1. Workshop on Grid Application Programming 19th-21st July 2004, Edinburgh, 1 Hartmut Kaiser The C Implementation of the GAT OO Specification Hartmut Kaiser hartmut.kaiser@aei.mpg.de MPI for Gravitational Physics, Albert-Einstein-Institut, Golm

2. Hartmut Kaiser Workshop on Grid Application Programming 19th-21st July 2004, Edinburgh, 2 Outline Design Principles Object orientation Derivation, Interfaces, Genericity Memory Management Object lifetime, Instance tracking, Allocation responsibilities Const Correctness Error Handling Portability Open Source Open problems

3. Hartmut Kaiser Workshop on Grid Application Programming 19th-21st July 2004, Edinburgh, 3 Object Orientation 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, /*…*/);

4. Hartmut Kaiser Workshop on Grid Application Programming 19th-21st July 2004, Edinburgh, 4 Derivation 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 *);

5. Hartmut Kaiser Workshop on Grid Application Programming 19th-21st July 2004, Edinburgh, 5 Derivation 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);

6. Hartmut Kaiser Workshop on Grid Application Programming 19th-21st July 2004, Edinburgh, 6 Interfaces Certain GAT object types have to implement different interfaces ( 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

7. Hartmut Kaiser Workshop on Grid Application Programming 19th-21st July 2004, Edinburgh, 7 GATFile Memory Layout

8. Hartmut Kaiser Workshop on Grid Application Programming 19th-21st July 2004, Edinburgh, 8 Genericity Possibility to call a function for an arbitrary GAT type not knowing the concrete type Representation in C For every interface function exist 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);

9. Hartmut Kaiser Workshop on Grid Application Programming 19th-21st July 2004, Edinburgh, 9 Memory Management 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_Destry(&pipe);

10. Hartmut Kaiser Workshop on Grid Application Programming 19th-21st July 2004, Edinburgh, 10 Memory Management 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.

11. Hartmut Kaiser Workshop on Grid Application Programming 19th-21st July 2004, Edinburgh, 11 Const correctness 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 As a result we’ve got: typedef struct GATFile_S * GATFile; typedef struct GATFile_S const * GATFile_const;

12. Hartmut Kaiser Workshop on Grid Application Programming 19th-21st July 2004, Edinburgh, 12 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 GATContext: Allows to print an error traceback of the full error history GATContext_GetCurrentStatus(context, &status); GATStatus_ErrorTrace(status);

13. Hartmut Kaiser Workshop on Grid Application Programming 19th-21st July 2004, Edinburgh, 13 Error Handling (Explicit) #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; }

14. Hartmut Kaiser Workshop on Grid Application Programming 19th-21st July 2004, Edinburgh, 14 Error Handling (macros) #include GATResult RemoteFile_GetFile (GATContext context, char const *source_url, char const *target_url) { GAT_USES_STATUS(“RemoteFile_GetFile”, context); 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(); }

15. Hartmut Kaiser Workshop on Grid Application Programming 19th-21st July 2004, Edinburgh, 15 Portability All you need is a C compiler and a POSIX conformant library. Dynamic (shared) library support is helpful GAT was compiled on a wide variety of systems Linux, IRIX etc. Windows (Cygwin and native) Macintosh …

16. Hartmut Kaiser Workshop on Grid Application Programming 19th-21st July 2004, Edinburgh, 16 Open Source Available at http://www.gridlab.org/wp-1 CVS access at cvs readonly@cvs.gridlab.org:/cvs/gridlab co wp-1

17. Hartmut Kaiser Workshop on Grid Application Programming 19th-21st July 2004, Edinburgh, 17 Open problems Memory management is very tedious User has to do a lot by himself Track all GAT object instance copies Free all GAT object instances Call constructor and destructor Error handling is complicated (even with macros) Always check for error codes, cluttered code. All of these problems are solved by the C++ wrapper Asynchronicity is (almost) completely missing Engine is not thread safe as of today