a function: int read_file (char *filename, void *abc) that reads the file, and puts the numbers in an array, which abc points to. I must use the void pointer - I know how I would do it if it were int *abc instead of void, as I could treat abc syntactically like an array, and do stuff like abc[0]=1 , but here I can't do that, as it's a void pointer.

I'm not too familiar with void pointers, and how I should get this to work. Please help! I prefer not to post code, as this is for a school assignment, and just want to know how I would put the information in the file into an array pointed to by abc , maybe with casting (not sure how to do that though).

I am already familiar with putting file information into an array, if it's given by int abc .

Answer 1

If read_file is always called with an int array for the abc parameter, you can just copy it to an int pointer and work with that.

int *p = abc;

In most cases, you need to cast when changing from one type to another, however a void * may be freely cast to or from any non-function pointer without a cast safely.

Answer 2

Underneath the hood all pointers are the same - they're integers which represent memory addresses. So you can cast them every which way. Just go:

int *p = (int *)abc;

And voila - you have your int * which you already know how to deal with. This is the "casting" thing, by the way - write the desired type in parentheses in front of your expression to "cast" it. It'll take the same bits and reinterpret them in a different way.

In a few cases C is actually smart enough to convert the data rather than blindly use the same bits. For example:

float f = 3.75;
int i = (int)f;

In this case i will contain 3 because it rounds down in this case. And an int is stored in memory differently than a float , so there is actual conversion going on here.

And in other cases it will forbid you to cast at all, because of how little sense it makes:

char *c = "Hello, world!";
float f = (float)c;

But quite often you can get away with it. Especially with pointers, everything is fair game. Now, mind you, with great power comes great responsibility. Although you can do it, doesn't mean that the result will be sensible and that using it won't crash your program. Be careful.