A "latest" symlink, is a symbolic link (on Linux, Unix etc) which links to the "latest" version of a file.
Suppose we have a file which takes some effort to create, which is generated periodically or in response to some stimulus (e.g. user activity). Then we want to create a "latest version" symlink.
Ideally the properties should be
- latest symlink always points at the latest version (duuh!)
- latest symlink always exists
- latest symlink never points at a partially completed, broken, missing or otherwise bad file
How to create a symlink
Dead easy, right? Just call the "symlink" function.
int symlink(const char *oldpath, const char *newpath);
symlink() creates a symbolic link named newpath which contains the
But if we call "symlink" and the newpath param points to an already existing file (including a symlink) then it will return EEXIST.
The wrong (obvious) way
The correct (not so obvious) way
templink = dest + '.temp'
Because we want to avoid a race condition where the destination symbolic link does not exist. Renaming files is atomic and will instantly replace the existing link with a new one; no other program can possibly see a non-existing file.
Other wrong ways
Some possibly common, but wrong (or even wronger) ways to do this
- Just create the "latest" file using our "do lots of work" process directly. This is really bad, as during file creation, another process can see a partially completed file. If your code looks like this: write file header; do lots of work to create file body; write file footer, then there is a really good chance that another process sees an incomplete file.
- Create a different file, then copy the file (file copying isn't atomic)