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| 1 | +# Debugger attributes |
| 2 | + |
| 3 | +The following [attributes] are used for enhancing the debugging experience when using third-party debuggers like GDB or WinDbg. |
| 4 | + |
| 5 | +## The `debugger_visualizer` attribute |
| 6 | + |
| 7 | +The *`debugger_visualizer` attribute* can be used to embed a debugger visualizer file into the debug information. |
| 8 | +This enables an improved debugger experience for displaying values in the debugger. |
| 9 | +It uses the [_MetaListNameValueStr_] syntax to specify its inputs, and must be specified as a crate attribute. |
| 10 | + |
| 11 | +### Using `debugger_visualizer` with Natvis |
| 12 | + |
| 13 | +Natvis is an XML-based framework for Microsoft debuggers (such as Visual Studio and WinDbg) that uses declarative rules to customize the display of types. |
| 14 | +For detailed information on the Natvis format, refer to Microsoft's [Natvis documentation]. |
| 15 | + |
| 16 | +This attribute only supports embedding Natvis files on `-windows-msvc` targets. |
| 17 | + |
| 18 | +The path to the Natvis file is specified with the `natvis_file` key, which is a path relative to the crate source file: |
| 19 | + |
| 20 | +<!-- ignore: requires external files, and msvc --> |
| 21 | +```rust ignore |
| 22 | +#![debugger_visualizer(natvis_file = "Rectangle.natvis")] |
| 23 | + |
| 24 | +struct FancyRect { |
| 25 | + x: f32, |
| 26 | + y: f32, |
| 27 | + dx: f32, |
| 28 | + dy: f32, |
| 29 | +} |
| 30 | + |
| 31 | +fn main() { |
| 32 | + let fancy_rect = FancyRect { x: 10.0, y: 10.0, dx: 5.0, dy: 5.0 }; |
| 33 | + println!("set breakpoint here"); |
| 34 | +} |
| 35 | +``` |
| 36 | + |
| 37 | +and `Rectangle.natvis` contains: |
| 38 | + |
| 39 | +```xml |
| 40 | +<?xml version="1.0" encoding="utf-8"?> |
| 41 | +<AutoVisualizer xmlns="http://schemas.microsoft.com/vstudio/debugger/natvis/2010"> |
| 42 | + <Type Name="foo::FancyRect"> |
| 43 | + <DisplayString>({x},{y}) + ({dx}, {dy})</DisplayString> |
| 44 | + <Expand> |
| 45 | + <Synthetic Name="LowerLeft"> |
| 46 | + <DisplayString>({x}, {y})</DisplayString> |
| 47 | + </Synthetic> |
| 48 | + <Synthetic Name="UpperLeft"> |
| 49 | + <DisplayString>({x}, {y + dy})</DisplayString> |
| 50 | + </Synthetic> |
| 51 | + <Synthetic Name="UpperRight"> |
| 52 | + <DisplayString>({x + dx}, {y + dy})</DisplayString> |
| 53 | + </Synthetic> |
| 54 | + <Synthetic Name="LowerRight"> |
| 55 | + <DisplayString>({x + dx}, {y})</DisplayString> |
| 56 | + </Synthetic> |
| 57 | + </Expand> |
| 58 | + </Type> |
| 59 | +</AutoVisualizer> |
| 60 | +``` |
| 61 | + |
| 62 | +When viewed under WinDbg, the `fancy_rect` variable would be shown as follows: |
| 63 | + |
| 64 | +```text |
| 65 | +> Variables: |
| 66 | + > fancy_rect: (10.0, 10.0) + (5.0, 5.0) |
| 67 | + > LowerLeft: (10.0, 10.0) |
| 68 | + > UpperLeft: (10.0, 15.0) |
| 69 | + > UpperRight: (15.0, 15.0) |
| 70 | + > LowerRight: (15.0, 10.0) |
| 71 | +``` |
| 72 | + |
| 73 | +### Using `debugger_visualizer` with GDB |
| 74 | + |
| 75 | +GDB supports the use of a structured Python script, called a *pretty printer*, that describes how a type should be visualized in the debugger view. |
| 76 | +For detailed information on pretty printers, refer to GDB's [pretty printing documentation]. |
| 77 | + |
| 78 | +Embedded pretty printers are not automatically loaded when debugging a binary under GDB. |
| 79 | +There are two ways to enable auto-loading embedded pretty printers: |
| 80 | +1. Launch GDB with extra arguments to explicitly add a directory or binary to the auto-load safe path: `gdb -iex "add-auto-load-safe-path safe-path path/to/binary" path/to/binary` |
| 81 | + For more information, see GDB's [auto-loading documentation]. |
| 82 | +1. Create a file named `gdbinit` under `$HOME/.config/gdb` (you may need to create the directory if it doesn't already exist). Add the following line to that file: `add-auto-load-safe-path path/to/binary`. |
| 83 | + |
| 84 | +These scripts are embedded using the `gdb_script_file` key, which is a path relative to the crate source file. |
| 85 | + |
| 86 | +<!-- ignore: requires external files --> |
| 87 | +```rust ignore |
| 88 | +#![debugger_visualizer(gdb_script_file = "printer.py")] |
| 89 | + |
| 90 | +struct Person { |
| 91 | + name: String, |
| 92 | + age: i32, |
| 93 | +} |
| 94 | + |
| 95 | +fn main() { |
| 96 | + let bob = Person { name: String::from("Bob"), age: 10 }; |
| 97 | + println!("set breakpoint here"); |
| 98 | +} |
| 99 | +``` |
| 100 | + |
| 101 | +and `printer.py` contains: |
| 102 | + |
| 103 | +```python |
| 104 | +import gdb |
| 105 | + |
| 106 | +class PersonPrinter: |
| 107 | + "Print a Person" |
| 108 | + |
| 109 | + def __init__(self, val): |
| 110 | + self.val = val |
| 111 | + self.name = val["name"] |
| 112 | + self.age = int(val["age"]) |
| 113 | + |
| 114 | + def to_string(self): |
| 115 | + return "{} is {} years old.".format(self.name, self.age) |
| 116 | + |
| 117 | +def lookup(val): |
| 118 | + lookup_tag = val.type.tag |
| 119 | + if lookup_tag is None: |
| 120 | + return None |
| 121 | + if "foo::Person" == lookup_tag: |
| 122 | + return PersonPrinter(val) |
| 123 | + |
| 124 | + return None |
| 125 | + |
| 126 | +gdb.current_objfile().pretty_printers.append(lookup) |
| 127 | +``` |
| 128 | + |
| 129 | +When the crate's debug executable is passed into GDB[^rust-gdb], `print bob` will display: |
| 130 | + |
| 131 | +```text |
| 132 | +"Bob" is 10 years old. |
| 133 | +``` |
| 134 | + |
| 135 | +[^rust-gdb]: Note: This assumes you are using the `rust-gdb` script which configures pretty-printers for standard library types like `String`. |
| 136 | + |
| 137 | +[auto-loading documentation]: https://sourceware.org/gdb/onlinedocs/gdb/Auto_002dloading-safe-path.html |
| 138 | +[attributes]: ../attributes.md |
| 139 | +[Natvis documentation]: https://docs.microsoft.com/en-us/visualstudio/debugger/create-custom-views-of-native-objects |
| 140 | +[pretty printing documentation]: https://sourceware.org/gdb/onlinedocs/gdb/Pretty-Printing.html |
| 141 | +[_MetaListNameValueStr_]: ../attributes.md#meta-item-attribute-syntax |
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