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## 4.11 Plug-in DLLs

The abilities of the NSIS scripting language can be extended by utilising functionality provided in a DLL file. Probably the best known example of this is the InstallOptions.dll bundled with every NSIS release.

When the NSIS compiler starts it scans the plug-ins directory for DLLs and makes a list of the plug-ins found and their exported functions. During compilation if a sequence such as fred::flintstone is encountered where the compiler expected to find a language keyword the compiler will look through this list. If a list entry specifies that fred.dll exports function flintstone NSIS will pack the fred.dll file into the created installer binary.

During execution of a plug-in command NSIS will unpack the necessary DLL to a temporary folder ($PLUGINSDIR), push all of the arguments specified (right-to-left order), and then execute the DLL function. If the /NOUNLOAD option is specified the DLL will not be unloaded until the installer exits or the next time you use the DLL without /NOUNLOAD. Please note that the last call to the plug-in must not have the /NOUNLOAD flag or the plug-in will not be deleted from $PLUGINSDIR, thus garbage will be left on the user's machine.

### 4.11.1 Using Plug-in Commands

A plug-in call looks like this:

    InstallOptions::dialog "ini_file_location.ini"

All parameters are pushed onto the stack (in this case, the plug-in function only needs one parameter). Some plug-in commands may not need any parameters on the stack, others might require more of them. To use a plug-in command you will need to read the documentation for the plug-in so that you know what parameters its functions require.

### 4.11.2 Disabling Plug-in Unloading

If you don't want to unload the DLL after calling a function, use /NOUNLOAD as the first parameter. For example:

    dll::function /NOUNLOAD "param"

You can also use SetPluginUnload alwaysoff to avoid writing /NOUNLOAD each and every time you use the same plug-in.

### 4.11.3 Calling plug-ins manually

If you want to call a plug-in that is stored on user's hard drive or somewhere else, use CallInstDLL. Almost all plug-ins provide installer functionality, so using plug-in commands is way easier. Using CallInstDLL can be useful when you have created plug-ins that should be linked to a certain version of your application and are being copied to the installation folder.

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