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mojo debug

Launches the Mojo debugger using the command-line interface or an external editor.

Synopsis​

mojo debug [debug-options]

Description​

This command, which underneath uses the LLDB debugger, or cuda-gdb, offers four basic debug session modes:

  • Build and debug a Mojo file.

      mojo debug [options] <file.mojo> [runtime args]

    Builds the Mojo file at the given path and launches it under the debugger. Options, which come before the Mojo file, can include any compilation options expected by the mojo run, as well as regular debuggingcommands. Runtime args, which come after the Mojo file, are passed directly to the debuggee upon launch. By default, this mode uses -O0 and --debug-level=full as compilation options.

  • Debug a precompiled program.

      mojo debug [options] <program> [runtime args]

    Launches the program at the given path in the debugger. Options, which come before the program path, cannot include compilation commands. Runtime args, which come after the program path, are passed directly to the debuggee upon launch.

  • Attach to a running process.

      mojo debug [options] [--pid <pid> | --process-name <process-name>]

    Attaches to the process specified by pid or name, which can be the full path of the process' executable. Options other than the process identifier cannot include compilation options.

  • Start the debugger command-line interface.

      mojo debug [options]

    Launches the debugger CLI with support for debugging Mojo programs. This command only supports LLDB or cuda-gdb options via the --X option.

You can also select one of two interfaces for the debug session:

  • CLI: By default, all debug session modes are launched using the regular debugger command-line interface.

  • VS Code Debug Server: If you add the --vscode option, the debug session is launched in VS Code via the Mojo extension. VS Code must be running and the Mojo extension must be enabled. Besides that, the environment variables and the current working directory of this invocation are preserved when launching programs in the debugger on VS Code.

Finally, it is worth mentioning that this debugger can debug programs written in other standard native languages like Rust, C and C++, as it is based on LLDB or cuda-gdb.

Debugger capabilitis:

  • LLDB: this is the default debugger and has great support for CPU Mojo code, but has no support at all for Mojo GPU code.

  • cuda-gdb: this is invoked via the --cuda-gdb option and has minimal support for CPU Mojo code but it has support for GPU Mojo code.

Options​

Attach options​

--pid <PID>​

Indicates the debugger to attach to the process with the given PID.

--process-name <NAME>​

Indicates the debugger to attach to the process with the given name or path.

cuda-gdb options​

--cuda-gdb​

Uses cuda-gdb instead of LLDB for debugging. In this mode, it's possible to step into GPU code, but the CPU debugging experience is degraded.

--cuda-gdb-path <CUDA_GDB_PATH>​

Uses the given CUDA_GDB_PATH instead of looking for cuda-gdb in the PATH environment variable.

--break-on-launch​

Set the breakOnLaunch option for cuda-gdb. This makes the debugger break on the first instruction of every launched kernel.

Compilation options​

--optimization-level <LEVEL>, -O, --no-optimization (LEVEL=0)​

Sets the level of optimization to use at compilation. The value must be a number between 0 and 3. The default is 3.

-I <PATH>​

Appends the given path to the list of directories to search for imported Mojo files.

-D <KEY=VALUE>​

Defines a named value that can be used from within the Mojo source file being executed. For example, -D foo=42 defines a name foo that, when queried with the sys.param_env module from within the Mojo program, would yield the compile-time value 42.

Target options​

--target-triple <TRIPLE>​

Sets the compilation target triple. Defaults to the host target.

--target-cpu <CPU>​

Sets the compilation target CPU. Defaults to the host CPU.

--target-features <FEATURES>​

Sets the compilation target CPU features. Defaults to the host features.

--march <ARCHITECTURE>​

Sets the architecture for which to generate code.

--mcpu <CPU>​

Sets the CPU for which to generate code.

--mtune <TUNE>​

Sets the CPU for which to tune code.

Compilation diagnostic options​

Controls how the Mojo compiler outputs diagnostics related to compiling and running Mojo source code.

--diagnose-missing-doc-strings​

Emits diagnostics for missing or partial doc strings.

--validate-doc-strings​

Emits errors for invalid doc strings instead of warnings.

--max-notes-per-diagnostic <INTEGER>​

When the Mojo compiler emits diagnostics, it sometimes also prints notes with additional information. This option sets an upper threshold on the number of notes that can be printed with a diagnostic. If not specified, the default maximum is 10.

Debugger options​

--X <ARG>​

Passes ARG as an argument to the debugger when the debug session is launched using the debugger command-line interface. This option can be specified multiple times. It is ignored when using the RPC mode.

Debug server options​

--vscode​

Launches the debug session on VS Code via the Mojo extension.

--rpc​

Alias for --vscode.

--terminal <TERMINAL>​

The type of terminal to use when starting a launch debug session.

  • console (default): the debuggee will be launched in the default environment for the editor. If using VS Code, this will be the Debug Console.
  • dedicated: the debuggee will be launched in a dedicated terminal within the editor.

--port <PORT>​

Uses the given PORT to communicate with the RPC debug server. Defaults to trying all ports from 12355 to 12364 inclusive.

Experimental compilation options​

--debug-level <LEVEL>, -g (LEVEL=full)​

Sets the level of debug info to use at compilation. The value must be one of: none (the default value), line-tables, or full. Please note that there are issues when generating debug info for some Mojo programs that have yet to be addressed.

--sanitize <CHECK>​

Turns on runtime checks. The following values are supported: address (detects memory issues), and thread (detects multi-threading issues). Please note that these checks are not currently supported when executing Mojo programs.

--debug-info-language <LANGUAGE>​

Sets the language to emit as part of the debug info. The supported languages are: Mojo, and C. C is the default, and is useful to enable rudimentary debugging and binary introspection in tools that don't understand Mojo.

Common options​

--help, -h​

Displays help information.

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