Building a DLL

PureBasic allows to create standard Microsoft Windows DLL (Dynamic Linked Library), shared objects (.so) on Linux, and dynamic libraries (.dylib) on MacOS X. The DLL code is like a PureBasic code excepts than no real code should be written outside of procedure.

When writing a DLL, all the code is done inside procedures. When a procedure should be public (ie: accessible by third programs which will use the DLL), the keyword ProcedureDLL (or ProcedureCDLL if the procedure needs to be in 'CDecl' format, which is not the case of regular Windows DLL) is used instead of Procedure (and DeclareDLL or DeclareCDLL if are used instead of Declare). This is the only change to do to a program.

When this is done, select 'Shared DLL' as output format ('Compiler Option' window in the PureBasic editor or /DLL switch in command line) and a DLL with the name you set (in the save-requester when using the IDE) will be created in the selected directory.


  ProcedureDLL MyFunction()
    MessageRequester("Hello", "This is a PureBasic DLL !", 0)
  ; Now the client program, which use the DLL
  If OpenLibrary(0, "PureBasic.dll")
    CallFunction(0, "MyFunction")
For advanced programmers: there is 4 special procedures which are called automatically by the OS when one of the following events happen:

- DLL is attached to a new process
- DLL is detached from a process
- DLL is attached to a new thread
- DLL is detached from a thread

To handle that, it's possible to declare 4 special procedures called: AttachProcess(Instance), DetachProcess(Instance), AttachThread(Instance) and DetachThread(Instance). This means these 4 procedures names are reserved and can't be used by the programmer for other purposes.

Notes about creating DLL's:

- The declaration of arrays, lists or map with Dim, NewList or NewMap must always be done inside the procedure AttachProcess.
- Don't write program code outside procedures. The only exception is the declaration of variables or structures.
- DirectX initialization routines must not be written in the AttachProcess procedure.

Note about returning strings from DLL's:

If you want to return a string out of a DLL, the string has to be declared as Global before using it.


  Global ReturnString$
  ProcedureDLL.s MyFunction(var.s)
    ReturnString$ = var + " test"
    ProcedureReturn ReturnString$
Without declaring it as Global first, the string is local to the ProcedureDLL and can't be accessed from outside.

When using CallFunction() (or one of its similar CallXXX functions) on a DLL function you will get a pointer on the return string, which you could read with PeekS().


  String.s = PeekS(CallFunction(0, "FunctionName", Parameter1, Parameter2))
Here a complete code example: