Question

So I have a bit of problem figuring what Perl does in the following case:

while(1){
$inputLine=<STDIN>

#parse $inputLine below
#BUT FIRST, I need to check if $inputLine = EOF

}

before I get the obvious answer of using while(<>){}, let me say that there is a very strong reason that I have to do the above (basically setting up an alarm to interrupt blocking and I didnt want that code to clutter the example).

Is there someway to compare $inputLine == undef (as I think that is what STDIN returns at the end).

Thanks.

Answer 1

Inside your loop, use

last unless defined $inputLine;

From the perlfunc documentation on defined:

defined EXPR
defined

Returns a Boolean value telling whether EXPR has a value other than the undefined value undef. If EXPR is not present, $_ will be checked.

Many operations return undef to indicate failure, end of file, system error, uninitialized variable, and other exceptional conditions. This function allows you to distinguish undef from other values. (A simple Boolean test will not distinguish among undef, zero, the empty string, and "0", which are all equally false.) Note that since undef is a valid scalar, its presence doesn’t necessarily indicate an exceptional condition: pop returns undef when its argument is an empty array, or when the element to return happens to be undef.