How $clientSocket->recv( $buffer, $length) pass the data back by a value-passed?

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

hi, all monks:

I notice that, $clientSocket->recv( $buffer, $length) can pass the data back to $buffer, even though the $buffer is value-passed, rather than ref-passed.

I write a test subroutine, if I want to pass the data back from a subroutine, I need to pass the variable by ref. like this:

my $buffer;
foosub( \$bufffer );
print $buffer, "\n";

sub foosub
{
    my $rs_buffer = shift;
    $$rs_buffer = "Hello, world";
}
[download]

if I pass it by value, it gets nothing back:

my $buffer;
foosub( $buffer);
print $buffer, "\n";

sub foosub
{
    my $buffer = shift;
    $buffer = "Hello, world";
}
[download]

so, why the recv() can pass the data back by a value-passed?

Comment on How $clientSocket->recv( $buffer, $length) pass the data back by a value-passed? Select or Download Code

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Re: How $clientSocket->recv( $buffer, $length) pass the data back by a value-passed? by GrandFather (Saint) on Sep 12, 2008 at 07:43 UTC
Consider: `my $value = 1; editValue ($value); print $value; sub editValue { $_[0]++; }` [download] prints '2' because @_ contains aliases to the parameters passed into the sub. If you edit the contents of elements of @_ you are editing the contents of the parameters. You can make it more explicit by taking a reference to an element: `sub editValue { my $value = \$_[0]; $$value++; }` [download] Perl reduces RSI - it saves typing	[reply] [d/l] [select]
Re: How $clientSocket->recv( $buffer, $length) pass the data back by a value-passed? by ikegami (Patriarch) on Sep 12, 2008 at 07:50 UTC
even though the $buffer is value-passed, rather than ref-passed. That's hard to believe since Perl always passes arguments by reference, never by value. You seem to be confusing "passing by reference" and "passing a reference". `@_` contains aliases to the values passed to the function (rather than the a copy of the values), so modifying elements of `@_` modifies the parameters in the caller.	[reply] [d/l] [select]
Re: How $clientSocket->recv( $buffer, $length) pass the data back by a value-passed? by sunshine_august (Scribe) on Sep 12, 2008 at 09:27 UTC
thx, I got it:) And, I find two paragraph about this in Programming Perl(3rd): 6.2. Semantics .... Any arguments passed to a Perl routine come in as the array @_. If you call a function with two arguments, they are accessible inside the function as the first two elements of that array: $_[0] and $_1. Since @_ is a just a regular array with an irregular name, you can do anything to it you'd normally do to an array.2 The array @_ is a local array, but its values are aliases to the actual scalar parameters. (This is known as pass-by-reference semantics.) 6.2.1. Tricks with Parameter Lists Perl does not yet have named formal parameters, but in practice all you do is copy the values of @_ to a my list, which serves nicely for a list of formal parameters. (Not coincidentally, copying the values changes the pass-by-reference semantics into pass-by-value, which is how people usually expect parameters to work anyway, even if they don't know the fancy computer science terms for it.)	[reply]