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

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

This compiles SDB and its dependencies, and puts everything in the bin directory. This directory can be moved around freely; when executed, the sdb script will set up the environment so all the dependency assemblies are found.

You could, for example, just add the bin directory to your PATH:

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

(sdb)

You can run the SDB test suite with:

$ make check

It's generally a good idea to do that to ensure that SDB works correctly on your system before you start using it.

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.
• 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.
• PKG_CONFIG: Path to the pkg-config utility.
• SED: Path to the sed POSIX utility.
• TAR: Path to the tar POSIX utility.
• XBUILD: Which XBuild executable to use.
• XBUILD_FLAGS: Flags to pass to XBuild.

Note that the F# tools are only necessary to run the test suite. Gendarme is also optional and is mostly used by the SDB developers. tar is also only used to package SDB releases.

Additionally, MODE can be set to Debug (default) or Release to indicate the kind of build desired.

Finally, MONO_PREFIX can be set to tell the test runner which Mono executable should be used. See the description of RuntimePrefix further down for more information.

Usage

Running a program is simple:

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

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

        Foo(str);
    }

    static void Foo(string str)
    {
        Console.WriteLine(str);

        Bar();
    }

    static void Bar()
    {
        Debugger.Break();
    }
}
$ 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
Foo!
Inferior process '5234' ('test.exe') suspended
#0 [0x00000001] Program.Bar at /home/alexrp/Projects/tests/cs/test.cs:22
        Debugger.Break();

A stack trace can be generated with bt:

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

We can select a frame and inspect locals:

(sdb) f up
#1 [0x00000007] Program.Foo at /home/alexrp/Projects/tests/cs/test.cs:17
        Bar();
(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
(sdb)

We can then exit SDB:

(sdb) q
Bye

For more commands, consult help in SDB.

Options

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"

Or:

$ 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.

Settings

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.

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.

Paths

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 and type formatter 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.

Environment

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.

Plugins

At the moment, SDB has one extension point which is the Command class and the associated 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).

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

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

[Command]
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:`which 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

  mycmd

Some sort of detailed help text goes here.

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

Issues

• There is no completion for commands - the default completion instead tries to complete file names which is not very useful most of the time.
• Decompilation is not implemented. The ICSharpCode.Decompiler library needs to be separated from ILSpy for this to be practical.
• The exit code of inferior processes is not shown. There is apparently no way to get it from Mono.Debugging.
• Attach support is not implemented. This requires special support in the debugging libraries.
• Some Mono versions throw a NullReferenceException when SDB shuts down. This is because of a bug in the finalizer of System.Timers.Timer in Mono. This bug has been fixed and should be available in whatever Mono version comes after 3.2.5.