This is just an accident of how some file systems are implemented and isn't actually garunteed. If you did this on xfs you could still end up with the a 0 length symlink if you crashed at just the right time.
I thought that requirement was from POSIX.
Does xfs not conform to POSIX?
The way I understand the article's proposal is this:
1. Create new symlink pointing to desired file (assumed to already exist in a stable state).
2. Move new symlink over old symlink.
They exist as directory entries with a small in-directory content only, thus syncing the directory they exist in is sufficient to persist them.
See e.g. https://ext4.wiki.kernel.org/index.php/Ext4_Disk_Layout#Symb...
I think it is more useful to think of the symbolic link as "inside" the directory it is in, because this tells the programmer what to do (and what to sync: the directory).
[1]: http://pubs.opengroup.org/onlinepubs/009695399/functions/rea...
A directory, historically (on systems with 14- or 30-character file name limits like we had in the 1990s, at least 4.2 or 4.3 BSD-derived), consisted of a file containing a list of (16-bit inode number, file name) structs, thus making a nice round 16 or 32 bytes per directory entry, evenly dividing a 512-byte block. On some older systems you could see this structure by opening and reading a directory with open and read (I think every system everywhere prohibited writes to a directory because modifying a directory would allow you to access files in unreadable directories, uncleanly delete any file given its inode number, etc). The only special thing about a directory on disk was that its inode said it was a dir, so the filesystem code would trust it as a source of inode numbers and otherwise use it as a dir.
Some of the 14-byte-name systems supported symlinks. As there was no space in the directory entry for anything other than an inode number, the link contents would have to be accessed through an inode. (I think I've heard of systems that could store very small file contents like typical symlink target names in the inode instead of allocating a data block, but can't name one).
The number you see in ls -i, which is sometimes called just the "inode" (and apparently also called the "file serial number"), is the number in the directory entry that's used to find the inode. I guess someone somewhere had or intended to have a system that could store symlinks in the directory instead of consuming an inode.
The fact that it does or does not should actually be in the documentation IMHO; probably a good enhancement request.
POSIX does (for some strange reason) permit a symbolic link to have an "inode" (well, d_ino), but there is no way to open this "inode", and no UNIX implementations to my knowledge do this.
HOWEVER, looking into it it seems that the target may actually be stored in the inode. This mean the symlink, though a file, has no contents and thus requires no fsync. Does this sound about right?
No they don't.
First of all, POSIX doesn't use the term "inode number" but "file serial number" which is silly wankery since there's no way to access a file using its "file serial number". They might as well say it was "implementation defined".
Secondly, POSIX used to specify[1] that their inode was "unspecified" because UNIX systems don't store symbolic links in separate inodes, but in the directory that contains them. Now, POSIX specifies symbolic links do have a "file serial number"[2], which could be implemented as a separate file block (confusingly Linux calls this an "inode" which has nothing to do with what UNIX called an inode -- a better term might be "virtual inode" but Linux uses that for something else entirely...)
To this end, I think the only sensible interpretation is the original one: UNIX symbolic links don't have inodes, and POSIX symbolic links might as well have a "hash" of the file contents in the "file serial number" since you can't do anything with it anyway.
[1]: http://pubs.opengroup.org/onlinepubs/009695399/functions/rea...
[2]: http://pubs.opengroup.org/onlinepubs/9699919799/functions/re...
> HOWEVER, looking into it it seems that the target may actually be stored in the inode. This mean the symlink, though a file, has no contents and thus requires no fsync. Does this sound about right?
You have to fsync() the directory that contains the symlink.
What UNIX called an inode is unfortunately not what Linux now calls an inode. That thing might have better names, but we are stuck with it.
I don't know if it is useful to say "Linux inode" because that can refer to both what "ext4" calls an inode and what the Linux kernel refers to as a "virtual inode".
What POSIX called a "file serial number" (and goes in the `d_ino` field) is also not what Linux calls an inode. I sometimes think this is okay to refer to as a "POSIX inode" because POSIX doesn't otherwise use the term, and up until SUS6 the POSIX file serial number was compatible with a UNIX inode.
However I'm not trying to nit: Trying to develop a simplified mental model is essential to determining what to do (and what to sync!). From an application point of view, knowing that a symbolic link has a name (and contents) and lives in a directory, can help guide us.
How so? Here's a v6 on-disk inode: https://github.com/hephaex/unix-v6/blob/master/ino.h
and here's an ext4 on-disk inode: http://lxr.free-electrons.com/source/fs/ext4/ext4.h#L704
The latter is considerably larger, to be sure, but still contains all of the fields from the former (many of them with the same name!).
UNIX called them "inode" and "permanent inode"; Linux calls them "in-memory inode" and "on-disk inode". The BSDs use "vnode" and "inode".
Edit: If you want to fsync the symlink you can do an open with nofollow iirc (on my phone so I can't check) so you get the actual symlink and not its target.
I think this is true in many filesytems, not just `xfs`.
If your atomicity requirement you never want the file to disappear from the POV of an external process, then the OP's method is sufficient. If you want crash-proofing as well, then you will need an fsync() -- preferrably on the tempfile BEFORE the rename().
d=open("."), unlink("t");
/* 1 */ symlink("new","t");
/* 2 */ rename("t", "link");
/* 3 */ fsync(d);
/* 4 */ close(d);
You can insert a second fsync() where you suggest at point (2), but all this will guarantee is that we will have "t" in the directory because the symlink contents are part of the directory they live in. This might be useful for some applications, but the cost of two disk writes is high enough it may be worth redesigning your application.
Now on modern filesystems, a non-huge symlink will be stored in the inode itself and presumably enjoys some sort of atomicity. But there is nothing in the standard about that.
No, I don't think you can, bugs notwithstanding. A "link" (§3.130) is what POSIX calls a directory entry.
> But there is nothing in the standard about that.
The "standard" (POSIX) doesn't talk much about crashing, however if mkdir("a") could destroy "b" – even during a system crash (§3.387), then users would complain.