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From: "gustaf.waldemarson at gmail dot com"
 <sourceware-bugzilla@sourceware.org>
To: gdb-prs@sourceware.org
Subject: [Bug gdb/30018] New: Garbage output from not-yet-existing local
 variables
Date: Tue, 17 Jan 2023 19:32:54 +0000
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https://sourceware.org/bugzilla/show_bug.cgi?id=3D30018

            Bug ID: 30018
           Summary: Garbage output from not-yet-existing local variables
           Product: gdb
           Version: unknown
            Status: UNCONFIRMED
          Severity: normal
          Priority: P2
         Component: gdb
          Assignee: unassigned at sourceware dot org
          Reporter: gustaf.waldemarson at gmail dot com
  Target Milestone: ---

Hello,

This was originally reported here:

 https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D107965

As a bug on the libstdc++ pretty printer, but given the comments, it seems a
bit unclear to me whether this is a GDB bug, if it is a bug in the pretty
printer framework, or both, hence this ticket.

In brief, doing the following yields creates large amount of garbage output:

Given the following simple `gdbinit` that only initializes the pretty print=
ers
and enables Python stack-traces:

    python
    import os
    import re
    import sys
    import os.path
    import textwrap
    import gdb
    import gdb.types
    import gdb.printing

    home          =3D os.environ.get("HOME", "~")
    default_path  =3D os.path.join(home, "git", "installs")
    objects_dir   =3D os.environ.get("objects_dir", default_path)
    python_addons =3D os.path.join(objects_dir, "gcc", "libstdc++-v3", "pyt=
hon")

    if os.path.isdir(python_addons):
      print("Installing libstdcxx printers...")
      sys.path.insert(0, python_addons)
      from libstdcxx.v6.printers import register_libstdcxx_printers
      register_libstdcxx_printers(None)
    end

    set python print-stack full

and this simple C++ file:

    #include <vector>
    #include <string>
    #include <iostream>

    using namespace std;

    int main(void)
    {
        vector<string> test{"test", "test2"};
        string blabla =3D "hello";
        int b =3D 2;

        std::cout << blabla << " "
                  << b << " "
                  << test[0] << " " << test[1] << std::endl;

        return 0;
    }

Compile it and start debugging:

    g++ -g3 test.cpp
    gdb -q a.out
    (gdb) start
    (gdb) info locals

At `info locals`, the garbage output is create, along with a large number of
Python exceptions from the pretty printers. Following the Python
print-stack-trace reveals a possible error here:

      File
"/home/xaldew/git/installs/gcc/libstdc++-v3/python/libstdcxx/v6/printers.py=
",
line 971, in to_string
        return ptr.lazy_string (length =3D length)
    OverflowError: int too big to convert

After which it disappears into various parts of the framework(s).

A number of comments from the original thread mentions that at the time of
`info
locals`, none of the local variables should exist yet somehow they do and t=
hus
causes these problems.

Frankly, I don't know enough about these low level details, but the comments
from the other thread were compelling enough that it probably should be
discussed here.

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