CODEWITHSUNDEEP
ccastinglinux-kernelintvoid-pointers
Mickey
Mickey

Reputation: 1525

C - best practice for casting a void* to int, kernel

I'm writing a module for Linux kernel, and I want to store a int value in the file private data.

Essentially, what I do is: file->private_data = (void*) x where x is some int value.

Now, I want to access the int back as a value.

Using int val = (int) file->private_data gives out a cast from pointer to integer of different size warning during compilation, which is reasonable since it may cause problems on a 64bit systems.

I also cannot use uintptr_t since I'm working in kernel and I do not have access to libraries.

Using double seems inappropriate.

My question is: What should be the best practice to do so?

Upvotes: 3

Views: 2092

Answers (2)

user4822941
user4822941

Reputation:

Can you please elaborate how come you got yourself into a situation where storing an int there is reasonable?

Normally this would be a pointer to a reference-counted object. In particular, if you are using that to look up another object, this very field should probably just point to that object.

Upvotes: 0

Tsyvarev
Tsyvarev

Reputation: 66118

In gcc world (Linux kernel is compiled by gcc), long (or unsigned long) has the the same size as a pointer. You may use this feature when convert pointers to integer and back:

// store
file->private_data = (void*)(long) x;
// load
int val = (int) (long) file->private_data;

Note: This answer addresses specifically Linux kernel programming.

For user-space application suggested approach could be treated as a bad practice, or simply being wrong.

Upvotes: 5

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