The death and possible rebirth of sysctl()

[Posted October 18, 2006 by corbet]

The sysctl() system call has had a rough life. It began as an idea imported from BSD; it allows a user-space process to tweak various kernel parameters using a set of integer indexes. People quickly discovered, however, that a text and filesystem-based interface (as seen under /proc/sys) is much easier to deal with. The /proc/sys hierarchy can be adjusted from the shell and manipulated by scripts - and nobody has to worry about sysctl numbers. So there are very few users of sysctl(), which has been considered deprecated for a long time. Recent kernels have issued warnings when sysctl() is called.

The 2.6.19-rc kernels take things one step further: for most configurations, sysctl() disappears altogether. In a strange sort of turnaround, only configurations with the "embedded" option set can enable sysctl() at all. This is all in accordance with the feature removal schedule, which calls for sysctl() to go away in January, 2007.

But sysctl() is part of the user-space API, which is never supposed to be broken for any reason. The removal of this function would appear to be a violation of the oft-repeated promise to keep this interface stable. So some developers have started to complain about the API change. There have been calls to back it out again, and to restore sysctl() to normal configurations. As Alan Cox put it: "We added it, we supported it, we get to keep it. We just stick notes in the docs saying 'please use /proc instead'."

Patches which restore sysctl() are circulating, though none have been merged. There appears to be some disagreement over whether removing sysctl() would truly break user-space applications or not. There are some uses of it in older C libraries, but, apparently, those libraries do the right thing when the attempt to use sysctl() fails, and applications operate normally. Linus has asked for an example of an application which truly breaks in the absence of sysctl(); none have been posted as of this writing. Interfaces which are not actually used on real systems are fair game for removal, so, unless somebody comes up with a a real-world problem soon, sysctl() will likely continue on its path out of the kernel.

Index entries for this article
Kernel	Development model/User-space ABI
Kernel	Sysctl