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SDB: Mono Soft Debugger Client

SDB is a command line client for Mono's soft debugger, a cooperative debugger that is part of the Mono VM. It tries to be similar in command syntax to tools such as GDB and LLDB.


Building and using SDB requires a basic POSIX-like environment, a Bash-like shell, the libedit library (or an API/ABI-compatible replacement), and an installed Mono framework.

First, clone the submodules:

$ git submodule update --init --recursive

To build, run:

$ make

If the build succeeds, you can install SDB with:

$ make install

(You may need to invoke it with sudo or some such command.)

You can also run SDB from within the build directory by appending it to PATH and invoking sdb-dev. This is mainly intended for development of SDB itself. An example:

$ export PATH=`pwd`/bin:$PATH
$ sdb-dev
Welcome to the Mono soft debugger (sdb 1.0.5058.39468)
Type 'help' for a list of commands or 'quit' to exit


You can run the SDB test suite with:

$ make check

(This requires an F# installation.)

The following variables can be set in your environment or on the make command line to affect the build:

  • CAT: Path to the cat POSIX utility.
  • CD: Path to the cd POSIX utility.
  • CHMOD: Path to the chmod POSIX utility.
  • CP: Path to the cp POSIX utility.
  • ECHO: Path to the echo POSIX utility.
  • FSHARPC: Which F# compiler executable to use.
  • FSHARPC_FLAGS: Flags to pass to the F# compiler.
  • FSHARPC_TEST_FLAGS: Flags to pass to the F# compiler for tests.
  • GENDARME: Which Gendarme executable to use (optional).
  • GENDARME_FLAGS: Flags to pass to Gendarme.
  • INSTALL: Path to the install POSIX utility.
  • MCS: Which C# compiler executable to use.
  • MCS_FLAGS: Flags to pass to the C# compiler.
  • MCS_TEST_FLAGS: Flags to pass to the C# compiler for tests.
  • MKDIR: Path to the mkdir POSIX utility.
  • NUGET: Which NuGet executable to use.
  • PKG_CONFIG: Path to the pkg-config utility.
  • RM: Path to the rm POSIX utility.
  • SED: Path to the sed POSIX utility.
  • TAR: Path to the tar POSIX utility.
  • MSBUILD: Which MSBuild executable to use.
  • MSBUILD_FLAGS: Flags to pass to MSBuild.

Additionally, MODE can be set to Debug (default) or Release to indicate the kind of build desired. PREFIX can be set to specify the path that the install and uninstall targets should operate within (defaults to /usr/local).

Finally, MONO_PREFIX and MONO_BINARY can be set to tell the check target which Mono executable should be used. See the description of RuntimePrefix and RuntimeExecutable further down for more information.


Running a program is simple:

$ cat test.cs
using System;
using System.Diagnostics;

static class Program
    static void Main()
        var str = "Foo!";


    static void Foo(string str)


    static void Bar()
$ mcs -debug test.cs
$ sdb
Welcome to the Mono soft debugger (sdb 1.0.5060.15368)
Type 'help' for a list of commands or 'quit' to exit

(sdb) r test.exe
Inferior process '5234' ('test.exe') started
Inferior process '5234' ('test.exe') suspended
#0 [0x00000001] Program.Bar at /home/alexrp/Projects/tests/cs/test.cs:22

A stack trace can be generated with bt:

(sdb) bt
#0 [0x00000001] Program.Bar at /home/alexrp/Projects/tests/cs/test.cs:22
#1 [0x00000007] Program.Foo at /home/alexrp/Projects/tests/cs/test.cs:17
#2 [0x00000008] Program.Main at /home/alexrp/Projects/tests/cs/test.cs:10

We can select a frame and inspect locals:

(sdb) f up
#1 [0x00000007] Program.Foo at /home/alexrp/Projects/tests/cs/test.cs:17
(sdb) p str
string it = "Foo!"

Or globals:

(sdb) p Environment.CommandLine
string it = "/home/alexrp/Projects/tests/cs/test.exe"

To continue execution, do:

(sdb) c
Inferior process '5234' ('test.exe') resumed
Inferior process '5234' ('test.exe') exited

We can then exit SDB:

(sdb) q

For more commands, consult help in SDB.


SDB has a few command line options that are useful for automation. For the full list, issue sdb --help.

First of all, all non-option arguments passed to SDB are treated as commands that SDB will execute at startup. For instance:

$ sdb "run test.exe"


$ sdb "args --foo --bar baz" "run test.exe"

This starts SDB and immediately executes test.exe with the given arguments.

The first option is -f. This option specifies files that SDB should read commands from. These commands are executed before any commands specified as non-option arguments. This option is useful for longer command sequences that are easier to maintain in a separate file. Example:

$ cat cmds.txt
args --foo --bar baz
run test.exe
$ sdb -f cmds.txt

The second option is -b. This runs SDB in batch mode; that is, it will exit as soon as all commands have finished and no inferior process is running. This goes well with -f for running programs regularly under SDB.


One configuration element that you almost certainly need to alter is the RuntimePrefix string value. It is set to /usr by default, regardless of OS, which is probably not desirable everywhere. For example, on Windows, you will want to set it to something like C:\Program Files (x86)\Mono-3.0.10. Or if you have Mono in some other directory, you might set it to e.g. /opt/mono.

The RuntimeExecutable element is another way to specify which Mono executable to use. If RuntimePrefix is empty, then RuntimeExecutable is taken as the full path to the Mono executable. If both elements are non-empty, they are combined as Path.Combine(RuntimePrefix, "bin", RuntimeExecutable). This configuration element is useful for switching between mono32 and mono64 on OS X in particular.

You may want to set DisableColors to true if you don't want the fancy ANSI color codes that SDB emits.

Finally, three useful settings for debugging SDB itself exist: DebugLogging can be set to true to make SDB spew a bunch of diagnostic information. LogInternalErrors can be set to true to log any internal errors that are encountered in the Mono debugging libraries. LogRuntimeSpew can be set to true to log all messages from the Mono VM.


When configuration elements are changed with config set, SDB will store the configuration data in ~/.sdb.cfg. The content of the file is the .NET binary serialization of the Mono.Debugger.Client.Configuration class. This file is read on startup if it exists.

At startup, SDB will scan the ~/.sdb directory for plugin assemblies. It will attempt to load all command definitions.

Finally, SDB will read ~/.sdb.rc and execute any commands (one per line) from it. This is useful if you prefer to change your settings with commands that you write down manually, rather than storing the data in a binary file.


The SDB_COLORS variable can be set to disable to tell SDB to not use colors in output. Normally, SDB will not use colors if it detects that stdout has been redirected, that TERM is set to dumb (or not set at all), or if the DisableColors configuration element is true.

SDB_CFG can be set to a specific configuration file to use instead of the default ~/.sdb.cfg. If set to the empty string (i.e. SDB_CFG="" sdb), SDB will not load any configuration file at all, and changed configuration values will not be saved.

The SDB_PATH variable can be set to a list of additional directories that SDB will scan for plugin assemblies in. Each directory should be separated by a semicolon (Windows) or a colon (POSIX).

SDB_DEBUG can be set to enable to make SDB print diagnostic information while debugging. This may be useful to debug SDB itself.


At the moment, SDB has one extension point which is the Mono.Debugger.Client.Command class and the related Mono.Debugger.Client.CommandAttribute class. A class implementing Command that is tagged with CommandAttribute will be instantiated at startup time and put into the root command list.

For SDB to find custom commands, they should be compiled into .dll assemblies and put in ~/.sdb (or some other directory specified in SDB_PATH). Plugin assemblies can also be loaded manually with the plugin command.

Here's an example of compiling and using a test plugin:

$ cat test.cs
using Mono.Debugger.Client;

public sealed class MyCommand : Command
    public override string[] Names
        get { return new[] { "mycmd" }; }

    public override string Summary
        get { return "Performs magic."; }

    public override string Syntax
        get { return "mycmd"; }

    public override string Help
        get { return "Some sort of detailed help text goes here."; }

    public override void Process(string args)
        Log.Info("Hello! I received: {0}", args);
$ mcs -debug -t:library test.cs -r:$(dirname $(which sdb))/../lib/sdb/sdb.exe -out:$HOME/.sdb/test.dll
$ sdb
Welcome to the Mono soft debugger (sdb 1.0.5061.14716)
Type 'help' for a list of commands or 'quit' to exit

(sdb) h mycmd


Some sort of detailed help text goes here.

(sdb) mycmd foo bar baz
Hello! I received: foo bar baz

You can look at SDB's own command classes for some examples of things that you can do in your commands.

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