Things UNIX can do atomically
rcrowley.org
Things UNIX can do atomically
1–10 of 41 posts
Re: Things UNIX can do atomically
#2The very first example ("mv -T" to retarget a symlink, something I didn't know about and that the manpage is silent on) is not atomic! The symlink(2) syscall will not overwrite an existing directory entry. You have to call unlink(2) first, which opens a race.
That doesn't mean this trick is useless, but calling it "atomic" is wrong, and opens up the possibility of bugs (likely security holes, even) if someone actually needs atomicity.
Re: Things UNIX can do atomically
#3One more: mkdir(2)
Re: Things UNIX can do atomically
#4I have a copy of "Advanced Programming in the UNIX Environment, Second Edition" (featuring very good description of C/POSIX/SuS/XSI protocols/api's and programming techniques for Linux/BSD/OSX/Solaris/other UNIX systems). Anyone who seriously wants to do thread safe multiprocessing or other complex low level stuff on UNIX platforms should have a copy of it. It's basically "the book" on the topic.
Re: Things UNIX can do atomically
#5IPC semaphore ops using the semop() function are also supposed to be atomic at the kernel level.
Re: Things UNIX can do atomically
#6The very first example ("mv -T" to retarget a symlink, something I didn't know about and that the manpage is silent on) is not atomic! The symlink(2) syscall will not overwrite an existing directory entry. You have to call unlink(2) first, which opens a race. That doesn't mean this trick is useless, but calling it "atomic" is wrong, and opens up the possibility of bugs (likely security holes, even) if someone actuall…
To change a symlink I use:
ln -snf NEW_SOURCE TARGET
Re: Things UNIX can do atomically
#7The very first example ("mv -T" to retarget a symlink, something I didn't know about and that the manpage is silent on) is not atomic! The symlink(2) syscall will not overwrite an existing directory entry. You have to call unlink(2) first, which opens a race. That doesn't mean this trick is useless, but calling it "atomic" is wrong, and opens up the possibility of bugs (likely security holes, even) if someone actuall…
> something I didn't know about and that the manpage is silent on
It's specific to GNU Coreutils mv(1).
Re: Things UNIX can do atomically
#8The very first example ("mv -T" to retarget a symlink, something I didn't know about and that the manpage is silent on) is not atomic! The symlink(2) syscall will not overwrite an existing directory entry. You have to call unlink(2) first, which opens a race. That doesn't mean this trick is useless, but calling it "atomic" is wrong, and opens up the possibility of bugs (likely security holes, even) if someone actuall…
To change a symlink I use: ln -snf NEW_SOURCE TARGET
Which works fine, but is still not atomic. To be clear, saying something is "atomic" means that there is no way for another process to see the process in an intermediate state (in the example in the article: a missing or incorrect symlink to a deployed web application). Underneath the hood, the /bin/ln program still needs to call unlink() on the path first, so there is a short period of time during which the desired link isn't present. So software written to assume that the link is always valid (for example, because its author was told that the update was atomic) will have hidden bugs.
This kind of tool merely runs really fast. The race condition is still there.
Re: Things UNIX can do atomically
#9 pipe(&pipefd);
assert(len Re: Things UNIX can do atomically
#10I have a copy of "Advanced Programming in the UNIX Environment, Second Edition" (featuring very good description of C/POSIX/SuS/XSI protocols/api's and programming techniques for Linux/BSD/OSX/Solaris/other UNIX systems). Anyone who seriously wants to do thread safe multiprocessing or other complex low level stuff on UNIX platforms should have a copy of it. It's basically "the book" on the topic.
[deleted]