# Calling DLL functions

You can import and call external DLL functions by adding special directives to the declaration of a [script](Scripting.md) routine. In addition to the function signature, the directive specifies the library name and, optionally, the calling convention.

External libraries are loaded automatically the first time one of their functions is called, unless the library is already loaded. To load and unload libraries explicitly, the Windows API functions **LoadLibrary** and **FreeLibrary** can be used.

**Note:** the DLL must match the bitness of SyncBackPro. A 64-bit installation of SyncBackPro can only load 64-bit DLLs, and a 32-bit installation can only load 32-bit DLLs. See [32-bit vs 64-bit](32bit64bit.md).

### Pascal Syntax

The declaration syntax when using the [Pascal scripting language](PascalScriptLanguage.md) is:

```
function functionName(arguments): resultType; [callingConvention]; external 'libName.dll' [name 'ExternalFunctionName'];
```

For example, the following declaration:

```
function MyFunction(arg: integer): integer; external 'CustomLib.dll';
```

imports a function called **MyFunction** from **CustomLib.dll**. The default calling convention, if not specified, is **register**. You can declare different calling conventions (**stdcall**, **register**, **pascal**, **cdecl** or **safecall**) and use a different name for the DLL function. For example:

```
function MessageBox(hwnd: pointer; text, caption: string; msgtype: integer): integer; stdcall; external 'User32.dll' name 'MessageBoxA';
```

This imports the **MessageBoxA** function from **User32.dll** (a Windows API library), but it is called as **MessageBox** in the script.

Declarations can be used for both functions and procedures (routines that do not return a value).

### Basic Syntax

The declaration syntax when using the [Basic scripting language](BasicScriptLanguage.md) is:

```
function lib "libName.dll" [alias "ExternalFunctionName"] [callingConvention] functionName(arguments) as resultType
```

For example, the following declaration:

```
function lib "CustomLib.dll" MyFunction(arg as integer) as integer
```

imports a function called **MyFunction** from **CustomLib.dll**. The default calling convention, if not specified, is **stdcall**. You can declare different calling conventions (**stdcall**, **register**, **pascal**, **cdecl** or **safecall**) and use a different name for the DLL function. For example:

```
function MessageBox lib "User32.dll" alias "MessageBoxA" stdcall
(hwnd as pointer, text as string, caption as string, msgtype as integer) as integer

```

This imports the **MessageBoxA** function from **User32.dll** (a Windows API library), but it is called as **MessageBox** in the script.

Declarations can be used for both functions and subs (routines that do not return a value).

### Supported Types

The following data types are supported for the arguments and result of external functions:

```
Integer
Boolean
Char
Extended
String
Pointer
PChar
Object
Class
WideChar
PWideChar
AnsiString
Currency
Variant
Interface
WideString
Longint
Cardinal
Longword
Single
Byte
Shortint
Word
Smallint
Double
Real
DateTime

```

Other types (records, arrays, etc.) are not supported. Arguments of the above types can be passed by reference by adding **var** (Pascal) or **byref** (Basic) to the parameter declaration.

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