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Date: Fri, 29 Jul 2016 16:22:55 +0300
From: Slawa Olhovchenkov <slw@zxy.spb.ru>
To: Adam Starak <starak.adam@gmail.com>
Cc: freebsd-hackers@freebsd.org
Subject: Re: Modify user space from kernel.
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On Fri, Jul 29, 2016 at 03:11:25PM +0200, Adam Starak wrote:

> Hello!
> 
> My name is Adam. I participate in Google Summer of Code this year. I came
> up with a big problem, which doesn't allow me to go further in my project.
> 
> I made a new syscall, which is going to retrieve sysctl data and put it
> inside the nvlist. And here my problem is. I need to move somehow this data
> (packed nvlist) into the user space. Is there any chance to pass data from
> kernel to user space without knowing the size of it?
> 
> Right now, the implementation of __sysctl() function requests void pointer
> and size in order to get data. If allocated memory is too low, it returns
> ENOMEM and you need to realloc the data. I wanted to avoid this situation.


char *intrname;
size_t inamlen;
sysctlbyname("hw.intrnames", NULL, &inamlen, NULL, 0);
intrname = malloc(inamlen);
sysctlbyname("hw.intrnames", intrname, &inamlen, NULL, 0);

NB: check error!