[CentOS] CentOS 5 file system read only issue

Wed Aug 21 19:53:09 UTC 2019
Peter <peter at pajamian.dhs.org>

On 22/08/19 2:20 AM, Warren Young wrote:
> On Aug 21, 2019, at 7:35 AM, Xinhuan Zheng <xzheng at christianbook.com> wrote:
>> my $s = IO::Select->new( $fh );
>> if ( $io->can_write( 10 ) {
> That’s not designed to do what you hope.  select(2) is a system call intended for use on network socket handles, not file handles.  Since socket handles and file handles are compatible on a Unix type system (including CentOS) the call doesn’t fail, but it *cannot* report the information you’re hoping to get.
> I would first try calling the -w operator:
>     print_to_file() if -w $fh;

First off I would be remiss if I didn't point out that CentOS 5 has been 
EOL for years, that said...

You're dealing with perl here and so this might be better off asking in 
a perl list.  The -X system of tests as documented with "perldoc -f -X" 
do not by default test actual ability to read and write files, but 
instead just check the file mode bits as returned by stat(), thus the -w 
test will not reflect the filesystem being in read-only mode.

There are two ways to get around this.  One is to to use the filetest 
pragma which changes the behavior of the -X tests to use the access(2) 
system function:
{ use filetest 'access';
   print_to_file() if -w $fh;

The other way is to use POSIX::access() directly (this requires the file 
name or path):

use POSIX qw(access W_OK);


print_to_file() if access($filepath, W_OK);

Note that there are caveats to either of the above approaches as per 
documentation.  See the following for additional info:

perldoc -f -X
perldoc filetest