Hi,
I have a writable and readable file on a small network file system
(Cisco NSLU2 Unslung; non-NTFS) that I access via a mounted drive on
Windows Vista. My problem could be due to a "funny" file
system/server, but here it goes:
> pathname <- "Q:/foo.txt"
> cat(file=pathname, "Hello world!\n")
> readLines(pathname)
[1] "Hello world!"
> file.info(pathname)
size isdir mode mtime ctime
Q:/foo.txt 14 FALSE 666 2008-11-26 11:45:53 2008-11-26 11:45:53
atime exe
Q:/foo.txt 2008-11-26 11:45:57 no
The mode == 666 reported by file.info() indicates that it is readable
& writable by all users. This is also what Windows Vista file
properties reports. So far so good. However, when I use
file.access() to test for file permissions, I get:
> file.access(pathname, 0) # 0 test for existence.
Q:/foo.txt
0
> file.access(pathname, 1) # 1 test for execute permission.
Q:/foo.txt
-1
> file.access(pathname, 2) # 2 test for write permission.
Q:/foo.txt
-1
> file.access(pathname, 4) # 4 test for read permission.
Q:/foo.txt
-1
I obviously can write to and read from the file, and this is what
file.info()$mode says too. However, file.access() tells a different
story. More troubleshooting: When I log into the file server and do:
# chmod ugo-w foo.txt
# ls -l foo.txt
-r-xr----- 1 admin everyone 14 Nov 26 11:48 foo.txt
The changes in permission are seen by file.info():
> file.info(pathname)
size isdir mode mtime ctime
Q:/foo.txt 14 FALSE 444 2008-11-26 11:48:50 2008-11-26 11:48:50
atime exe
Q:/foo.txt 2008-11-26 11:56:40 no
The output from file.access() remains the same though.
>From help(file.info) I read:
File modes are probably only useful on NTFS file systems, and it
seems all three digits refer to the file's owner. The
execute/search bits are set for directories, and for files based
on their extensions (e.g., '.exe', '.com', '.cmd' and '.bat'
files). 'file.access' will give a more reliable view of
read/write access availability to the R process.
>From what I conclude, file.access() is not reliable in this case. Is
this a feature or a bug?
I need a cross-platform test for file permissions, and I am looking
for safer workaround. For instance, could it be that a zero result
from file.access() can be trusted, but a -1 could occur either from a
true lack of permission as well as a failure to test for the
permission? If that would be case, I could try other measures (e.g.
try to open the file) whenever I receive a -1 before throwing an
exception.
Any feedback or suggestions would be great.
Thanks
/Henrik
> sessionInfo()
R version 2.8.0 Patched (2008-10-21 r46766)
i386-pc-mingw32
locale:
LC_COLLATE=English_United States.1252;LC_CTYPE=English_United States.1252;LC_MON
ETARY=English_United States.1252;LC_NUMERIC=C;LC_TIME=English_United States.1252
attached base packages:
[1] stats graphics grDevices utils datasets methods base
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