Re: How do I tell if a handle is open for writing?

Try writing nothing at all to the file, and see if it fails:


sub is_writable_fh
{
  my($fh)=@_;
  local $\='';
  return print $fh '';
}

open(F1,"< $0")
  or die "Couldn't open '$0' for read: $!\n";
print "F1 is".(is_writable_fh(\*F1)?"":"n't")." writable.\n";

open(F2,">> $0")
  or die "Couldn't open '$0' for append: $!\n";
print "F2 is".(is_writable_fh(\*F2)?"":"n't")." writable.\n";
[download]

Zaxo's solution will tell you whether you have write permissions to the file, but not whether you have write permissions to the filehandle.

Comment on Re: How do I tell if a handle is open for writing? Download Code

Replies are listed 'Best First'.
Re^2: How do I tell if a handle is open for writing? by DrWhy (Chaplain) on Nov 07, 2005 at 17:44 UTC
Excellent! I had thought about just trying to write something to the file but didn't want to modify the file. It didn't occur to me to write nothing to the file :) I was worried that even this solution might have unintended side effects, e.g., changing the modified time on the file or (less likely in my mind) writing a null character to the file. I tried it on my WinXP system and it doesn't appear to modify the file in any way -- modified date remains unchanged. Thanks! --DrWhy "If God had meant for us to think for ourselves he would have given us brains. Oh, wait..."	[reply]
Re^3: How do I tell if a handle is open for writing? by Anonymous Monk on Mar 01, 2006 at 17:02 UTC
It should write an EOF at the position (on Win that is, or was it FAT-filesystem only?) and everything after it would be lost. Of course perl could intervene and take it to mean a check for writability, and then use other means to check and get you the result. But I would first seek to EOF.	[reply]