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7.2. Opening Files with Unusual Filenames
7.2.1. Problem
You want to open a file with a funny
filename, such as "-", or one that starts with
<, >, or
|; has leading or trailing whitespace; or ends
with |. You don t want these to trigger
open s do-what-I-mean behavior, since in this
case, thats not what you mean.
7.2.2. Solution
When open is
called with three arguments, not two, placing the mode in the second
argument:
open(HANDLE, "<", $filename) or die "cannot open $filename : $!\n";
Or simply use sysopen:
sysopen(HANDLE, $filename, O_RDONLY) or die "cannot open $filename: $!\n";
7.2.3. Discussion
When open is called with three arguments, the
access mode and the filename are kept separate. But when called with
only two arguments, open has to extract the access
mode and the filename from a single string. If your filename begins
with the same characters used to specify an access mode,
open could easily do something unexpected. Imagine
the following code:
$filename = shift @ARGV;
open(INPUT, $filename) or die "Couldn''t open $filename : $!\n";
If the user gave ">/etc/passwd" as the filename
on the command line, this code would attempt to open
/etc/passwd for writing. We can try to give an
explicit mode, say for writing:
open(OUTPUT, ">$filename")
or die "Couldn''t open $filename for writing: $!\n";
but even this would let the user give a filename of
">data", and the code would append to the file
data instead of erasing the old contents.The easiest solution is to pass three arguments to
open, where the second argument is the mode and
the third the path. Now there can be neither confusion nor
subterfuge.
open(OUTPUT, ">", $filename)
or die "Couldn''t open $filename for writing: $!\n";
Another solution is sysopen, which also takes the
mode and filename as distinct arguments:
use Fcntl; # for file constants
sysopen(OUTPUT, $filename, O_WRONLY|O_TRUNC)
or die "Can't open $filename for writing: $!\n";
This special way that open interprets filenames,
sometimes referred to as magic open, is a matter
of convenience—and usually a good thing. You don't have to
worry about a space or two between the access mode and the path. You
never have to use the special case of "-" to mean
standard input or output. If you write a filter and use a simple
open, users can pass "gzip -dc
bible.gz|" as a filename, and your filter will
automatically run the decoding program.It's only those programs that run under special privilege that should
worry about security with open. When designing
programs that will be run on someone else's behalf, such as setuid
programs or CGI scripts, the prudent programmer always considers
whether the user can supply their own filename and thereby cajole
what would otherwise appear to be a normal open
used for simple reading into overwriting a file or even running
another program. Perl's -T
command-line flag to enable taint-checking would take care of this.In versions of Perl without three-argument open
(those before v5.6.0), one had little recourse but to resort to the
following sort of chicanery to cope with filenames with leading or
trailing whitespace:
$file =~ s#^(\s)#./$1#;
open(OUTPUT, "> $file\0")
or die "Couldn't open $file for OUTPUT : $!\n";
The substitution protects initial whitespace (this cannot occur in
fully specified filenames like "/etc/passwd", but
only in relative filenames like ">passwd"). The
NUL byte (ASCII 0, "\0") isn't considered part of
the filename by open, but it does prevent trailing
whitespace from being ignored.
7.2.4. See Also
The open and sysopen functions
in perlfunc(1) and Chapter 29 of
Programming Perl; Recipe 7.1; Recipe 7.14; Recipe 16.2; Recipe 19.4;
Recipe 19.5
