TracePoint

Class

A class that provides the functionality of Kernel#set_trace_func in a well-structured Object-Oriented API.

Example

Use TracePoint to gather information specifically for exceptions:

trace = TracePoint.new(:raise) do |tp|
  p [tp.lineno, tp.event, tp.raised_exception]
end
#=> #<TracePoint:disabled>

trace.enable  #=> false

0 / 0
#=> [5, :raise, #<ZeroDivisionError: divided by 0>]

Events

If you don’t specify the types of events you want to listen for, TracePoint will include all available events.

Note: Do not depend on the current event set, as this list is subject to change. Instead, it is recommended to specify the types of events you want to use.

To filter what is traced, you can pass any of the following as events:

:line

Execute an expression or statement on a new line.

:class

Start a class or module definition.

:end

Finish a class or module definition.

:call

Call a Ruby method.

:return

Return from a Ruby method.

:c_call

Call a C-language routine.

:c_return

Return from a C-language routine.

:raise

Raise an exception.

:rescue

Rescue an exception.

:b_call

Event hook at block entry.

:b_return

Event hook at block ending.

:a_call

Event hook at all calls (call, b_call, and c_call).

:a_return

Event hook at all returns (return, b_return, and c_return).

:thread_begin

Event hook at thread beginning.

:thread_end

Event hook at thread ending.

:fiber_switch

Event hook at fiber switch.

:script_compiled

New Ruby code compiled (with eval, load, or require).

Class Methods

TracePoint.allow_reentry { block }
::

Generally, while a TracePoint callback is running, other registered callbacks are not called to avoid confusion from reentrance. This method allows reentrance within a given block. Use this method carefully to avoid infinite callback invocation.

If called when reentrance is already allowed, it raises a RuntimeError.

Example:

# Without reentry
# ---------------

line_handler = TracePoint.new(:line) do |tp|
  next if tp.path != __FILE__ # Only works in this file
  puts "Line handler"
  binding.eval("class C; end")
end.enable

class_handler = TracePoint.new(:class) do |tp|
  puts "Class handler"
end.enable

class B
end

# This script will print "Class handler" only once: when inside the :line
# handler, all other handlers are ignored.

# With reentry
# ------------

line_handler = TracePoint.new(:line) do |tp|
  next if tp.path != __FILE__ # Only works in this file
  next if (__LINE__..__LINE__+3).cover?(tp.lineno) # Prevent infinite calls
  puts "Line handler"
  TracePoint.allow_reentry { binding.eval("class C; end") }
end.enable

class_handler = TracePoint.new(:class) do |tp|
  puts "Class handler"
end.enable

class B
end

# This will print "Class handler" twice: inside the allow_reentry block in the :line
# handler, other handlers are enabled.

Note that the example shows the principal effect of the method, but its practical usage is for debugging libraries that sometimes require other libraries’ hooks to not be affected by the debugger being inside trace point handling. Precautions should be taken against infinite recursion in this case (note that we needed to filter out calls by itself from the :line handler, otherwise it would call itself infinitely).

TracePoint.new(*events) { |tp| block } → tp
::

Returns a new TracePoint object, not enabled by default.

To activate the TracePoint object, use TracePoint#enable:

trace = TracePoint.new(:call) do |tp|
  p [tp.lineno, tp.defined_class, tp.method_id, tp.event]
end
#=> #<TracePoint:disabled>

trace.enable  #=> false

puts "Hello, TracePoint!"
# ...
# [48, IRB::Notifier::AbstractNotifier, :printf, :call]
# ...

To deactivate the trace, use TracePoint#disable.

trace.disable

See Events at TracePoint for possible events and more information.

A block must be given; otherwise, an ArgumentError is raised.

If the trace method isn’t included in the given events filter, a RuntimeError is raised.

TracePoint.trace(:line) do |tp|
  p tp.raised_exception
end
#=> RuntimeError: 'raised_exception' not supported by this event

If the trace method is called outside a block, a RuntimeError is raised.

TracePoint.trace(:line) do |tp|
  $tp = tp
end
$tp.lineno #=> access from outside (RuntimeError)

Access from other threads is also forbidden.

TracePoint.stat → obj
::

Returns internal information of TracePoint.

The contents of the returned value are implementation-specific and may change in the future.

This method is only for debugging TracePoint itself.

TracePoint.trace(*events) { |tp| block } → obj
::

A convenience method for TracePoint.new that activates the trace automatically.

trace = TracePoint.trace(:call) { |tp| [tp.lineno, tp.event] }
#=> #<TracePoint:enabled>

trace.enabled?  #=> true
Instance Methods

binding
#

Returns the generated binding object from the event.

Note that for :c_call and :c_return events, the method returns nil, since C methods themselves do not have bindings.

callee_id
#

Returns the called name of the method being called.

defined_class
#

Returns the class or module of the method being called.

class C; def foo; end; end
trace = TracePoint.new(:call) do |tp|
  p tp.defined_class #=> C
end.enable do
  C.new.foo
end

If the method is defined by a module, then that module is returned.

module M; def foo; end; end
class C; include M; end
trace = TracePoint.new(:call) do |tp|
  p tp.defined_class #=> M
end.enable do
  C.new.foo
end

Note: defined_class returns the singleton class.

The 6th block parameter of Kernel#set_trace_func passes the original class attached by the singleton class.

This is a difference between Kernel#set_trace_func and TracePoint.

class C; def self.foo; end; end
trace = TracePoint.new(:call) do |tp|
  p tp.defined_class #=> #<Class:C>
end.enable do
  C.foo
end

trace.disable → true or false

trace.disable { block } → obj
#

Deactivates the trace.

Returns true if the trace was enabled. Returns false if the trace was disabled.

trace.enabled?  #=> true
trace.disable   #=> true (previous status)
trace.enabled?  #=> false
trace.disable   #=> false

If a block is given, the trace will only be disabled within the scope of the block.

trace.enabled?  #=> true

trace.disable do
  trace.enabled?
  # Only disabled for this block
end

trace.enabled?  #=> true

Note: You cannot access event hooks within the block.

trace.disable { p tp.lineno }
#=> RuntimeError: access from outside

trace.enable(target: nil, target_line: nil, target_thread: nil) → true or false

trace.enable(target: nil, target_line: nil, target_thread: :default) { block } → obj
#

Activates the trace.

Returns true if the trace was enabled. Returns false if the trace was disabled.

trace.enabled?  #=> false
trace.enable    #=> false (previous state)
                #   trace is enabled
trace.enabled?  #=> true
trace.enable    #=> true (previous state)
                #   trace is still enabled

If a block is given, the trace will only be enabled during the block execution. If target and target_line are both nil, then target_thread will default to the current thread if a block is given.

trace.enabled?  #=> false

trace.enable do
  trace.enabled?
  # Only enabled for this block and thread
end

trace.enabled?  #=> false

The target, target_line, and target_thread parameters are used to limit tracing to specified code objects. target should be a code object for which RubyVM::InstructionSequence.of will return an instruction sequence.

t = TracePoint.new(:line) { |tp| p tp }

def m1
  p 1
end

def m2
  p 2
end

t.enable(target: method(:m1))

m1
# Prints #<TracePoint:line test.rb:4 in `m1'>
m2
# Prints nothing

Note: You cannot access event hooks within the enable block.

trace.enable { p tp.lineno }
#=> RuntimeError: access from outside

trace.enabled? → true or false
#

Returns the current status of the trace.

eval_script
#

Returns the compiled source code (String) from eval methods on the :script_compiled event. If loaded from a file, it returns nil.

event
#

Returns the type of event.

See Events at TracePoint for more information.

trace.inspect → string
#

Returns a string containing a human-readable TracePoint status.

instruction_sequence
#

Returns the compiled instruction sequence represented by a RubyVM::InstructionSequence instance on the :script_compiled event.

Note that this method is CRuby-specific.

lineno
#

Returns the line number of the event.

method_id
#

Returns the name at the definition of the method being called.

parameters
#

Returns the parameter definitions of the method or block that the current hook belongs to. The format is the same as for Method#parameters.

path
#

Returns the path of the file being executed.

raised_exception
#

Returns the exception raised on the :raise event or rescued on the :rescue event.

return_value
#

Returns the return value from :return, :c_return, and :b_return events.

self
#

Returns the trace object during the event.

Similar to the following, but it returns the correct object (the method receiver) for :c_call and :c_return events:

trace.binding.eval('self')