Digging into the GAT API Comparing C, C++ and Python API‘s Hartmut Kaiser

October, 24th 2005Digging into the GAT API2 Design Principles –Object orientation Derivation, Interfaces, Genericity –Memory Management Object lifetime, Instance tracking, Allocation responsibilities –Const Correctness –Error Handling

October, 24th 2005Digging into the GAT API3 Object Orientation - C GAT Specification is object oriented What’s an ‘Object’ –Some data and a set of related functions Representation in C –Internal struct GATFile_S { /*…*/ }; –External typedef struct GATFile_S *GATFile; –Constructor/Destructor GATFile_Create(), GATFile_Destroy() –Naming convention GATResult GATFile_Copy(GATFile, /*…*/);

October, 24th 2005Digging into the GAT API4 Object Orientation – C++ Separate namespaces: GAT (GAT::Adaptors) Defines reference counted wrappers on top of the C API Natural representation –Create new GAT object GAT::File file(name);// constructor, destructor –Wrap existing C objects (handles) GAT::File file = GAT::MakeFile(c_file); Naming convention: GATResult GATFile_Copy(GATFile, /*…*/); GAT::Result GAT::File::Copy(/*…*/);

October, 24th 2005Digging into the GAT API5 Object Orientation – Python Separate namespaces (modules): GAT (GAT.Adaptors) Defines Python classes on top of the C API Representation is natural –Create new GAT object file = GAT.File(name)// constructor, destructor –Wrapping not necessary, done by the PyGAT runtime (which is written in C/C++) Naming convention: GATResult GATFile_Copy(GATFile, /*…*/); GAT.Result GAT.File.Copy(/*…*/)

October, 24th 2005Digging into the GAT API6 Interfaces - C Certain GAT object types have to implement different interfaces (GATObject, GATStreamable, GATMonitorable) What’s an interface –A set of related functions, which may be called even not knowing the type of the object Representation in C –Emulation of virtual functions. Every object has a table of function pointers, one table for each interface –GetInterface(): helper function to get at the different function pointer tables

October, 24th 2005Digging into the GAT API7 GATFile Memory Layout

October, 24th 2005Digging into the GAT API8 Interfaces – C++ Implemented through static polymorphism and multiple inheritance from different base class templates (GAT::Streamable<>, GAT::Serialisable<>, GAT::Monitorable<> etc.)

October, 24th 2005Digging into the GAT API9 Interfaces – Python All classes simply have interface functions available, no derivation needed (although internally used for the implementation) Integration of serialisation into the Python language (pickling).

October, 24th 2005Digging into the GAT API10 Derivation - C Every GAT object type has to be derived from the GATObject What’s ‘Derivation’ –Reuse of common functionality –Conversion from and to GATObject should be possible Representation in C –Every GAT object type has a common set of functions with an identical signature GATType GATFile_GetType(GATFile_const); GATResult GATFile_Destroy(GATFile *); GATResult GATFile_Clone(GATFile_const, GATFile *); GATResult GATFile_GetInterface(GATFile_const, void **); GATResult GATFile_Equals(GATFile_const, GATFile_const, GATBool *);

October, 24th 2005Digging into the GAT API11 Derivation - C Conversion from any GAT type to GATObject should be possible, since all these ‘derived’ from GATObject. Conversion from GATObject to the corresponding GAT type should be possible as well. Representation in C –For every GAT type the following functions exist: Succeeds always: GATObject GATFile_ToGATObject(GATFile); GATObject_constGATFile_ToGATObject_const(GATFile_const); Succeeds only, if type matches: GATFile GATObject_ToGATFile(GATObject); GATFile_const GATObject_ToGATFile_const(GATObject_const);

October, 24th 2005Digging into the GAT API12 Derivation – C++ Representation in C++ –Every GAT++ object type is derived from the GAT::Object template: GAT::Type GAT::Object ::GetType(Derived const); GAT::Object ::Object(Derived const); bool GAT::Object ::operator==(Derived const); –Conversion is handled automatically by derivation

October, 24th 2005Digging into the GAT API13 Derivation - Python Representaion in Python –Every GAT. has a similar set of functions implemented: GAT.File.GetType() GAT.File.Clone() GAT.File.__cmp__() –Typeless language, no conversion problems, handled by PyGAT wrapper runtime

October, 24th 2005Digging into the GAT API14 Genericity - C Possibility to call a function for an arbitrary GAT type not knowing the concrete type Representation in C –For every interface function exists Concrete function implementation for every GAT type, which realises this interface GATResult GATFile_Serialise(GATFile file, GATObject stream, GATBool cleardirty); Generic function allowing to call the type specific function GATResult GATSerialisable_Serialise(GATObject object, GATObject stream, GATBool cleardirty);

October, 24th 2005Digging into the GAT API15 Genericity – C++ Representation in C++ –Works based on the inheritance scheme

October, 24th 2005Digging into the GAT API16 Genericity – Python No special support needed for that (Python is typeless)

October, 24th 2005Digging into the GAT API17 Memory Management - C All GAT object types have a …_Create() function, which returns a new instance of this type. All GAT object types have a …_Destroy function, which frees all associated memory. You are responsible to call …_Destroy! –for all objects you’ve created GATFile file = GATFile_Create(location); … /* do something useful with ‘file’ */ GATFile_Destroy(&file); –for all non const objects you get back from the engine GATPipe pipe = NULL; GATEndpoint_Connect(endpoint, &pipe); … /* do something useful with ‘pipe’ */ GATPipe_Destroy(&pipe);

October, 24th 2005Digging into the GAT API18 Memory Management - C The GAT objects returned from the engine are handles! (well actually pointers, but …) typedef struct GATFile_S * GATFile; You’re free to copy around those ‘objects’ without performance harm. But watch out! Don’t free any of these objects while you’re holding copies of it, which you still want to use. Never free a GATObject with free(). If you are using casting functions (as GATObject_ToGATFile ) please note, that the result refers to the same object, so don’t free twice.

October, 24th 2005Digging into the GAT API19 Memory Management – C++ The GAT++ objects handle all memory management issues (reference counted) You’re free to copy around those objects without performance harm. GAT::File file1(...);// creates C ‚object‘ GAT::File file2 = file; Underlying C handle get‘s free‘d only after file1 and file2 are destructed No need for explicit destruction, language (compiler) keeps track of that

October, 24th 2005Digging into the GAT API20 Memory Management – Python The PyGAT objects handle all memory management issues (Python is reference counted/garbage collected) You’re free to copy around those objects without performance harm. file1 = GAT.File(...);// creates C ‚object‘ file2 = file; Underlying C handle get‘s free‘d only after file1 and file2 are out of scope and got garbage collected Runtime keeps track of not needed instances

October, 24th 2005Digging into the GAT API21 Const correctness - C Const correctness introduced wherever possible Helps to enforce semantics, especially for memory management –You’ll have to free by yourself all objects and memory blocks given back from the engine, which are not const –Objects and memory blocks which are const are controlled by the GAT engine, you don’t want to free these Representation in C –First temptation to have: GATFile and GATFile const but this doesn’t give, what we want: typedef struct GATFile_S * GATFile; so ‚GATFile‘ const would be ‚GATFile_S * const‘ --- wrong! –As a result we’ve got: typedef struct GATFile_S * GATFile; typedef struct GATFile_S const * GATFile_const;

October, 24th 2005Digging into the GAT API22 Const correctness – C++ Const correctness introduced wherever possible, allows compiler to catch errors early Natural representation in C++ GAT::File, GAT::File const

October, 24th 2005Digging into the GAT API23 Const correctness - Python Const correctness not an issue (not supported by the language)

October, 24th 2005Digging into the GAT API24 Error Handling Every method (except constructor, destructor and certain simple accessors) return a GATResult value –Is a structured 32 bit unsigned int: Every CPI based object has additional error tracking inside the associated GAT Context: –Allows to print an error trace back of the full error history GATContext_GetCurrentStatus(context, &status); GATStatus_ErrorTrace(status);

October, 24th 2005Digging into the GAT API25 Error Handling (Explicit) - C #include GATResult RemoteFile_GetFile (GATContext context, char const *source_url, char const *target_url) { GATResult rc = GAT_FAIL; GATStatus status = NULL; GATLocation source = GATLocation_Create (source_url); GATLocation target = GATLocation_Create (target_url); GATFile file = GATFile_Create (context, source, NULL); if (NULL == source || NULL == target || NULL == file) { GATCreateStatus(“RemoteFile_GetFile”, &status, GAT_MEMORYFAILURE, context, __FILE__, __LINE__); return GATContext_SetCurrentStatus (context, &status); } rc = GATFile_Copy(file, target, GATFileMode_Overwrite); if (GAT_FAILED(rc)) { GATCreateStatus(“RemoteFile_GetFile”, &status, rc, context, __FILE__, __LINE__); return GATContext_SetCurrentStatus (context, &status); } GATFile_Destroy (&file); GATLocation_Destroy (&target); GATLocation_Destroy (&source); return GAT_SUCCEEDED; }

October, 24th 2005Digging into the GAT API26 Error Handling (macros) - C #include GATResult RemoteFile_GetFile (GATContext context, char const *source_url, char const *target_url) { GAT_USES_STATUS(context, “RemoteFile_GetFile”); GATLocation source = GATLocation_Create (source_url); GATLocation target = GATLocation_Create (target_url); GATFile file = GATFile_Create (context, source, NULL); if (NULL == source || NULL == target || NULL == file) { GAT_CREATE_STATUS(GAT_MEMRORYFAILURE); } else { GAT_CREATE_STATUS(GATFile_Copy(file, target, GATFileMode_Overwrite)); } GATFile_Destroy (&file); GATLocation_Destroy (&target); GATLocation_Destroy (&source); return GAT_RETURN_STATUS(); }

October, 24th 2005Digging into the GAT API27 Error Handling – C++ #include GAT::Result RemoteFile_GetFile (GAT::Context context, char const *source_url, char const *target_url) { try { GAT::Location source (source_url); GAT::Location target (target_url); GAT::File file (context, source); file.Copy(target); } catch (GAT::Exception const &e) { std::cout << e.what() << std::endl; return e.GetResult(); } return GAT_SUCCEEDED; }

October, 24th 2005Digging into the GAT API28 Error Handling – Python import GAT def RemoteFile_GetFile (context, source_url, target_url): try: source = GAT.Location(source_url); target = GAT.Location(target_url); file = GAT.File(context, source); file.Copy(target); except GAT.Status, err: print err.args[0].message print err.args[0].traceback return err.args[0].errcode return GAT.SUCCEEDED;

October, 24th 2005Digging into the GAT API29 Demo File Transfer Application –Using different languages (C, C++, Python)