The problem was that this script would fail every so often with ‘Not numeric subtraction’. So I started saving snapshots of /proc/net/dev and noticed that the script would fail after when the values were around 4 billion something. This I knew to be in the neighborhood of 2^32 (The max of a positive only 32-bit integer value). To confirm my thoughts that this was the max value for this counter, I decided to have a poke around the kernel source code.

Into the Kernel:
I didn’t know where to look in the source for this, but /proc/net/dev has the string ‘Inter-|’ which I figured would be a unique enough string to give me a place to start. Sure enough, a recursive grep for this string returned only 3 lines of code. The function I wanted was dev_seq_printf_stats in dev/core/dev.c:

static void dev_seq_printf_stats(struct seq_file *seq, struct net_device *dev)
{
        struct net_device_stats *stats = dev->get_stats(dev);
        seq_printf(seq, "%6s:%8lu %7lu %4lu %4lu %4lu %5lu %10lu %9lu "
                   "%8lu %7lu %4lu %4lu %4lu %5lu %7lu %10lu\n",
                   dev->name, stats->rx_bytes, stats->rx_packets,
                   stats->rx_errors,
                   ///.....

Looking at the printf specifiers for this they were %ul — unsigned long integer, which on my system was indeed a max of 4294967295 ( 32^2 – 1). I wanted to be extra sure, so I traced the net_device_stats struct to include/linux/netdevice.h and confirmed that the net_device_stats->rx_bytes member was in fact an unsigned long integer. So now I knew the error happened when the counter maxed out and then reset to zero, but why a non-numeric subtraction error?

Problem Found:
%8lu as a ANSI C standard library printf specifier defaults to 8 characters wide, and also defaults to right justify since there is no hyphen flag. To find out if the kernel did the same I traced seq_printf to lib/vsprintf.c and saw that the Linux kernel version formatted this in the same way. When the bytes value was less than 8 characters long, there was leading white space that threw off my parser. All I needed was to add the extra line at line 9 to eliminate any leading whitespace:

sub parseBandwidth {
    my $interface = shift;
    my @ifconfigOutput = @_;
    foreach my $line (@ifconfigOutput) {
        if ( $line =~ /:/ ) {
            my @interfaceLine = split( /:/, $line);
            if ($interfaceLine[0] =~ /$interface/) {
                # Next line is to sanitize leading whitespace
                $interfaceLine[1] =~ s/^\s+//;
                my @interfaceStats = split( /\s+/, $interfaceLine[1] );
                print( LOG "DEBUG I have parsed out: @interfaceStats\n") if $debug;
                return @interfaceStats;
            }
        }
    }
}