Creating DLLs in C++Builder is the same as in standard C++:

1. Choose
2. If you want the DLL to be multi-threaded, check the Multi-threaded option.
3. Click OK.

Exported functions in the code should be identified with the __declspec(dllexport) modifier as they must be in Embarcadero C or Microsoft Visual C .
For example, the following code is legal in C Builder and other Windows C compilers:
In the code above, the function changeValue is exported, and therefore made available to calling applications. The functions dblValue and halfValue are internal, and cannot be called from outside of the DLL.
Additional information on creating DLLs can be found in the Win32 MSDK Reference.

資料來源：Creating DLLs in C Builder