On Jun 10, 2011, at 12:51 PM, Jerry Krinock wrote:
> do {
> newPath = [[[newPath stringByDeletingPathExtension]
> stringByAppendingString:suffix]
> stringByAppendingPathExtension:[self pathExtension]] ;
> } while ([[NSFileManager defaultManager] fileExistsAtPath:newPath]) ;
This will work most of the time, but it has a race condition: just because
there was no file at that path when this method returns, is no guarantee that
there’s still no such file when the caller attempts to create a file at the
returned path. (This seems unlikely, but it can happen when two apps using the
same code are trying to create a file at the same time. It’s also sometimes
been exploited by malware to substitute a malicious file for the real one a
system task thought it was creating.)
This race condition is pretty well known, and it’s generally considered wrong
to test for existence prior to operating on a file. Instead, do the operation
without checking and handle the error. (Sometimes called “ask for forgiveness,
not permission”.)
So the correct code would look like:
do
compute a new filename to use on this iteration
try to copy to that filename
while the copy failed with a ‘duplicate filename’ error
—Jens
smime.p7s
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