Recipe 10.4 Determining Current Function Name

10.4.1 Problem

You want to determine the name of the currently running function. This is useful for creating error messages that don't need to be changed if you copy and paste the subroutine code.

10.4.2 Solution

Use the caller function:

$this_function = (caller(0))[3];

10.4.3 Discussion

Code can always determine the current source line number via the special symbol _ _LINE_ _, the current file via _ _FILE_ _, and the current package via _ _PACKAGE_ _. But no such symbol for the current subroutine name exists, let alone the name for the subroutine that called this one.

The built-in function caller handles all of these. In scalar context it returns the calling function's package name, but in list context it returns much more. You can also pass it a number indicating how many frames (nested subroutine calls) back you'd like information about: 0 is your own function, 1 is your caller, and so on.

Here's the full syntax, where $i is how far back you're interested in:

($package, $filename, $line, $subr, $has_args, $wantarray  #   0         1         2       3       4          5  $evaltext, $is_require, $hints, $bitmask #   6         7            8       9 )= caller($i);

Here's what each of those return values means:

$package

The package in which the code was compiled.

$filename

The name of the file in which the code was compiled, reporting -e if launched from that command-line switch, or - if the script was read from standard input.

$line

The line number from which that frame was called.

$subr

The name of that frame's function, including its package. Closures are indicated by names like main::_ _ANON_ _, which are not callable. In an eval, it contains (eval).

$has_args

Whether the function had its own @_ variable set up. It may be that there are no arguments, even if true. The only way for this to be false is if the function was called using the &fn notation instead of fn( ) or &fn( ).

$wantarray

The value the wantarray function would return for that stack frame; either true, false but defined, or else undefined. This tells whether the function was called in list, scalar, or void context (respectively).

$evaltext

The text of the current eval STRING, if any.

$is_require

Whether the code is currently being loaded by a do, require, or use.

$hints, $bitmask

These both contain pragmatic hints that the caller was compiled with. Consider them to be for internal use only by Perl itself.

Rather than using caller directly as in the Solution, you might want to write functions instead:

$me  = whoami( ); $him = whowasi( ); sub whoami  { (caller(1))[3] } sub whowasi { (caller(2))[3] }

These use arguments of 1 and 2 for parent and grandparent functions because the call to whoami or whowasi would itself be frame number 0.

10.4.4 See Also

The wantarray and caller functions in Chapter 29 of Programming Perl and in perlfunc(1); Recipe 10.6