uninitialized value warning and duplicating file handles

perlplayer has asked for the wisdom of the Perl Monks concerning the following question:

i was trying to duplicate two file handles for the same file and faced some issue.
the file:-newfile.c file contents :- malay halder

my program:-

open (HANDLE1,"+< newfile.c");
my $line=<HANDLE1>;
print $line."\n";
open(HANDLE2,"+< &HANDLE1");
$line=<HANDLE2>
print "hello $line\n";

OUTPUT

--------------
malay halder
Use of uninitialized value in concatenation (.) or string at ./test.pl line 14. hello

??

what is the problem with the duplicated file handle i have also tried interchanging their positions in the code only the first print statement is working

please help me i am newbie to perl programming

Comment on uninitialized value warning and duplicating file handles

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Re: uninitialized value warning and duplicating file handles by kyle (Abbot) on Feb 27, 2008 at 06:27 UTC
I ran your code on my system (with a "newfile.c" that had four lines), and I had no problem. If I run with a file that has only one line, I get the output you get. In that case, when you do: `$line=<HANDLE2>;` [download] ...`$line` contains undef because `HANDLE2` is, like `HANDLE1`, at end of file. Then when you use line in the following print: `print "hello $line\n";` [download] That's when you get the warning. Note that you'd get the warning any time you try to interpolate that variable (carrying the special undef value), even if it's not being printed.	[reply] [d/l] [select]
Re: uninitialized value warning and duplicating file handles by pc88mxer (Vicar) on Feb 27, 2008 at 06:33 UTC
You are using `warnings` which will give that error when you use an undefined value in a print statement (like `print "hello $line\n"` and `$line` is `undef` `$line` is `undef` because at the time that you dup-ed `HANDLE1` it was already at the end of the file (newfile.c only has one line in it.) So reading from `HANDLE2` set `$line` to `undef`. If you want to `HANDLE2` to read from the beginning of the file you'll have to `seek` to the beginning ala `seek(HANDLE2, 0, SEEK_SET)`. Note that doing this will not affect the position of `HANDLE1`.	[reply] [d/l] [select]