VorpalBlade / layout_of

GDB extension to display memory layout

GDB extension to display layout of a type

This is a pair of simple GDB commands to display layout of types. There are two commands in this repo:

• layout-of [-r] TYPE_NAME displays the memory layout of TYPE_NAME, optionally showing details recursively. It will show byte ranges for each member as well as any holes due to padding.
• offsets-of TYPE_NAME will just show the starting offset of each member.

Example

Below is an example of using the commands in Rust (they work for C and C++ as well, other languages are untested).

NOTES!

• For Rust, the command might not be accurate for types with niches, which mostly affects enums, but the commands should be correct for structs.
• For Rust, the total padding can be wrong with enums, as it can get double counted. Unions in C and C++ could have the same issue, but those are much less commonly used than enums in Rust.
• If the type of interest contains spaces put quotes around the argument

Please be aware that in the real terminal, colours will be used to enhance readability (not visible here in the README).

(gdb) offsets-of ini_merge::Property
ini_merge::Property {
    section => 0
    key => 16
    val => 48
    raw => 32
}

(gdb) layout-of ini_merge::Property
ini_merge::Property {
   section => 0 - 16
   key => 16 - 32
   raw => 32 - 48
   val => 48 - 64
}
Total size: 64

(gdb) layout-of -r ini_merge::Property
ini_merge::Property {
   section => 0 - 16 (&str) {
      data_ptr => 0 - 8
      length => 8 - 16
   }
   key => 16 - 32 (&str) {
      data_ptr => 0 - 8
      length => 8 - 16
   }
   raw => 32 - 48 (&str) {
      data_ptr => 0 - 8
      length => 8 - 16
   }
   val => 48 - 64 (core::option::Option<&str>) {
      None => 0 - 8
      None => 0 - 16 (core::option::Option<&str>::None) {

         --- Padding: 16 bytes ---   

      }
      Some => 0 - 16 (core::option::Option<&str>::Some) {
         __0 => 0 - 16 (&str) {
            data_ptr => 0 - 8
            length => 8 - 16
         }
      }
   }
}
Total padding size: 16
Total size: 64

(gdb) layout-of "ini_merge::actions::Actions<ini_merge::filter::FilterAction, ini_merge::filter::FilterAction>"
ini_merge::actions::Actions<ini_merge::filter::FilterAction, ini_merge::filter::FilterAction> {
   section_actions => 0 - 48
   literal_actions => 48 - 96
   regex_matches => 96 - 128
   regex_actions => 128 - 152
}
Total size: 152

Installation

First, you should install the ansicolors package for the Python version your GDB uses. For example:

# Install with pip:
pip install -U --user ansicolors
# Install using pacman on Arch Linux:
sudo pacman -S python-ansicolors

In your .gdbinit add:

python
import sys
sys.path.insert(0, '<absolute checkout path of this repo>')
import layout_of
end