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Debugging scripts

AngelScript offers a rich interface to support the debugging of scripts. It is easy to build an embedded debugger that can set break points, inspect/manipulate variables in functions, visualize the call stack, etc.

Observe that the CDebugMgr class used in the examples below doesn't exist. It is only used as an abstraction to avoid having to write fictional debug routines.

See also
  Debugger for a standard implementation

# Setting line breaks

In order to break at a specified line in the code the debugger can set the line callback function in the script context. The VM will then invoke the callback for each statement executed, allowing the debugger to decide whether to proceed to the next statement or not.

// An example line callback
void DebugLineCallback(asIScriptContext *ctx, CDebugMgr *dbg)
{
// Determine if we have reached a break point
const char *scriptSection;
int line = ctx->GetLineNumber(0, 0, &scriptSection);
asIScriptFunction *function = ctx->GetFunction();
// Now let the debugger check if a breakpoint has been set here
if( dbg->IsBreakpoint(scriptSection, line, function) )
{
// A break point has been reached so the execution of the script should be suspended
ctx->Suspend();
}
}

The line callback is set on the context with the following call:

// Set the line callback with the address of the debug manager as parameter
ctx->SetLineCallback(asFUNCTION(DebugLineCallback), dbg, asCALL_CDECL);

When the line callback suspends the execution the context's Execute function will return with the code asEXECUTION_SUSPENDED. The application can then go into a special message loop where the debug routines can be handled, e.g. to view the call stack, examine variables, etc. Once the execution should continue, simply call the Execute method again to resume it.

An alternative to suspending the script execution might be to start the message loop directly within the line callback, in which case resuming the execution is done simply by returning from the line callback function. Which is the easiest to implement depends on how you have implemented your application.

# Viewing the call stack

The asIScriptContext exposes the call stack for viewing purposes, so that you can easily track the origin of calls. It is also possible to print the value of variables at each level in the callstack.

Here's an example of how the entire call stack can be printed:

void PrintCallstack(asIScriptContext *ctx)
{
// Show the call stack
for( asUINT n = 0; n < ctx->GetCallstackSize(); n++ )
{
asIScriptFunction *func;
const char *scriptSection;
int line, column;
func = ctx->GetFunction(n);
line = ctx->GetLineNumber(n, &column, &scriptSection);
printf("%s:%s:%d,%d\n", scriptSection,
func->GetDeclaration(),
line, column);
}
}

# Inspecting variables

Through the context interface it is possible to inspect and even modify the value of the local variables on the stack. This can be done for each level in the call stack, and not just the current function that is being executed.

Here is an example for how the variables may be printed:

void PrintVariables(asIScriptContext *ctx, asUINT stackLevel)
{
asIScriptEngine *engine = ctx->GetEngine();
// First print the this pointer if this is a class method
int typeId = ctx->GetThisTypeId(stackLevel);
void *varPointer = ctx->GetThisPointer(stackLevel);
if( typeId )
{
printf(" this = 0x%x\n", varPointer);
}
// Print the value of each variable, including parameters
int numVars = ctx->GetVarCount(stackLevel);
for( int n = 0; n < numVars; n++ )
{
int typeId = ctx->GetVarTypeId(n, stackLevel);
void *varPointer = ctx->GetAddressOfVar(n, stackLevel);
if( typeId == asTYPEID_INT32 )
{
printf(" %s = %d\n", ctx->GetVarDeclaration(n, stackLevel), *(int*)varPointer);
}
else if( typeId == asTYPEID_FLOAT )
{
printf(" %s = %f\n", ctx->GetVarDeclaration(n, stackLevel), *(float*)varPointer);
}
else if( typeId & asTYPEID_SCRIPTOBJECT )
{
asIScriptObject *obj = (asIScriptObject*)varPointer;
if( obj )
printf(" %s = {...}\n", ctx->GetVarDeclaration(n, stackLevel));
else
printf(" %s = <null>\n", ctx->GetVarDeclaration(n, stackLevel));
}
else if( typeId == engine->GetTypeIdByDecl("string") )
{
string *str = (string*)varPointer;
if( str )
printf(" %s = '%s'\n", ctx->GetVarDeclaration(n, stackLevel), str->c_str());
else
printf(" %s = <null>\n", ctx->GetVarDeclaration(n, stackLevel));
}
else
{
printf(" %s = {...}\n", ctx->GetVarDeclaration(n, stackLevel));
}
}
}

The above code is only an example to give an idea of how it can be done. It is not complete and only recognizes a few types. To make it useful it would have to be expanded to recognize all types, and perhaps add some generic way of converting an object to human readable string for printing.

For script objects that conversion can be done by enumerating the members of an object through the asIScriptObject interface.

The debugger may also need to be able to inspect the global variables that the functions access. As the global variables are stored in the module, there is the place to look for them. The asIScriptModule interface can be obtained by querying the module name from the function, and then getting the module pointer from the engine. Once the module is determined the global variables are enumerated much the same way as in the example above, except that the appropriate methods on the asIScriptModule interface are used instead.

# Debugging internally executed scripts

Some script execution is not initiated by the application, e.g. the initialization of global variables or the call to the script class destructor when destroying objects from the garbage collector. If these executions should be debugged, the application must set the context callback functions with a call to asIScriptEngine::SetContextCallbacks. The engine will invoke these callbacks to request a context from the application when it will execute a script internally. The application can then
attach the debugger to the context it provides to the engine.

asTYPEID_SCRIPTOBJECT
The bit that shows if the type is a script class.
Definition: angelscript.h:494
asIScriptContext
The interface to the virtual machine.
Definition: angelscript.h:2654
asIScriptContext::GetFunction
virtual asIScriptFunction * GetFunction(asUINT stackLevel=0)=0
Returns the function at the specified callstack level.
asTYPEID_FLOAT
The type id for float.
Definition: angelscript.h:482
asIScriptContext::GetAddressOfVar
virtual void * GetAddressOfVar(asUINT varIndex, asUINT stackLevel=0)=0
Returns a pointer to a local variable at the specified callstack level.
asIScriptContext::GetLineNumber
virtual int GetLineNumber(asUINT stackLevel=0, int *column=0, const char **sectionName=0)=0
Returns the line number at the specified callstack level.
asIScriptContext::GetVarCount
virtual int GetVarCount(asUINT stackLevel=0)=0
Returns the number of local variables at the specified callstack level.
asTYPEID_INT32
The type id for int.
Definition: angelscript.h:470
asIScriptEngine
The engine interface.
Definition: angelscript.h:1060
asIScriptEngine::GetTypeIdByDecl
virtual int GetTypeIdByDecl(const char *decl) const =0
Returns a type id by declaration.
asIScriptContext::GetEngine
virtual asIScriptEngine * GetEngine() const =0
Returns a pointer to the engine.
asIScriptFunction::GetDeclaration
virtual const char * GetDeclaration(bool includeObjectName=true, bool includeNamespace=false, bool includeParamNames=false) const =0
Returns the function declaration.
asIScriptContext::GetCallstackSize
virtual asUINT GetCallstackSize() const =0
Returns the size of the callstack, i.e. the number of functions that have yet to complete.
asIScriptContext::GetVarDeclaration
virtual const char * GetVarDeclaration(asUINT varIndex, asUINT stackLevel=0, bool includeNamespace=false)=0
Returns the declaration of a local variable at the specified callstack level.
asFUNCTION
#define asFUNCTION(f)
Returns an asSFuncPtr representing the function specified by the name.
Definition: angelscript.h:673
asIScriptContext::SetLineCallback
virtual int SetLineCallback(asSFuncPtr callback, void *obj, int callConv)=0
Sets a line callback function. The function will be called for each executed script statement.
asIScriptContext::Suspend
virtual int Suspend()=0
Suspends the execution, which can then be resumed by calling Execute again.
asUINT
unsigned int asUINT
32 bit unsigned integer
Definition: angelscript.h:598
asIScriptFunction
The interface for a script function description.
Definition: angelscript.h:3757
asIScriptObject
The interface for an instance of a script object.
Definition: angelscript.h:3348
asIScriptContext::GetThisTypeId
virtual int GetThisTypeId(asUINT stackLevel=0)=0
Returns the type id of the object, if a class method is being executed.
asCALL_CDECL
A cdecl function.
Definition: angelscript.h:226
asIScriptContext::GetThisPointer
virtual void * GetThisPointer(asUINT stackLevel=0)=0
Returns a pointer to the object, if a class method is being executed.
asIScriptContext::GetVarTypeId
virtual int GetVarTypeId(asUINT varIndex, asUINT stackLevel=0)=0
Returns the type id of a local variable at the specified callstack level.