CODEWITHSUNDEEP
perl
user2683229
user2683229

Reputation: 67

Default integer length in Perl?

I am reading some script which is written in Perl and I don't understand it. I have never used Perl before. I've read someting about scalars and that confuses me. For an example look at this code:

my $sim_packets = 5;
my $sim_length = $payload_length * 2 * $sim_packets;
push @data, (1...$sim_length * 10);
my $data_32bit = 0;

if I use after this:

$data_32bit = shift @data;

What is length of $data_32bit in bits?

I ask this because I have another array in this code: @payload, and this line confuses me:

push @payload, ($data_32bit >> 24) & 0xff, 
               ($data_32bit >> 16) & 0xff, 
               ($data_32bit >> 8)  & 0xff, 
               ($data_32bit)       & 0xff;

$data_32bit is 32 bit long?

Upvotes: 1

Views: 902

Answers (2)

ikegami
ikegami

Reputation: 386541

If I understand correctly third line of this code created array: 1..50?

It added to an array rather than creating one.

The scalars added consist of numbers starting with 1 and going up to and including $payload_length * 2 * $sim_packets * 10, which is $payload_length * 100.

$payload_length is unlikely to be 1/2, so I suspect the number of scalars added is more than the 50 you mentioned.

What is length of $data_32bit in bits?

What does that even mean?

The size of the scalar of 24 bytes on one of my system.

$ perl -MDevel::Size=total_size -E'$i=123; say total_size $i'
24

The amount of bits required to store the value:

ceil(log($payload_length * 100) / log(2))

In this case, the author appears to be indicating the value will/should fit in 32 bits. That will be the case unless $payload_length exceeds some number larger than 40,000,000.

and this line confuses me:

It adds four values to the array. The four values correspond to the bytes of $data_32bit when stored as a unsigned two's complement number with the most significant byte first.

Upvotes: 2

Ingo
Ingo

Reputation: 36339

Oh boy,

 push @payload, ($data_32bit >> 24) & 0xff, ($data_32bit >> 16) & 0xff, ($data_32bit >> 8) & 0xff, ($data_32bit) & 0xff;

Somebody apparently needs to

perldoc -f pack
perldoc -f unpack

Regarding your question, $data_32bit is not 32bit long, just becasue the term 32bit appears in its name. If you need to know how exactly it is represented, you should go for Data::Dumper.

Perl stores integers in the mantissa of a native floating point number, so it really depends on the machine architecture. With IEEE, it should be something like 53 bits.

Upvotes: 3

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