Difference between revisions of "Dynamically loading headers"

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Line 84: Line 84:
 
someheader.pas
 
someheader.pas
 
   unit someheader;
 
   unit someheader;
 
+
 
 
   {$i someheader.inc}
 
   {$i someheader.inc}
 
+
 
 
   end.
 
   end.
  
 
someheaderdyn.pas
 
someheaderdyn.pas
 
   unit someheader;
 
   unit someheader;
 
+
 
 
   {$DEFINE LOAD_DYNAMICALLY}
 
   {$DEFINE LOAD_DYNAMICALLY}
 
   {$i someheader.inc}
 
   {$i someheader.inc}
 
+
 
 
   end.
 
   end.
  

Revision as of 00:07, 1 November 2009

Template:points

Abstract

Currently, most library headers in fpc/packages support only static linking. Some exceptions are mysql, ibase, sqlite (and some other database headers). The idea is now to add dynamic linking support for other headers. This page should provide some place to list pro/contra, implementation details, etc...

Since dynamic linking is not supported by the compiler directly, it's required to define some framework, so that all headers are "implemented" in an uniform way and can be used easily.

Pro

  • loading libraries at runtime (plugin support)

Contra

Implementation (suggestions)

Framework

dynlibs.pas

 type
   PLibHandler = ^TLibHandler;
   
   TLibEventLoading = function(User: Pointer; Handler: PLibHandler): Boolean;
   TLibEventUnloading = procedure(User: Pointer; Handler: PLibHandler);
   
   PPLibSymbol = ^PLibSymbol;
   PLibSymbol = ^TLibSymbol;
   TLibSymbol = record
     pvar: PPointer;  { pointer to Symbol variable }
     name: String;    { name of the Symbol }
     weak: Boolean;   { weak }
   end;
   
   TLibHandler = record
     InterfaceName: String;                { abstract name of the library }
     Defaults     : array of String;       { list of default library filenames }
     Filename     : String;                { handle of the current loaded library }
     Handle       : TLibHandle;            { filename of the current loaded library }
     Loading      : TLibEventLoading;      { loading event, called after the unit is loaded }
     Unloading    : TLibEventUnloading;    { unloading event, called before the unit is unloaded }
     SymCount     : Integer;               { number of symbols }
     Symbols      : PLibSymbol;            { symbol address- and namelist }
     ErrorMsg     : String;                { last error message }
     RefCount     : Integer;               { reference counter }
   end;
   
   
 { handler definition }
 function LibraryHandler(const InterfaceName: String; const DefaultLibraries: array of String;
   const Symbols: PLibSymbol; const SymCount: Integer; const AfterLoading: TLibEventLoading = nil;
   const BeforeUnloading: TLibEventUnloading = nil): TLibHandler;
 
 { initialization/finalization }
 function TryInitializeLibrary(var Handler: TLibHandler; const LibraryNames: array of String;
   const User: Pointer = nil; const NoSymbolErrors: Boolean = False): Integer;
 function TryInitializeLibrary(var Handler: TLibHandler; const LibraryName: String = ;
   const User: Pointer = nil; const NoSymbolErrors: Boolean = False): Integer;
 function InitializeLibrary(var Handler: TLibHandler; const LibraryNames: array of String;
   const User: Pointer = nil; const NoSymbolErrors: Boolean = False): Integer;
 function InitializeLibrary(var Handler: TLibHandler; const LibraryName: String = ;
   const User: Pointer = nil; const NoSymbolErrors: Boolean = False): Integer;
 function ReleaseLibrary(var Handler: TLibHandler; User: Pointer = nil): Integer;
 
 { errors }
 procedure AppendLibraryError(var Handler: TLibHandler; const Msg: String);
 function GetLastLibraryError(var Handler: TLibHandler): String;
 procedure RaiseLibraryException(var Handler: TLibHandler);
 
 { symbol load/clear }
 function LoadLibrarySymbols(const Lib: TLibHandle; const Symbols: PLibSymbol; const Count: Integer;
   const ErrorSym: PPLibSymbol = nil): Boolean;
 procedure ClearLibrarySymbols(const Symbols: PLibSymbol; const Count: Integer);

Naming conventions

For the dynamic headers the suffix "dyn" is added

  • static: someheader.pas
  • dynamic: someheaderdyn.pas

Header implementation

someheader.pas

 unit someheader;
 
 {$i someheader.inc}
 
 end.

someheaderdyn.pas

 unit someheader;
 
 {$DEFINE LOAD_DYNAMICALLY}
 {$i someheader.inc}
 
 end.

someheaderdyn.inc

Example

Others