src/runtime/runtime-gdb.py - go - Git at Google

 # Copyright 2010 The Go Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style
 # license that can be found in the LICENSE file.

 """GDB Pretty printers and convenience functions for Go's runtime structures.

 This script is loaded by GDB when it finds a .debug_gdb_scripts
 section in the compiled binary. The [68]l linkers emit this with a
 path to this file based on the path to the runtime package.
 """

 # Known issues:
 #    - pretty printing only works for the 'native' strings. E.g. 'type
 #      foo string' will make foo a plain struct in the eyes of gdb,
 #      circumventing the pretty print triggering.


 from __future__ import print_function
 import re
 import sys
 import gdb

 print("Loading Go Runtime support.", file=sys.stderr)
 #http://python3porting.com/differences.html
 if sys.version > '3':
 	xrange = range
 # allow to manually reload while developing
 goobjfile = gdb.current_objfile() or gdb.objfiles()[0]
 goobjfile.pretty_printers = []

 # G state (runtime2.go)

 def read_runtime_const(varname, default):
   try:
     return int(gdb.parse_and_eval(varname))
   except Exception:
     return int(default)


 G_IDLE = read_runtime_const("'runtime._Gidle'", 0)
 G_RUNNABLE = read_runtime_const("'runtime._Grunnable'", 1)
 G_RUNNING = read_runtime_const("'runtime._Grunning'", 2)
 G_SYSCALL = read_runtime_const("'runtime._Gsyscall'", 3)
 G_WAITING = read_runtime_const("'runtime._Gwaiting'", 4)
 G_MORIBUND_UNUSED = read_runtime_const("'runtime._Gmoribund_unused'", 5)
 G_DEAD = read_runtime_const("'runtime._Gdead'", 6)
 G_ENQUEUE_UNUSED = read_runtime_const("'runtime._Genqueue_unused'", 7)
 G_COPYSTACK = read_runtime_const("'runtime._Gcopystack'", 8)
 G_SCAN = read_runtime_const("'runtime._Gscan'", 0x1000)
 G_SCANRUNNABLE = G_SCAN+G_RUNNABLE
 G_SCANRUNNING = G_SCAN+G_RUNNING
 G_SCANSYSCALL = G_SCAN+G_SYSCALL
 G_SCANWAITING = G_SCAN+G_WAITING

 sts = {
     G_IDLE: 'idle',
     G_RUNNABLE: 'runnable',
     G_RUNNING: 'running',
     G_SYSCALL: 'syscall',
     G_WAITING: 'waiting',
     G_MORIBUND_UNUSED: 'moribund',
     G_DEAD: 'dead',
     G_ENQUEUE_UNUSED: 'enqueue',
     G_COPYSTACK: 'copystack',
     G_SCAN: 'scan',
     G_SCANRUNNABLE: 'runnable+s',
     G_SCANRUNNING: 'running+s',
     G_SCANSYSCALL: 'syscall+s',
     G_SCANWAITING: 'waiting+s',
 }


 #
 #  Value wrappers
 #

 class SliceValue:
 	"Wrapper for slice values."

 	def __init__(self, val):
 		self.val = val

 	@property
 	def len(self):
 		return int(self.val['len'])

 	@property
 	def cap(self):
 		return int(self.val['cap'])

 	def __getitem__(self, i):
 		if i < 0 or i >= self.len:
 			raise IndexError(i)
 		ptr = self.val["array"]
 		return (ptr + i).dereference()


 #
 #  Pretty Printers
 #

 # The patterns for matching types are permissive because gdb 8.2 switched to matching on (we think) typedef names instead of C syntax names.
 class StringTypePrinter:
 	"Pretty print Go strings."

 	pattern = re.compile(r'^(struct string( \*)?|string)$')

 	def __init__(self, val):
 		self.val = val

 	def display_hint(self):
 		return 'string'

 	def to_string(self):
 		l = int(self.val['len'])
 		return self.val['str'].string("utf-8", "ignore", l)


 class SliceTypePrinter:
 	"Pretty print slices."

 	pattern = re.compile(r'^(struct \[\]|\[\])')

 	def __init__(self, val):
 		self.val = val

 	def display_hint(self):
 		return 'array'

 	def to_string(self):
 		t = str(self.val.type)
 		if (t.startswith("struct ")):
 			return t[len("struct "):]
 		return t

 	def children(self):
 		sval = SliceValue(self.val)
 		if sval.len > sval.cap:
 			return
 		for idx, item in enumerate(sval):
 			yield ('[{0}]'.format(idx), item)


 class MapTypePrinter:
 	"""Pretty print map[K]V types.

 	Map-typed go variables are really pointers. dereference them in gdb
 	to inspect their contents with this pretty printer.
 	"""

 	pattern = re.compile(r'^map\[.*\].*$')

 	def __init__(self, val):
 		self.val = val

 	def display_hint(self):
 		return 'map'

 	def to_string(self):
 		return str(self.val.type)

 	def children(self):
 		B = self.val['B']
 		buckets = self.val['buckets']
 		oldbuckets = self.val['oldbuckets']
 		flags = self.val['flags']
 		inttype = self.val['hash0'].type
 		cnt = 0
 		for bucket in xrange(2 ** int(B)):
 			bp = buckets + bucket
 			if oldbuckets:
 				oldbucket = bucket & (2 ** (B - 1) - 1)
 				oldbp = oldbuckets + oldbucket
 				oldb = oldbp.dereference()
 				if (oldb['overflow'].cast(inttype) & 1) == 0:  # old bucket not evacuated yet
 					if bucket >= 2 ** (B - 1):
 						continue    # already did old bucket
 					bp = oldbp
 			while bp:
 				b = bp.dereference()
 				for i in xrange(8):
 					if b['tophash'][i] != 0:
 						k = b['keys'][i]
 						v = b['values'][i]
 						if flags & 1:
 							k = k.dereference()
 						if flags & 2:
 							v = v.dereference()
 						yield str(cnt), k
 						yield str(cnt + 1), v
 						cnt += 2
 				bp = b['overflow']


 class ChanTypePrinter:
 	"""Pretty print chan[T] types.

 	Chan-typed go variables are really pointers. dereference them in gdb
 	to inspect their contents with this pretty printer.
 	"""

 	pattern = re.compile(r'^chan ')

 	def __init__(self, val):
 		self.val = val

 	def display_hint(self):
 		return 'array'

 	def to_string(self):
 		return str(self.val.type)

 	def children(self):
 		# see chan.c chanbuf(). et is the type stolen from hchan<T>::recvq->first->elem
 		et = [x.type for x in self.val['recvq']['first'].type.target().fields() if x.name == 'elem'][0]
 		ptr = (self.val.address["buf"]).cast(et)
 		for i in range(self.val["qcount"]):
 			j = (self.val["recvx"] + i) % self.val["dataqsiz"]
 			yield ('[{0}]'.format(i), (ptr + j).dereference())


 #
 #  Register all the *Printer classes above.
 #

 def makematcher(klass):
 	def matcher(val):
 		try:
 			if klass.pattern.match(str(val.type)):
 				return klass(val)
 		except Exception:
 			pass
 	return matcher

 goobjfile.pretty_printers.extend([makematcher(var) for var in vars().values() if hasattr(var, 'pattern')])
 #
 #  Utilities
 #

 def pc_to_int(pc):
 	# python2 will not cast pc (type void*) to an int cleanly
 	# instead python2 and python3 work with the hex string representation
 	# of the void pointer which we can parse back into an int.
 	# int(pc) will not work.
 	try:
 		# python3 / newer versions of gdb
 		pc = int(pc)
 	except gdb.error:
 		# str(pc) can return things like
 		# "0x429d6c <runtime.gopark+284>", so
 		# chop at first space.
 		pc = int(str(pc).split(None, 1)[0], 16)
 	return pc


 #
 #  For reference, this is what we're trying to do:
 #  eface: p *(*(struct 'runtime.rtype'*)'main.e'->type_->data)->string
 #  iface: p *(*(struct 'runtime.rtype'*)'main.s'->tab->Type->data)->string
 #
 # interface types can't be recognized by their name, instead we check
 # if they have the expected fields.  Unfortunately the mapping of
 # fields to python attributes in gdb.py isn't complete: you can't test
 # for presence other than by trapping.


 def is_iface(val):
 	try:
 		return str(val['tab'].type) == "struct runtime.itab *" and str(val['data'].type) == "void *"
 	except gdb.error:
 		pass


 def is_eface(val):
 	try:
 		return str(val['_type'].type) == "struct runtime._type *" and str(val['data'].type) == "void *"
 	except gdb.error:
 		pass


 def lookup_type(name):
 	try:
 		return gdb.lookup_type(name)
 	except gdb.error:
 		pass
 	try:
 		return gdb.lookup_type('struct ' + name)
 	except gdb.error:
 		pass
 	try:
 		return gdb.lookup_type('struct ' + name[1:]).pointer()
 	except gdb.error:
 		pass


 def iface_commontype(obj):
 	if is_iface(obj):
 		go_type_ptr = obj['tab']['_type']
 	elif is_eface(obj):
 		go_type_ptr = obj['_type']
 	else:
 		return

 	return go_type_ptr.cast(gdb.lookup_type("struct reflect.rtype").pointer()).dereference()


 def iface_dtype(obj):
 	"Decode type of the data field of an eface or iface struct."
 	# known issue: dtype_name decoded from runtime.rtype is "nested.Foo"
 	# but the dwarf table lists it as "full/path/to/nested.Foo"

 	dynamic_go_type = iface_commontype(obj)
 	if dynamic_go_type is None:
 		return
 	dtype_name = dynamic_go_type['string'].dereference()['str'].string()

 	dynamic_gdb_type = lookup_type(dtype_name)
 	if dynamic_gdb_type is None:
 		return

 	type_size = int(dynamic_go_type['size'])
 	uintptr_size = int(dynamic_go_type['size'].type.sizeof)	 # size is itself an uintptr
 	if type_size > uintptr_size:
 			dynamic_gdb_type = dynamic_gdb_type.pointer()

 	return dynamic_gdb_type


 def iface_dtype_name(obj):
 	"Decode type name of the data field of an eface or iface struct."

 	dynamic_go_type = iface_commontype(obj)
 	if dynamic_go_type is None:
 		return
 	return dynamic_go_type['string'].dereference()['str'].string()


 class IfacePrinter:
 	"""Pretty print interface values

 	Casts the data field to the appropriate dynamic type."""

 	def __init__(self, val):
 		self.val = val

 	def display_hint(self):
 		return 'string'

 	def to_string(self):
 		if self.val['data'] == 0:
 			return 0x0
 		try:
 			dtype = iface_dtype(self.val)
 		except Exception:
 			return "<bad dynamic type>"

 		if dtype is None:  # trouble looking up, print something reasonable
 			return "({typename}){data}".format(
 				typename=iface_dtype_name(self.val), data=self.val['data'])

 		try:
 			return self.val['data'].cast(dtype).dereference()
 		except Exception:
 			pass
 		return self.val['data'].cast(dtype)


 def ifacematcher(val):
 	if is_iface(val) or is_eface(val):
 		return IfacePrinter(val)

 goobjfile.pretty_printers.append(ifacematcher)

 #
 #  Convenience Functions
 #


 class GoLenFunc(gdb.Function):
 	"Length of strings, slices, maps or channels"

 	how = ((StringTypePrinter, 'len'), (SliceTypePrinter, 'len'), (MapTypePrinter, 'count'), (ChanTypePrinter, 'qcount'))

 	def __init__(self):
 		gdb.Function.__init__(self, "len")

 	def invoke(self, obj):
 		typename = str(obj.type)
 		for klass, fld in self.how:
 			if klass.pattern.match(typename):
 				return obj[fld]


 class GoCapFunc(gdb.Function):
 	"Capacity of slices or channels"

 	how = ((SliceTypePrinter, 'cap'), (ChanTypePrinter, 'dataqsiz'))

 	def __init__(self):
 		gdb.Function.__init__(self, "cap")

 	def invoke(self, obj):
 		typename = str(obj.type)
 		for klass, fld in self.how:
 			if klass.pattern.match(typename):
 				return obj[fld]


 class DTypeFunc(gdb.Function):
 	"""Cast Interface values to their dynamic type.

 	For non-interface types this behaves as the identity operation.
 	"""

 	def __init__(self):
 		gdb.Function.__init__(self, "dtype")

 	def invoke(self, obj):
 		try:
 			return obj['data'].cast(iface_dtype(obj))
 		except gdb.error:
 			pass
 		return obj

 #
 #  Commands
 #

 def linked_list(ptr, linkfield):
 	while ptr:
 		yield ptr
 		ptr = ptr[linkfield]


 class GoroutinesCmd(gdb.Command):
 	"List all goroutines."

 	def __init__(self):
 		gdb.Command.__init__(self, "info goroutines", gdb.COMMAND_STACK, gdb.COMPLETE_NONE)

 	def invoke(self, _arg, _from_tty):
 		# args = gdb.string_to_argv(arg)
 		vp = gdb.lookup_type('void').pointer()
 		for ptr in SliceValue(gdb.parse_and_eval("'runtime.allgs'")):
 			if ptr['atomicstatus'] == G_DEAD:
 				continue
 			s = ' '
 			if ptr['m']:
 				s = '*'
 			pc = ptr['sched']['pc'].cast(vp)
 			pc = pc_to_int(pc)
 			blk = gdb.block_for_pc(pc)
 			status = int(ptr['atomicstatus'])
 			st = sts.get(status, "unknown(%d)" % status)
 			print(s, ptr['goid'], "{0:8s}".format(st), blk.function)


 def find_goroutine(goid):
 	"""
 	find_goroutine attempts to find the goroutine identified by goid.
 	It returns a tuple of gdb.Value's representing the stack pointer
 	and program counter pointer for the goroutine.

 	@param int goid

 	@return tuple (gdb.Value, gdb.Value)
 	"""
 	vp = gdb.lookup_type('void').pointer()
 	for ptr in SliceValue(gdb.parse_and_eval("'runtime.allgs'")):
 		if ptr['atomicstatus'] == G_DEAD:
 			continue
 		if ptr['goid'] == goid:
 			break
 	else:
 		return None, None
 	# Get the goroutine's saved state.
 	pc, sp = ptr['sched']['pc'], ptr['sched']['sp']
 	status = ptr['atomicstatus']&~G_SCAN
 	# Goroutine is not running nor in syscall, so use the info in goroutine
 	if status != G_RUNNING and status != G_SYSCALL:
 		return pc.cast(vp), sp.cast(vp)

 	# If the goroutine is in a syscall, use syscallpc/sp.
 	pc, sp = ptr['syscallpc'], ptr['syscallsp']
 	if sp != 0:
 		return pc.cast(vp), sp.cast(vp)
 	# Otherwise, the goroutine is running, so it doesn't have
 	# saved scheduler state. Find G's OS thread.
 	m = ptr['m']
 	if m == 0:
 		return None, None
 	for thr in gdb.selected_inferior().threads():
 		if thr.ptid[1] == m['procid']:
 			break
 	else:
 		return None, None
 	# Get scheduler state from the G's OS thread state.
 	curthr = gdb.selected_thread()
 	try:
 		thr.switch()
 		pc = gdb.parse_and_eval('$pc')
 		sp = gdb.parse_and_eval('$sp')
 	finally:
 		curthr.switch()
 	return pc.cast(vp), sp.cast(vp)


 class GoroutineCmd(gdb.Command):
 	"""Execute gdb command in the context of goroutine <goid>.

 	Switch PC and SP to the ones in the goroutine's G structure,
 	execute an arbitrary gdb command, and restore PC and SP.

 	Usage: (gdb) goroutine <goid> <gdbcmd>

 	You could pass "all" as <goid> to apply <gdbcmd> to all goroutines.

 	For example: (gdb) goroutine all <gdbcmd>

 	Note that it is ill-defined to modify state in the context of a goroutine.
 	Restrict yourself to inspecting values.
 	"""

 	def __init__(self):
 		gdb.Command.__init__(self, "goroutine", gdb.COMMAND_STACK, gdb.COMPLETE_NONE)

 	def invoke(self, arg, _from_tty):
 		goid_str, cmd = arg.split(None, 1)
 		goids = []

 		if goid_str == 'all':
 			for ptr in SliceValue(gdb.parse_and_eval("'runtime.allgs'")):
 				goids.append(int(ptr['goid']))
 		else:
 			goids = [int(gdb.parse_and_eval(goid_str))]

 		for goid in goids:
 			self.invoke_per_goid(goid, cmd)

 	def invoke_per_goid(self, goid, cmd):
 		pc, sp = find_goroutine(goid)
 		if not pc:
 			print("No such goroutine: ", goid)
 			return
 		pc = pc_to_int(pc)
 		save_frame = gdb.selected_frame()
 		gdb.parse_and_eval('$save_sp = $sp')
 		gdb.parse_and_eval('$save_pc = $pc')
 		# In GDB, assignments to sp must be done from the
 		# top-most frame, so select frame 0 first.
 		gdb.execute('select-frame 0')
 		gdb.parse_and_eval('$sp = {0}'.format(str(sp)))
 		gdb.parse_and_eval('$pc = {0}'.format(str(pc)))
 		try:
 			gdb.execute(cmd)
 		finally:
 			# In GDB, assignments to sp must be done from the
 			# top-most frame, so select frame 0 first.
 			gdb.execute('select-frame 0')
 			gdb.parse_and_eval('$pc = $save_pc')
 			gdb.parse_and_eval('$sp = $save_sp')
 			save_frame.select()


 class GoIfaceCmd(gdb.Command):
 	"Print Static and dynamic interface types"

 	def __init__(self):
 		gdb.Command.__init__(self, "iface", gdb.COMMAND_DATA, gdb.COMPLETE_SYMBOL)

 	def invoke(self, arg, _from_tty):
 		for obj in gdb.string_to_argv(arg):
 			try:
 				#TODO fix quoting for qualified variable names
 				obj = gdb.parse_and_eval(str(obj))
 			except Exception as e:
 				print("Can't parse ", obj, ": ", e)
 				continue

 			if obj['data'] == 0:
 				dtype = "nil"
 			else:
 				dtype = iface_dtype(obj)

 			if dtype is None:
 				print("Not an interface: ", obj.type)
 				continue

 			print("{0}: {1}".format(obj.type, dtype))

 # TODO: print interface's methods and dynamic type's func pointers thereof.
 #rsc: "to find the number of entries in the itab's Fn field look at
 # itab.inter->numMethods
 # i am sure i have the names wrong but look at the interface type
 # and its method count"
 # so Itype will start with a commontype which has kind = interface

 #
 # Register all convenience functions and CLI commands
 #
 GoLenFunc()
 GoCapFunc()
 DTypeFunc()
 GoroutinesCmd()
 GoroutineCmd()
 GoIfaceCmd()
	# Copyright 2010 The Go Authors. All rights reserved.
	# Use of this source code is governed by a BSD-style
	# license that can be found in the LICENSE file.

	"""GDB Pretty printers and convenience functions for Go's runtime structures.

	This script is loaded by GDB when it finds a .debug_gdb_scripts
	section in the compiled binary. The [68]l linkers emit this with a
	path to this file based on the path to the runtime package.
	"""

	# Known issues:
	# - pretty printing only works for the 'native' strings. E.g. 'type
	# foo string' will make foo a plain struct in the eyes of gdb,
	# circumventing the pretty print triggering.


	from __future__ import print_function
	import re
	import sys
	import gdb

	print("Loading Go Runtime support.", file=sys.stderr)
	#http://python3porting.com/differences.html
	if sys.version > '3':
	xrange = range
	# allow to manually reload while developing
	goobjfile = gdb.current_objfile() or gdb.objfiles()[0]
	goobjfile.pretty_printers = []

	# G state (runtime2.go)

	def read_runtime_const(varname, default):
	try:
	return int(gdb.parse_and_eval(varname))
	except Exception:
	return int(default)


	G_IDLE = read_runtime_const("'runtime._Gidle'", 0)
	G_RUNNABLE = read_runtime_const("'runtime._Grunnable'", 1)
	G_RUNNING = read_runtime_const("'runtime._Grunning'", 2)
	G_SYSCALL = read_runtime_const("'runtime._Gsyscall'", 3)
	G_WAITING = read_runtime_const("'runtime._Gwaiting'", 4)
	G_MORIBUND_UNUSED = read_runtime_const("'runtime._Gmoribund_unused'", 5)
	G_DEAD = read_runtime_const("'runtime._Gdead'", 6)
	G_ENQUEUE_UNUSED = read_runtime_const("'runtime._Genqueue_unused'", 7)
	G_COPYSTACK = read_runtime_const("'runtime._Gcopystack'", 8)
	G_SCAN = read_runtime_const("'runtime._Gscan'", 0x1000)
	G_SCANRUNNABLE = G_SCAN+G_RUNNABLE
	G_SCANRUNNING = G_SCAN+G_RUNNING
	G_SCANSYSCALL = G_SCAN+G_SYSCALL
	G_SCANWAITING = G_SCAN+G_WAITING

	sts = {
	G_IDLE: 'idle',
	G_RUNNABLE: 'runnable',
	G_RUNNING: 'running',
	G_SYSCALL: 'syscall',
	G_WAITING: 'waiting',
	G_MORIBUND_UNUSED: 'moribund',
	G_DEAD: 'dead',
	G_ENQUEUE_UNUSED: 'enqueue',
	G_COPYSTACK: 'copystack',
	G_SCAN: 'scan',
	G_SCANRUNNABLE: 'runnable+s',
	G_SCANRUNNING: 'running+s',
	G_SCANSYSCALL: 'syscall+s',
	G_SCANWAITING: 'waiting+s',
	}


	#
	# Value wrappers
	#

	class SliceValue:
	"Wrapper for slice values."

	def __init__(self, val):
	self.val = val

	@property
	def len(self):
	return int(self.val['len'])

	@property
	def cap(self):
	return int(self.val['cap'])

	def __getitem__(self, i):
	if i < 0 or i >= self.len:
	raise IndexError(i)
	ptr = self.val["array"]
	return (ptr + i).dereference()


	#
	# Pretty Printers
	#

	# The patterns for matching types are permissive because gdb 8.2 switched to matching on (we think) typedef names instead of C syntax names.
	class StringTypePrinter:
	"Pretty print Go strings."

	pattern = re.compile(r'^(struct string( \*)?\|string)$')

	def __init__(self, val):
	self.val = val

	def display_hint(self):
	return 'string'

	def to_string(self):
	l = int(self.val['len'])
	return self.val['str'].string("utf-8", "ignore", l)


	class SliceTypePrinter:
	"Pretty print slices."

	pattern = re.compile(r'^(struct \[\]\|\[\])')

	def __init__(self, val):
	self.val = val

	def display_hint(self):
	return 'array'

	def to_string(self):
	t = str(self.val.type)
	if (t.startswith("struct ")):
	return t[len("struct "):]
	return t

	def children(self):
	sval = SliceValue(self.val)
	if sval.len > sval.cap:
	return
	for idx, item in enumerate(sval):
	yield ('[{0}]'.format(idx), item)


	class MapTypePrinter:
	"""Pretty print map[K]V types.

	Map-typed go variables are really pointers. dereference them in gdb
	to inspect their contents with this pretty printer.
	"""

	pattern = re.compile(r'^map\[.\].$')

	def __init__(self, val):
	self.val = val

	def display_hint(self):
	return 'map'

	def to_string(self):
	return str(self.val.type)

	def children(self):
	B = self.val['B']
	buckets = self.val['buckets']
	oldbuckets = self.val['oldbuckets']
	flags = self.val['flags']
	inttype = self.val['hash0'].type
	cnt = 0
	for bucket in xrange(2 ** int(B)):
	bp = buckets + bucket
	if oldbuckets:
	oldbucket = bucket & (2 ** (B - 1) - 1)
	oldbp = oldbuckets + oldbucket
	oldb = oldbp.dereference()
	if (oldb['overflow'].cast(inttype) & 1) == 0: # old bucket not evacuated yet
	if bucket >= 2 ** (B - 1):
	continue # already did old bucket
	bp = oldbp
	while bp:
	b = bp.dereference()
	for i in xrange(8):
	if b['tophash'][i] != 0:
	k = b['keys'][i]
	v = b['values'][i]
	if flags & 1:
	k = k.dereference()
	if flags & 2:
	v = v.dereference()
	yield str(cnt), k
	yield str(cnt + 1), v
	cnt += 2
	bp = b['overflow']


	class ChanTypePrinter:
	"""Pretty print chan[T] types.

	Chan-typed go variables are really pointers. dereference them in gdb
	to inspect their contents with this pretty printer.
	"""

	pattern = re.compile(r'^chan ')

	def __init__(self, val):
	self.val = val

	def display_hint(self):
	return 'array'

	def to_string(self):
	return str(self.val.type)

	def children(self):
	# see chan.c chanbuf(). et is the type stolen from hchan<T>::recvq->first->elem
	et = [x.type for x in self.val['recvq']['first'].type.target().fields() if x.name == 'elem'][0]
	ptr = (self.val.address["buf"]).cast(et)
	for i in range(self.val["qcount"]):
	j = (self.val["recvx"] + i) % self.val["dataqsiz"]
	yield ('[{0}]'.format(i), (ptr + j).dereference())


	#
	# Register all the *Printer classes above.
	#

	def makematcher(klass):
	def matcher(val):
	try:
	if klass.pattern.match(str(val.type)):
	return klass(val)
	except Exception:
	pass
	return matcher

	goobjfile.pretty_printers.extend([makematcher(var) for var in vars().values() if hasattr(var, 'pattern')])
	#
	# Utilities
	#

	def pc_to_int(pc):
	# python2 will not cast pc (type void*) to an int cleanly
	# instead python2 and python3 work with the hex string representation
	# of the void pointer which we can parse back into an int.
	# int(pc) will not work.
	try:
	# python3 / newer versions of gdb
	pc = int(pc)
	except gdb.error:
	# str(pc) can return things like
	# "0x429d6c <runtime.gopark+284>", so
	# chop at first space.
	pc = int(str(pc).split(None, 1)[0], 16)
	return pc


	#
	# For reference, this is what we're trying to do:
	# eface: p ((struct 'runtime.rtype'*)'main.e'->type_->data)->string
	# iface: p ((struct 'runtime.rtype'*)'main.s'->tab->Type->data)->string
	#
	# interface types can't be recognized by their name, instead we check
	# if they have the expected fields. Unfortunately the mapping of
	# fields to python attributes in gdb.py isn't complete: you can't test
	# for presence other than by trapping.


	def is_iface(val):
	try:
	return str(val['tab'].type) == "struct runtime.itab " and str(val['data'].type) == "void "
	except gdb.error:
	pass


	def is_eface(val):
	try:
	return str(val['_type'].type) == "struct runtime._type " and str(val['data'].type) == "void "
	except gdb.error:
	pass


	def lookup_type(name):
	try:
	return gdb.lookup_type(name)
	except gdb.error:
	pass
	try:
	return gdb.lookup_type('struct ' + name)
	except gdb.error:
	pass
	try:
	return gdb.lookup_type('struct ' + name[1:]).pointer()
	except gdb.error:
	pass


	def iface_commontype(obj):
	if is_iface(obj):
	go_type_ptr = obj['tab']['_type']
	elif is_eface(obj):
	go_type_ptr = obj['_type']
	else:
	return

	return go_type_ptr.cast(gdb.lookup_type("struct reflect.rtype").pointer()).dereference()


	def iface_dtype(obj):
	"Decode type of the data field of an eface or iface struct."
	# known issue: dtype_name decoded from runtime.rtype is "nested.Foo"
	# but the dwarf table lists it as "full/path/to/nested.Foo"

	dynamic_go_type = iface_commontype(obj)
	if dynamic_go_type is None:
	return
	dtype_name = dynamic_go_type['string'].dereference()['str'].string()

	dynamic_gdb_type = lookup_type(dtype_name)
	if dynamic_gdb_type is None:
	return

	type_size = int(dynamic_go_type['size'])
	uintptr_size = int(dynamic_go_type['size'].type.sizeof) # size is itself an uintptr
	if type_size > uintptr_size:
	dynamic_gdb_type = dynamic_gdb_type.pointer()

	return dynamic_gdb_type


	def iface_dtype_name(obj):
	"Decode type name of the data field of an eface or iface struct."

	dynamic_go_type = iface_commontype(obj)
	if dynamic_go_type is None:
	return
	return dynamic_go_type['string'].dereference()['str'].string()


	class IfacePrinter:
	"""Pretty print interface values

	Casts the data field to the appropriate dynamic type."""

	def __init__(self, val):
	self.val = val

	def display_hint(self):
	return 'string'

	def to_string(self):
	if self.val['data'] == 0:
	return 0x0
	try:
	dtype = iface_dtype(self.val)
	except Exception:
	return "<bad dynamic type>"

	if dtype is None: # trouble looking up, print something reasonable
	return "({typename}){data}".format(
	typename=iface_dtype_name(self.val), data=self.val['data'])

	try:
	return self.val['data'].cast(dtype).dereference()
	except Exception:
	pass
	return self.val['data'].cast(dtype)


	def ifacematcher(val):
	if is_iface(val) or is_eface(val):
	return IfacePrinter(val)

	goobjfile.pretty_printers.append(ifacematcher)

	#
	# Convenience Functions
	#


	class GoLenFunc(gdb.Function):
	"Length of strings, slices, maps or channels"

	how = ((StringTypePrinter, 'len'), (SliceTypePrinter, 'len'), (MapTypePrinter, 'count'), (ChanTypePrinter, 'qcount'))

	def __init__(self):
	gdb.Function.__init__(self, "len")

	def invoke(self, obj):
	typename = str(obj.type)
	for klass, fld in self.how:
	if klass.pattern.match(typename):
	return obj[fld]


	class GoCapFunc(gdb.Function):
	"Capacity of slices or channels"

	how = ((SliceTypePrinter, 'cap'), (ChanTypePrinter, 'dataqsiz'))

	def __init__(self):
	gdb.Function.__init__(self, "cap")

	def invoke(self, obj):
	typename = str(obj.type)
	for klass, fld in self.how:
	if klass.pattern.match(typename):
	return obj[fld]


	class DTypeFunc(gdb.Function):
	"""Cast Interface values to their dynamic type.

	For non-interface types this behaves as the identity operation.
	"""

	def __init__(self):
	gdb.Function.__init__(self, "dtype")

	def invoke(self, obj):
	try:
	return obj['data'].cast(iface_dtype(obj))
	except gdb.error:
	pass
	return obj

	#
	# Commands
	#

	def linked_list(ptr, linkfield):
	while ptr:
	yield ptr
	ptr = ptr[linkfield]


	class GoroutinesCmd(gdb.Command):
	"List all goroutines."

	def __init__(self):
	gdb.Command.__init__(self, "info goroutines", gdb.COMMAND_STACK, gdb.COMPLETE_NONE)

	def invoke(self, _arg, _from_tty):
	# args = gdb.string_to_argv(arg)
	vp = gdb.lookup_type('void').pointer()
	for ptr in SliceValue(gdb.parse_and_eval("'runtime.allgs'")):
	if ptr['atomicstatus'] == G_DEAD:
	continue
	s = ' '
	if ptr['m']:
	s = '*'
	pc = ptr['sched']['pc'].cast(vp)
	pc = pc_to_int(pc)
	blk = gdb.block_for_pc(pc)
	status = int(ptr['atomicstatus'])
	st = sts.get(status, "unknown(%d)" % status)
	print(s, ptr['goid'], "{0:8s}".format(st), blk.function)


	def find_goroutine(goid):
	"""
	find_goroutine attempts to find the goroutine identified by goid.
	It returns a tuple of gdb.Value's representing the stack pointer
	and program counter pointer for the goroutine.

	@param int goid

	@return tuple (gdb.Value, gdb.Value)
	"""
	vp = gdb.lookup_type('void').pointer()
	for ptr in SliceValue(gdb.parse_and_eval("'runtime.allgs'")):
	if ptr['atomicstatus'] == G_DEAD:
	continue
	if ptr['goid'] == goid:
	break
	else:
	return None, None
	# Get the goroutine's saved state.
	pc, sp = ptr['sched']['pc'], ptr['sched']['sp']
	status = ptr['atomicstatus']&~G_SCAN
	# Goroutine is not running nor in syscall, so use the info in goroutine
	if status != G_RUNNING and status != G_SYSCALL:
	return pc.cast(vp), sp.cast(vp)

	# If the goroutine is in a syscall, use syscallpc/sp.
	pc, sp = ptr['syscallpc'], ptr['syscallsp']
	if sp != 0:
	return pc.cast(vp), sp.cast(vp)
	# Otherwise, the goroutine is running, so it doesn't have
	# saved scheduler state. Find G's OS thread.
	m = ptr['m']
	if m == 0:
	return None, None
	for thr in gdb.selected_inferior().threads():
	if thr.ptid[1] == m['procid']:
	break
	else:
	return None, None
	# Get scheduler state from the G's OS thread state.
	curthr = gdb.selected_thread()
	try:
	thr.switch()
	pc = gdb.parse_and_eval('$pc')
	sp = gdb.parse_and_eval('$sp')
	finally:
	curthr.switch()
	return pc.cast(vp), sp.cast(vp)


	class GoroutineCmd(gdb.Command):
	"""Execute gdb command in the context of goroutine <goid>.

	Switch PC and SP to the ones in the goroutine's G structure,
	execute an arbitrary gdb command, and restore PC and SP.

	Usage: (gdb) goroutine <goid> <gdbcmd>

	You could pass "all" as <goid> to apply <gdbcmd> to all goroutines.

	For example: (gdb) goroutine all <gdbcmd>

	Note that it is ill-defined to modify state in the context of a goroutine.
	Restrict yourself to inspecting values.
	"""

	def __init__(self):
	gdb.Command.__init__(self, "goroutine", gdb.COMMAND_STACK, gdb.COMPLETE_NONE)

	def invoke(self, arg, _from_tty):
	goid_str, cmd = arg.split(None, 1)
	goids = []

	if goid_str == 'all':
	for ptr in SliceValue(gdb.parse_and_eval("'runtime.allgs'")):
	goids.append(int(ptr['goid']))
	else:
	goids = [int(gdb.parse_and_eval(goid_str))]

	for goid in goids:
	self.invoke_per_goid(goid, cmd)

	def invoke_per_goid(self, goid, cmd):
	pc, sp = find_goroutine(goid)
	if not pc:
	print("No such goroutine: ", goid)
	return
	pc = pc_to_int(pc)
	save_frame = gdb.selected_frame()
	gdb.parse_and_eval('$save_sp = $sp')
	gdb.parse_and_eval('$save_pc = $pc')
	# In GDB, assignments to sp must be done from the
	# top-most frame, so select frame 0 first.
	gdb.execute('select-frame 0')
	gdb.parse_and_eval('$sp = {0}'.format(str(sp)))
	gdb.parse_and_eval('$pc = {0}'.format(str(pc)))
	try:
	gdb.execute(cmd)
	finally:
	# In GDB, assignments to sp must be done from the
	# top-most frame, so select frame 0 first.
	gdb.execute('select-frame 0')
	gdb.parse_and_eval('$pc = $save_pc')
	gdb.parse_and_eval('$sp = $save_sp')
	save_frame.select()


	class GoIfaceCmd(gdb.Command):
	"Print Static and dynamic interface types"

	def __init__(self):
	gdb.Command.__init__(self, "iface", gdb.COMMAND_DATA, gdb.COMPLETE_SYMBOL)

	def invoke(self, arg, _from_tty):
	for obj in gdb.string_to_argv(arg):
	try:
	#TODO fix quoting for qualified variable names
	obj = gdb.parse_and_eval(str(obj))
	except Exception as e:
	print("Can't parse ", obj, ": ", e)
	continue

	if obj['data'] == 0:
	dtype = "nil"
	else:
	dtype = iface_dtype(obj)

	if dtype is None:
	print("Not an interface: ", obj.type)
	continue

	print("{0}: {1}".format(obj.type, dtype))

	# TODO: print interface's methods and dynamic type's func pointers thereof.
	#rsc: "to find the number of entries in the itab's Fn field look at
	# itab.inter->numMethods
	# i am sure i have the names wrong but look at the interface type
	# and its method count"
	# so Itype will start with a commontype which has kind = interface

	#
	# Register all convenience functions and CLI commands
	#
	GoLenFunc()
	GoCapFunc()
	DTypeFunc()
	GoroutinesCmd()
	GoroutineCmd()
	GoIfaceCmd()