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caller

Perl 5 version 26.0 documentation
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caller

Perl functions A-Z | Perl functions by category | The 'perlfunc' manpage
  • caller EXPR

  • caller

    Returns the context of the current pure perl subroutine call. In scalar context, returns the caller's package name if there is a caller (that is, if we're in a subroutine or eval or require) and the undefined value otherwise. caller never returns XS subs and they are skipped. The next pure perl sub will appear instead of the XS sub in caller's return values. In list context, caller returns

    1.        # 0         1          2
    2.     my ($package, $filename, $line) = caller;

    With EXPR, it returns some extra information that the debugger uses to print a stack trace. The value of EXPR indicates how many call frames to go back before the current one.

    1.     #  0         1          2      3            4
    2.  my ($package, $filename, $line, $subroutine, $hasargs,
    4.     #  5          6          7            8       9         10
    5.     $wantarray, $evaltext, $is_require, $hints, $bitmask, $hinthash)
    6.   = caller($i);

    Here, $subroutine is the function that the caller called (rather than the function containing the caller). Note that $subroutine may be (eval) if the frame is not a subroutine call, but an eval. In such a case additional elements $evaltext and $is_require are set: $is_require is true if the frame is created by a require or use statement, $evaltext contains the text of the eval EXPR statement. In particular, for an eval BLOCK statement, $subroutine is (eval) , but $evaltext is undefined. (Note also that each use statement creates a require frame inside an eval EXPR frame.) $subroutine may also be (unknown) if this particular subroutine happens to have been deleted from the symbol table. $hasargs is true if a new instance of @_ was set up for the frame. $hints and $bitmask contain pragmatic hints that the caller was compiled with. $hints corresponds to $^H , and $bitmask corresponds to ${^WARNING_BITS} . The $hints and $bitmask values are subject to change between versions of Perl, and are not meant for external use.

    $hinthash is a reference to a hash containing the value of %^H when the caller was compiled, or undef if %^H was empty. Do not modify the values of this hash, as they are the actual values stored in the optree.

    Furthermore, when called from within the DB package in list context, and with an argument, caller returns more detailed information: it sets the list variable @DB::args to be the arguments with which the subroutine was invoked.

    Be aware that the optimizer might have optimized call frames away before caller had a chance to get the information. That means that caller(N) might not return information about the call frame you expect it to, for N > 1 . In particular, @DB::args might have information from the previous time caller was called.

    Be aware that setting @DB::args is best effort, intended for debugging or generating backtraces, and should not be relied upon. In particular, as @_ contains aliases to the caller's arguments, Perl does not take a copy of @_ , so @DB::args will contain modifications the subroutine makes to @_ or its contents, not the original values at call time. @DB::args , like @_ , does not hold explicit references to its elements, so under certain cases its elements may have become freed and reallocated for other variables or temporary values. Finally, a side effect of the current implementation is that the effects of shift @_ can normally be undone (but not pop @_ or other splicing, and not if a reference to @_ has been taken, and subject to the caveat about reallocated elements), so @DB::args is actually a hybrid of the current state and initial state of @_ . Buyer beware.