While working on Devel::Profiler I've stumbled into a dark pit. Basically, Devel::Profiler works by instrumenting every sub it can find by walking symbol tables. When it finds a sub it wraps it in a profiling routine and installs it in the sub's slot in the symbol table. This is generally working fine. The problem occurs when that subroutine isn't a real subroutine, but a fake produced by exporting a symbol that doesn't exist. This exported sub will then call AUTOLOAD when it's called. Fcntl, for example, uses this pattern.

Here's a bit of code that shows a simplified version of what I'm doing. CAUTION: this code loops infinitely printing "LOCK_EX called" - it might lock up your machine if you're not quick with Ctrl-C.

#!/usr/bin/perl -w
use Fcntl qw(LOCK_EX);

# instrument LOCK_EX
my $lock_ex_coderef = \&Fcntl::LOCK_EX;
*Fcntl::LOCK_EX = sub { 
    print "LOCK_EX called\n";
    return &$lock_ex_coderef; 
};

# call LOCK_EX
$_ = LOCK_EX;

[download]

Now, what I'd expected was that this code would print "LOCK_EX called" just once and then call Fcntl::AUTOLOAD as usual. Can anyone think of a way to get that behavior? Or perhaps I just need to understand why this method isn't working. Can anyone explain why a calling code-ref to a "fake" sub causes this recursion?

Alternately, can anyone propose a way to detect "fake" subroutines so I could skip them for now? B:: solutions are acceptable.

Thanks!
-sam

In reply to How to instrument a "fake" subroutine? by samtregar

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