Return values from system calls

[Fewmitz]

I've implemented a few custom system calls in my Ubuntu kernel, but I'm having some issues with the return values. Each function returns a variety of non-negative integers depending on which error is encountered. However, when I'm testing the system calls, the only negative value that gets returned to the user program is -1, regardless of what I have in the code.

Is there some special path I have to take to get the proper return values?

Thanks.

[ragnaroktomorrow]

I'm having the exact same problem. This wouldn't be for a school project would it?

I have verified that the code returning other negative values is getting executed by placing printk()'s before those return statements.

Yet the -1 is all I can get.

EDIT: I may have solved this. I've looked into errno. I may be wrong about this, but I think the calls will only return -1 specifying that an error has occurred, then the caller must check errno to see what error that was.

[Fewmitz]

No, it's not for a school project.

I thought that it might be an errno issue, but I've only recently started tinkering with linux under the hood. Do I have to explicitly set errno in the system call? Or will a negative value jump into it automatically?

[ragnaroktomorrow]

Or will a negative value jump into it automatically?

I believe so yes, but it won't be negative.