On Wed, 2011-09-07 at 23:17 -0700, Philip Guenther wrote:
> Symlinks, particularly relative symlinks, are weird in UNIX and a
> general wart on the design that has never been fully smoothed over,
> with special handling required by many systems calls and programs.
Although this has nothing to do with make, I don't agree with that.
They work the only way they could possibly work when it comes to
relative paths. Nothing else would make sense.
Also, symlinks are not weird nor a wart on the design: they are fairly
excellent. And the most excellent thing about symlinks is that
virtually NO system calls or programs require special handling for them.
They "just work", 99.99% of the time.
My $0.02.
For Mark: the rule for expanding symlinks is this: as you're walking a
directory path and you come across a symlink, then:
If the symlink target is a fully-qualified path (starts with "/"),
replace the entire current path with the contents of the symlink.
If the symlink target is NOT fully-qualified (doesn't start with "/"),
then remove the symlink element (the last filename in the path) and
replace it with the contents of the symlink.
So if you create a symlink:
/my/special/foo -> /etc/passwd
then when the kernel wants to open "/my/special/foo" it walks the path
and when it gets to "foo" it sees it's a symlink, and the target is
fully-qualified, and it replaces the entire path with "/etc/passwd".
And if you create a symlink:
/my/special/foo -> bar
then when the kernel gets to "foo" and sees it's a symlink, and the
target is NOT fully-qualified, it removes the link element ("foo") and
replaces it with the contents ("bar") giving "/my/special/bar".
And if you create a symlink:
/my/special/foo -> ordinary/bar
Then the kernel replaces "foo" with "ordinary/bar" and you get
"/my/special/ordinary/bar".
And of course:
/my/special/foo -> ../../your/handy/bar
replaces "foo" with "../../your/handy/bar" yielding
"/my/special/../../your/handy/bar", or just "/your/handy/bar".
One last thing: it is NOT necessary that the target of a symlink exist!
Just because you can create the link without error doesn't meant that
the link can be resolved. In fact a number of programs use this feature
to good advantage, since creating a symlink is an atomic operation on
the filesystem, for creating lock files, etc. where the target of the
symlink is not a file at all, but rather some useful information related
to the lock (etc.) If the information you want to keep is small this is
MUCH more efficient than open/write/close for a real file: it's one
(atomic) system call.
Anyway. Symlinks rule! :-)
--
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Paul D. Smith <[email protected]> Find some GNU make tips at:
http://www.gnu.org http://make.mad-scientist.net
"Please remain calm...I may be mad, but I am a professional." --Mad Scientist
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