Using Export Clause in Libraries

The main source for a dynamically loadable library is identical to that of a program, except that it begins with the reserved word library (instead of program).

Only routines that a library explicitly exports are available for importing by other libraries or programs. The following example shows a library with two exported functions, Min and Max.

library MinMax;

function Min(X, Y: Integer): Integer; stdcall; begin if X < Y then Min := X else Min := Y; end;

function Max(X, Y: Integer): Integer; stdcall; begin if X > Y then Max := X else Max := Y; end;

exports Min, Max; begin end.

If you want your library to be available to applications written in other languages, it's safest to specify stdcall in the declarations of exported functions. Other languages may not support Delphi's default register calling convention.

Libraries can be built from multiple units. In this case, the library source file is frequently reduced to a uses clause, an exports clause, and the initialization code. For example, library Editors;

uses EdInit, EdInOut, EdFormat, EdPrint; exports

InitEditors, DoneEditors name Done, InsertText name Insert, DeleteSelection name Delete,

FormatSelection, PrintSelection name Print,

SetErrorHandler; begin

InitLibrary; end.

You can put exports clauses in the interface or implementation section of a unit. Any library that includes such a unit in its uses clause automatically exports the routines listed the unit's exports clauses without the need for an exports clause of its own.

The directive local, which marks routines as unavailable for export, is platform-specific and has no effect in Windows programming.

On Linux, the local directive provides a slight performance optimization for routines that are compiled into a library but are not exported. This directive can be specified for stand-alone procedures and functions, but not for methods. A routine declared with localfor example, function Contraband(I: Integer): Integer; local;

does not refresh the EBX register and hence

  • cannot be exported from a library.
  • cannot be declared in the interface section of a unit.
  • cannot have its address taken or be assigned to a procedural-type variable.
  • if it is a pure assembler routine, cannot be called from another unit unless the caller sets up EBX.

A routine is exported when it is listed in an exports clause, which has the form exports entryl, ..., entryn;

where each entry consists of the name of a procedure, function, or variable (which must be declared prior to the exports clause), followed by a parameter list (only if exporting a routine that is overloaded), and an optional name specifier. You can qualify the procedure or function name with the name of a unit.

(Entries can also include the directive resident, which is maintained for backward compatibility and is ignored by the compiler.)

On the Win32 platform, an index specifier consists of the directive index followed by a numeric constant between 1 and 2,147,483,647. (For more efficient programs, use low index values.) If an entry has no index specifier, the routine is automatically assigned a number in the export table.

Note: Use of index specifiers, which are supported for backward compatibility only, is discouraged and may cause problems for other development tools. A name specifier consists of the directive name followed by a string constant. If an entry has no name specifier, the routine is exported under its original declared name, with the same spelling and case. Use a name clause when you want to export a routine under a different name. For example, exports

DoSomethingABC name 'DoSomething';

When you export an overloaded function or procedure from a dynamically loadable library, you must specify its parameter list in the exports clause. For example, exports

Divide(X, Y: Integer) name 'Divide Ints', Divide(X, Y: Real) name 'Divide Reals';

On Win32, do not include index specifiers in entries for overloaded routines.

An exports clause can appear anywhere and any number of times in the declaration part of a program or library, or in the interface or implementation section of a unit. Programs seldom contain an exports clause.

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