Stacktrace::Configurable - a configurable Perl stack trace


 use Stacktrace::Configurable;



The idea for Stacktrace::Configurable came when I needed a easily readable stack trace in Log::Log4perl output. That distribution's pattern layout can give you a stack trace but it's not very readable. There are other modules out there that provide a caller stack, like Devel::StackTrace and Carp. Choose what suits you best.

A stack trace is basically a list of stack frames starting with the place where the get_trace method is called down to the main program. The first element in that list is also called the topmost frame.

Each frame of the list collected by get_trace is a Stacktrace::Configurable::Frame object which provides simple accessors for the information returned by caller. Additionally, a frame has a "nr" in Stacktrace::Configurable::Frame attribute which contains its position in the list starting from 1 (topmost).


The constructor Stacktrace::Configurable->new is called with a list of key/value pairs as parameters. After constructing an empty object it uses each of those keys as method name and calls it passing the value as parameter.




Attributes are simple accessor methods that provide access to scalar variables stored in the object. If called with a parameter the new value is stored. The return value is always the new or current value.

These attributes are implemented:


the format specification, see below


the stack trace. It is an arrayref of Stacktrace::Configurable::Frame objects usually initialized by the get_trace method.

Public Methods


collects the stack trace with the caller of get_trace as the topmost frame and stores it as $obj->frames.

Returns the object itself to allow for chained calls like


formats the stack trace according to the current format and returns the resulting string.

Methods interesting for subclassing


returns the empty list. If overwritten by subclasses, it should return a regular expression matching package names which is used to skip stack frames from the top of the stack. get_trace starts to collect stack frames from the top of the stack. If skip_package_re returns a regexp, it drops those frames as long as their package matches the regexp. Once a non-matching package is discovered all remaining frames are included in the trace no matter what package.

This allows you to skip frames internal to your subclass from the top of the stack if you are not sure of the nesting level at which get_trace is called.


Similar to skip_package_re, only it specifies the actual nesting level. For the base class (Stacktrace::Configurable) 1 is returned.


this method returns a constant that is used by the constructor to initialize the format attribute if omitted.

The current default format is:

 '%[nr=1,s=    ==== START STACK TRACE ===]b%[nr=1,n]b'.
 '%4b[%*n] at %f line %l%[n]b'.
 '%12b%[skip_package]s %[env=STACKTRACE_CONFIG_A]a'.
 '%[nr!STACKTRACE_CONFIG_MAX,c=%n    ... %C frames cut off]b'.
 '%[nr=$,n]b%[nr=$,s=    === END STACK TRACE ===]b%[nr=$,n]b'

these methods format a certain portion of a stack frame. They are called as methods. So, the first parameter is the object itself. The following parameters are:


the frame to format


the width part of the format specification


the param part of the format specification

Return value: the formatted string

Private Methods



The format used by Stacktrace::Configurable is inspired by printf and Log::Log4perl::Layout::PatternLayout.

The first format component is an optional string starting with env= and ending in a comma, like


If this component is found as_string consults the specified environment variable for instructions. If the variable is off, no or 0, no stack trace at all is created and as_string returns the empty string.

If after stripping of that first component, the format becomes the empty string the value of the environment variable or, if also empty, the default format is used as format specification.

The rest of the format is a string with embedded format specifications or fspec. An fspec starts with a percent sign, %. Then follows an optional width component, an optional parameter component and the mandatory format letter.

The width component is just an integer number optionally prepended with a minus sign or an asterisk (*). Not every fspec uses the width component or does something useful for *.

The parameter component is surrounded by brackets ([]).

The parsing of an fspec is kept simple. So, it does not support nested brackets or similar.

The following format letters are implemented:


Originally the name b was chosen because s was already in use. It stands for blank or empty space. Though, it can generate arbitrary output and be based on conditions.

The simplest form %b just outputs one space. Add a width component, %20b and you get 20 spaces.

The parameter component is a set of 2 optional items separated by comma. The first item specifies a condition. The second modifies the string used in place of the space.

Let's first look at examples where the condition part is omitted. The n parameter tells to use a newline instead of a space. So, %20[n]b inserts 20 newline characters. The parameter t does the same only that a tabulator character is used. %20[t]b results in 20 tabs. The 3rd option is the s= parameter. It allows you to specify arbitrary strings. %4[s=ab]b results in


Now, let's look at conditional output. The nr= parameter matches a specific stack frame given by its number. It is most useful at the start and the end of the stack trace.


 %[nr=1,s=stack trace start]b

if given at the beginning of the format, this specification prints the string stack trace start but only for the topmost frame.

 %[nr=$,s=stack trace end]b

nr=$ matches only for the last stack frame. So, the fspec above prints stack trace end at the end of the trace.

The nr! condition also matches a specific frame given by its number. But in addition to generate output it cuts off the trace after the current frame. It is used if you want to print only the topmost N frames. It is often used with the empty string as what to print, like


This prints nothing but cuts off the stack trace after the 10th frame.

If the part after the exclamation mark is not a number but matches \w+, it is taken as the name of an environment variable. If set and if it is a number, that number is taken instead of the literal number above.

In combination with this condition there is another parameter to specify the string, c= or the cutoff message. It is printed only if there has been cut off at least one frame. Also, the cutoff message can contain %n and %C (capital C). The former is replaced by a newline, the latter by the number of frames cut off.

This allows for the following pattern:

 %[nr!MAX,c=%ncutting off remaining %C frames]n

Now, let's assume $ENV{MAX}=4 but the actual stack is 20 frames deep. The specification tells to insert an additional newline for the 4th frame followed by the string cutting off remaining 16 frames.

The last condition is nr%N and nr%N=M. It can be used to insert a special delimiter after every N stack frames.


prints a delimiter consisting of a newline and 80 equal signs after every 10th frame.

The condition is true if frame_number % M == N where N defaults to 0.


inserts the frame number. This format ignores the parameter component. Width can be given as positive or negative number an is interpreted just like in sprintf. If width is * or -*, the actual width is taken to fit the largest frame number.



inserts the subroutine. The width component is ignored and only one parameter is known, skip_package. If specified, the package where the function belongs to is omitted.


 %s                   # might print "Pack::Age::fun"
 %[skip_package]s     # prints only "fun"

inserts the subroutine arguments. The width component is ignored. The parameter component is a comma separated list of


all arguments are dumped using Data::Dumper. The dumper object is configured in a way to print the whole thing in one line.

This may cause very verbose stack traces.


all arguments for which ref returns Pack::Age are dumped using Data::Dumper.

You can, of course, also dump simple ARRAYs, HASHes etc.


all arguments for which ref matches the regexp are dumped using Data::Dumper.

If multiple such parameters are given, an argument that matches at least one regexp is dumped.


if dump or dump=CODE is also specified, the dumper object is configured to deparse the subroutine that is passed in the argument.


normally, all arguments are printed in one line separated by comma and space. With this parameter every argument is printed on a separate line.

A format containing %s %[multiline]a would for instance generate this output:

 main::function (

The surrounding parentheses are part of the %a output.

N and M are indentation specifications. N tells how many positions the closing parenthesis is indented. M tells how many positions further each parameter is indented. The default value for both is 4.


This parameter reads the environment variable ENVVAR and appends it to the parameter list.


 %[dump,deparse,multiline]a    # very verbose

inserts the file name. This fspec recognizes the following parameters:


if the file name of the stack frame begins with PREFIX, it is cut off.

For instance, if your personal Perl modules are installed in /usr/local/perl, then you might specify


cuts off the directory part of the file name.

If a width component is specified and the file name is longer than the absolute value of the given width, then if the width is positive, the file name is cut at the end to meet the given width. If the width is negative, the file name is cut at the start to meet the width. Then an ellipsis (3 dots) is appended or prepended.


inserts the line number. The width component is interpreted like in sprintf.


prints the context of the subroutine call as void, scalar or list.


prints the package name of the stack frame.

The %p fspec recognizes the skip_prefix parameter just like %f.

The width component is also interpreted the same way as for %f.


 env=T,%f(%l)   # one line of "" for each frame
                # unless $ENV{T} is "off", "no" or "0"

 env=T,         # use the format given in $ENV{T}
                # unless $ENV{T} is "off", "no" or "0"




Torsten Förtsch <>


Copyright 2014- Torsten Förtsch


This library is free software; you can redistribute it and/or modify it under the same terms as Perl itself.


Carp, Devel::StackTrace, Log::Log4perl::Layout::PatternLayout::Stacktrace