What does the for(;;) (2 semi colons) loop mean?

Question

I'm working on a script for the VoIP software, Freeswitch. The script will run as an instance listening for inbound messages to a socket.

I used an example script provided with Freeswitch, to start with, and everything works fine.

However, one bit is throwing me off.

use IO::Socket::INET;
use warnings;
use strict;


my $sock = new IO::Socket::INET ( LocalHost => '127.0.0.1',  LocalPort => '8060',  Proto => 'tcp',  Listen => 1,  Reuse => 1 );
die "Could not create socket: $!\n" unless $sock;


for(;;) {
    my $new_sock = $sock->accept();
    my $pid = fork();
    if ($pid) {
      close($new_sock);
      next;
    } 
    close($new_sock); 
}

What exactly does the ;; mean? Is it a special operator that is defined when a socket is created? Struggling to find documentation!

Answer 1

for(;;) is a C-ish idiom which is read as for ever, and is a loop that never terminates. People who don't come from C would write while (1) instead.

Normally, this C-style for loop has three statements: initialization, looping condition, and some step:

for (init; condition; step) {
    body;
}

which is exactly equivalent to

{
    init;
    while (condition) {
        body;
    }
    continue {
        step;
    }
}

An empty condition is taken to be always true.

A common usage would be to iterate over all numbers in a range:

for (my $i = 0; $i < @array; $i++) {
    say $array[$i];
}

However, this would be written more idiomatically with a foreach loop:

for my $i (0 .. $#array) {
    say $array[$i];
}

or

for my $item (@array) {
    say $item;
}

which is why you see the C-style for loop very rarely in idiomatic Perl code.

This construct is documented in perldoc perlsyn.