How can a function pass all the arguments it received transparently to another function?

The title of this post says it all. The point is that the calling function X should be agnostic about the signature of the called function Y. IOW, X should just pass to Y, en masse, all the arguments it (X) received, at leave it up to Y to complain if the signature is off.


I thought this was just a matter of

sub X { return Y( @_ ); }

...but apparently not:

sub getpwuid_wrapper { return getpwuid( @_ ); }

The wrapper above is doing more than merely wrapping:

DB<7> p getpwuid( 5 ) gamesx560games/usr/games/usr/sbin/nologin DB<8> p getpwuid_wrapper( 5 ) daemonx11daemon/usr/sbin/usr/sbin/nologin

The next bit gives a clue to what may be going on:

DB<9> p getpwuid( 1 ) daemonx11daemon/usr/sbin/usr/sbin/nologin

I figure that getpwuid( @_ ) is getting interpreted as getpwuid( scalar @_ ), for reasons beyond my comprehension.

<strong>NOTE:</strong> getpwuid is just an example to illustrate the problem. In practice one needs this sort of pass-through functionality primarily when the function that will be called is not known until runtime; e.g. when this function is a callback.


The problem you are facing is because getpwuid has a prototype of $, which causes it to interpret its parameter in scalar context.

You can see this from the following one-liner:

perl -e 'print prototype("CORE::getpwuid"), "\n"'

...which prints $.

So when you pass @_ to getpwuid, you are correct; it's being taken in scalar context and passing a count of how many elements @_ contains.

One solution is to use the subroutine form of goto, which calls its operand with the same call stack and parameters that were passed to its caller.

Here's an example in the form of a one-liner:

perl -e 'print getpwuid(5), "\n"; print sub{ getpwuid(@_) }->(5), "\n"; print sub { goto &CORE::getpwuid }->(5), "\n";'

In this example, the first and third calls will pass '5' to getpwuid, but the second call will pass scalar(@_), which is 1 in this case.

Another hackish option would be to detect whether the function you are calling has a prototype using the prototype function, and react accordingly. But that's problematic. First, you already know you're calling getpwuid -- no need to detect at runtime. And even if you had to detect at runtime for some reason, some CORE:: functions will return undef for their prototype because they use one that cannot be expressed in terms of the prototype options available to us (system, for example).

In this specific case since you already know what function you are calling, and already know its prototype, it's probably just easiest to do this:

perl -e 'print sub { getpwuid(shift) }->(5), "\n";'

<strong>Update:</strong> Where the subroutine being called isn't known until runtime goto may be a good means. One of the things you have to look out for with goto, however, is that you never return back to the parent sub; the subroutine being gone-to will return directly to the caller of the parent sub.

Another option is to call the target sub with the & prefix and no parameters. This also serves to pass @_ through to it, but in a way that bypasses prototypes. Here's an example:

perl -e 'print sub { &CORE::getpwuid }->(5)'

This will appear similar to the goto approach, but will not tinker with the call stack, and will return back to the parent sub.


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