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Urs Thuermann
*nix forums beginner
Joined: 15 Mar 2005
Posts: 40
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 Posted: Mon May 02, 2005 3:33 pm Post subject:
rename(2) without unlinking old file
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Is there a way to rename(2) a file only if the new name does not
already exist, i.e. without unlinking the file that had the new name
before? When I do
touch a b
mv a b
the old file named b is unlinked and the file a replaces b, atomically
with a call to rename(2). Is there a way to make sure, no file is
unlinked when I call rename(2)?
First checking with stat(2) and calling rename(2) only if stat returns
ENOENT doesn't do this, because it's not atomic. Between
stat("new-name", ...) and rename("old-name", "new-name") the file
"new-name" might be created.
urs |
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Lew Pitcher
*nix forums Guru
Joined: 21 Feb 2005
Posts: 675
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| Posted: Mon May 02, 2005 4:22 pm Post subject:
Re: rename(2) without unlinking old file
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-----BEGIN PGP SIGNED MESSAGE-----
Hash: SHA1
Urs Thuermann wrote:
| Quote: | Is there a way to rename(2) a file only if the new name does not
already exist, i.e. without unlinking the file that had the new name
before? When I do
touch a b
mv a b
the old file named b is unlinked and the file a replaces b, atomically
with a call to rename(2). Is there a way to make sure, no file is
unlinked when I call rename(2)?
First checking with stat(2) and calling rename(2) only if stat returns
ENOENT doesn't do this, because it's not atomic. Between
stat("new-name", ...) and rename("old-name", "new-name") the file
"new-name" might be created.
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I would think that you could accomplish this through judicious use of link(2)
and unlink(2). Something like
IF link(old_name,new_name) succeeds
unlink(old_name)
link(2) should fail if the new_name already exists
- --
Lew Pitcher
IT Specialist, Enterprise Data Systems,
Enterprise Technology Solutions, TD Bank Financial Group
(Opinions expressed are my own, not my employers')
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Andrew Smallshaw
*nix forums beginner
Joined: 23 Feb 2005
Posts: 26
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| Posted: Mon May 02, 2005 7:06 pm Post subject:
Re: rename(2) without unlinking old file
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In article <m24qdlzg5a.fsf@janus.isnogud.escape.de>, Urs Thuermann wrote:
| Quote: | Is there a way to rename(2) a file only if the new name does not
already exist, i.e. without unlinking the file that had the new name
before? When I do
touch a b
mv a b
the old file named b is unlinked and the file a replaces b, atomically
with a call to rename(2). Is there a way to make sure, no file is
unlinked when I call rename(2)?
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If we're talking about within the same directory, or even the same filesystem,
a link(), followed by an unlink() will do what you want since link() is
non-clobbering. Across filesystems it doesn't really matter since the file
will need to be copied over before you can delete the original. [1]
One health warning though: some systems allow you to 'merge' an existing
directory with a freshly mounted filesytem. I've never played around with
this because I've always considered it dangerous, but if the directory in
question is somewhere that might be a mount point do your own research.
--
Andrew Smallshaw
andrews@sdf.lonestar.org
[1] Yes, you can have an open file that has been unlinked. But if we're
talking about data integrity are you going to take that chance? |
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Urs Thuermann
*nix forums beginner
Joined: 15 Mar 2005
Posts: 40
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| Posted: Thu May 05, 2005 5:07 am Post subject:
Re: rename(2) without unlinking old file
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Lew Pitcher <Lew.Pitcher@td.com> writes:
| Quote: | I would think that you could accomplish this through judicious use of link(2)
and unlink(2). Something like
IF link(old_name,new_name) succeeds
unlink(old_name)
link(2) should fail if the new_name already exists
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Thanks. As it is that simple I wonder why I hadn't seen this myself.
The reason for my question was that GNU mv does the following when
calling mv foo bar, which is not safe:
...
umask(0) = 022
stat64("bar", 0xbffffa38) = -1 ENOENT (No such file or directory)
brk(0x8053000) = 0x8053000
lstat64("foo", {st_mode=S_IFREG|0644, st_size=0, ...}) = 0
lstat64("bar", 0xbffff904) = -1 ENOENT (No such file or directory)
rename("foo", "bar") = 0
_exit(0)
urs |
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