On Tue, 4 Nov 2025 at 14:42, Tom Tromey <[email protected]> wrote:
>
> A user pointed out that, in DAP mode, gdb would hang while trying to
> display a certain vector. See
>
> https://sourceware.org/bugzilla/show_bug.cgi?id=33594
>
> This is caused by a combination of things: the vector is
> uninitialized, DAP requires a count of the number of children of a
> variable, and libstdc++ printers don't implement the 'num_children'
> method, so gdb tries to count children by iterating.
>
> In this case, the vector has a nonsensical size:
>
> (gdb) p myVector
> $1 = std::vector of length -34979931, capacity -33992726
>
> This patch adds a 'num_children' method to a subset of the
> pretty-printers, in particular ones where I thought the length might
> be arbitrarily large and susceptible to being garbage when the object
> isn't initialized.
>
> I've also specifically added a check to the vector printer for the
> case where the length is negative.
>
> These container printers could be further improved by adding the
> 'child' method, allowing random access to child objects. However I
> haven't done that here.
>
> libstc++-v3/ChangeLog
s/libstc++/libstdc++/
OK with that change (which would probably be caught by the receive hook anyway).
>
> * python/libstdcxx/v6/printers.py (StdVectorPrinter._bounds):
> New method.
> (StdVectorPrinter.to_string): Use it.
> (StdVectorPrinter.num_children): New method.
> (StdStackOrQueuePrinter.num_children): New method.
> (StdMapPrinter.num_children): New method.
> (StdSetPrinter.num_children): New method.
> (StdDequePrinter._size): New method.
> (StdDequePrinter.to_string): Use it.
> (StdDequePrinter.num_children): New method.
> (Tr1UnorderedSetPrinter.num_children): New method.
> (Tr1UnorderedMapPrinter.num_children): New method.
> (StdSpanPrinter.num_children): New method.
> ---
> libstdc++-v3/python/libstdcxx/v6/printers.py | 62 ++++++++++++++++----
> 1 file changed, 52 insertions(+), 10 deletions(-)
>
> diff --git a/libstdc++-v3/python/libstdcxx/v6/printers.py
> b/libstdc++-v3/python/libstdcxx/v6/printers.py
> index 7a5a627479f..f7f0489abf9 100644
> --- a/libstdc++-v3/python/libstdcxx/v6/printers.py
> +++ b/libstdc++-v3/python/libstdcxx/v6/printers.py
> @@ -572,7 +572,10 @@ class StdVectorPrinter(printer_base):
> self._val['_M_impl']['_M_finish'],
> self._is_bool)
>
> - def to_string(self):
> + # Helper to compute the bounds of the vector.
> + # Returns a tuple: (length, capacity, suffix)
> + # SUFFIX is a type-name suffix to print.
> + def _bounds(self):
> start = self._val['_M_impl']['_M_start']
> finish = self._val['_M_impl']['_M_finish']
> end = self._val['_M_impl']['_M_end_of_storage']
> @@ -582,13 +585,27 @@ class StdVectorPrinter(printer_base):
> fo = self._val['_M_impl']['_M_finish']['_M_offset']
> itype = start.dereference().type
> bl = 8 * itype.sizeof
> - length = bl * (finish - start) + fo
> - capacity = bl * (end - start)
> - return ('%s<bool> of length %d, capacity %d'
> - % (self._typename, int(length), int(capacity)))
> + length = int(bl * (finish - start) + fo)
> + capacity = int(bl * (end - start))
> + suffix = '<bool>'
> else:
> - return ('%s of length %d, capacity %d'
> - % (self._typename, int(finish - start), int(end -
> start)))
> + length = int(finish - start)
> + capacity = int(end - start)
> + suffix = ''
> + if length < 0:
> + # Probably uninitialized.
> + length = 0
> + capacity = 0
> + return (length, capacity, suffix)
> +
> + def to_string(self):
> + (length, capacity, suffix) = self._bounds()
> + return ('%s%s of length %d, capacity %d'
> + % (self._typename, suffix, length, capacity))
> +
> + def num_children(self):
> + (length, capacity, suffix) = self._bounds()
> + return length
>
> def display_hint(self):
> return 'array'
> @@ -733,6 +750,11 @@ class StdStackOrQueuePrinter(printer_base):
> return '%s wrapping: %s' % (self._typename,
> self._visualizer.to_string())
>
> + def num_children(self):
> + if hasattr(self._visualizer, 'num_children'):
> + return self._visualizer.num_children()
> + return None
> +
> def display_hint(self):
> if hasattr(self._visualizer, 'display_hint'):
> return self._visualizer.display_hint()
> @@ -876,6 +898,9 @@ class StdMapPrinter(printer_base):
> node = lookup_node_type('_Rb_tree_node', self._val.type).pointer()
> return self._iter(RbtreeIterator(self._val), node)
>
> + def num_children(slf):
> + return len(RbtreeIterator(self._val))
> +
> def display_hint(self):
> return 'map'
>
> @@ -915,6 +940,8 @@ class StdSetPrinter(printer_base):
> node = lookup_node_type('_Rb_tree_node', self._val.type).pointer()
> return self._iter(RbtreeIterator(self._val), node)
>
> + def num_children(slf):
> + return len(RbtreeIterator(self._val))
>
> class StdBitsetPrinter(printer_base):
> """Print a std::bitset."""
> @@ -1006,7 +1033,8 @@ class StdDequePrinter(printer_base):
> else:
> self._buffer_size = 1
>
> - def to_string(self):
> + # Helper to compute the size.
> + def _size(self):
> start = self._val['_M_impl']['_M_start']
> end = self._val['_M_impl']['_M_finish']
>
> @@ -1014,9 +1042,11 @@ class StdDequePrinter(printer_base):
> delta_s = start['_M_last'] - start['_M_cur']
> delta_e = end['_M_cur'] - end['_M_first']
>
> - size = self._buffer_size * delta_n + delta_s + delta_e
> + return long(self._buffer_size * delta_n + delta_s + delta_e)
>
> - return '%s with %s' % (self._typename, num_elements(long(size)))
> + def to_string(self):
> + size = self._size()
> + return '%s with %s' % (self._typename, num_elements(size))
>
> def children(self):
> start = self._val['_M_impl']['_M_start']
> @@ -1024,6 +1054,9 @@ class StdDequePrinter(printer_base):
> return self._iter(start['_M_node'], start['_M_cur'],
> start['_M_last'],
> end['_M_cur'], self._buffer_size)
>
> + def num_children(self):
> + return self._size()
> +
> def display_hint(self):
> return 'array'
>
> @@ -1210,6 +1243,9 @@ class Tr1UnorderedSetPrinter(printer_base):
> return izip(counter, Tr1HashtableIterator(self._hashtable()))
> return izip(counter, StdHashtableIterator(self._hashtable()))
>
> + def num_children(self):
> + return int(self._hashtable()['_M_element_count'])
> +
>
> class Tr1UnorderedMapPrinter(printer_base):
> """Print a std::unordered_map or tr1::unordered_map."""
> @@ -1254,6 +1290,9 @@ class Tr1UnorderedMapPrinter(printer_base):
> # Zip the two iterators together.
> return izip(counter, data)
>
> + def num_children(self):
> + return int(self._hashtable()['_M_element_count'])
> +
> def display_hint(self):
> return 'map'
>
> @@ -1949,6 +1988,9 @@ class StdSpanPrinter(printer_base):
> def children(self):
> return self._iterator(self._val['_M_ptr'], self._size)
>
> + def num_children(self):
> + return int(self._size)
> +
> def display_hint(self):
> return 'array'
>
> --
> 2.51.0
>
