Pretty Printers

Pretty printers allow debuggers to display a uint256 in human-readable form instead of showing the four raw words. The library contains pretty printers for LLDB, GDB, and Visual Studio in the extra/ folder.

LLDB

To use this, add the following line to your ~/.lldbinit file:

command script import /path/to/int256/extra/int256_printer_lldb.py

If this is successful, you should see the following message in your debugger upon startup:

"uint256 pretty printer loaded successfully"

A value is shown as the decimal number with digit grouping, followed by the full 64-digit hexadecimal form, and the four words remain available as children:

(lldb) frame variable x
(const boost::int256::uint256) x = 514,631,507,721,405,312,519,378,913,364,952,599,460,579,264,246,569,804,738,786,927,427,030,286,080 (0x0123456789abcdeffedcba9876543210112233445566778899aabbccddeeff00) {
  [0] = 11072869122414935808
  [1] = 1234605616436508552
  [2] = 18364758544493064720
  [3] = 81985529216486895
}
(lldb) frame variable small
(const boost::int256::uint256) small = 42 (0x000000000000000000000000000000000000000000000000000000000000002a) {
  [0] = 42
  [1] = 0
  [2] = 0
  [3] = 0
}

GDB

To load the pretty printer, add the following line to your .gdbinit:

source /path/to/int256/extra/int256_printer_gdb.py

or you can source it manually in GDB.

(gdb) source /path/to/int256/extra/int256_printer_gdb.py

The display format is the same as for LLDB: the grouped decimal value, the 64-digit hexadecimal value in parentheses, and words[0] through words[3] as children.

Visual Studio (NATVIS)

The extra/int256.natvis file provides visualization for the Visual Studio debugger. There are several ways to register it:

Per-Project

Add the .natvis file to your Visual Studio project. In Solution Explorer, right-click the project, select Add > Existing Item, and choose int256.natvis. Visual Studio will automatically use it when debugging that project.

Per-User (All Projects)

Copy int256.natvis to your per-user Visualizers directory:

%USERPROFILE%\Documents\Visual Studio 2022\Visualizers\

Replace 2022 with your Visual Studio version. All projects debugged with that installation will use the visualizer.

CMake Projects

Add the .natvis file as a source file in your CMakeLists.txt:

target_sources(my_target PRIVATE /path/to/int256/extra/int256.natvis)

Display Format

Values that fit in 64 bits are displayed in decimal. Larger values are displayed as a synthesized hexadecimal value, most significant word first, with every nibble present.

Value Display

42

42

2^64 + 1

0x0000000000000000000000000000000000000000000000010000000000000001

uint256_max

0xFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFF

The four words are also expandable, most significant first.

Full decimal display for values beyond 64 bits is not possible in plain NATVIS. The NATVIS expression evaluator does not support 256-bit arithmetic, so values that exceed the 64-bit range are shown in hexadecimal.

MSVC Debugger Add-In

For a full decimal display of any value, extra/ also carries a native debugger add-in: int256_printer_msvc.cpp builds a small DLL that formats a uint256 by calling the library’s own detail::mini_to_chars, and int256_printer_msvc.natvis points the debugger at it.

Build the DLL with the supplied batch file from a directory where the compiler can find the library headers:

cd extra
int256_printer_msvc.bat

Then register int256_printer_msvc.natvis the same way as int256.natvis, and keep int256_printer_msvc.dll next to it. The add-in exports two formatters, formatter_u256_dec and formatter_u256_hex, which the .natvis selects with the dec and hex display views, so x,view(hex) in a watch window switches base.

The add-in uses the LegacyAddin NATVIS mechanism, which is supported on x64 and x86 only. Use the plain int256.natvis on ARM64.
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