Macro vs Function

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Macros and Functions in NSIS are powerful tools to help make coding your installer easier and more flexible. They can also be very powerful tools, allowing you to extend the NSIS scripting language to an extent - LogicLib is a great example of this.

Definitions

Macros

From the NSIS help file: "Macros are used to insert code at compile time, depending on defines and using the values of the defines. The macro's commands are inserted at compile time. This allows you to write a general code only once and use it a lot of times but with a few changes."

If you are new to macros in any scripting language, then this may sound a bit confusing. Let's go through a few examples to show what the above means.

"Macros are used to insert code at compile time"

What this means is that the code you define in a macro will simply be inserted at the location of your !insertmacro, as if copy/pasted, when you compile your installer script.

!macro Hello
  DetailPrint "Hello world"
!macroend
 
Section Test
  !insertmacro Hello
SectionEnd

Example 1.1

Could be seen as just:

Section Test
  DetailPrint "Hello world"
SectionEnd

Example 1.2

The above is obviously just a simple example with a single line and really wouldn't be a good use of macros. But if you have multiple lines of code that you may have to use over and over again, it may be a good idea to start using a macro for it; it allows you to just make the any changes once in the macro, and it will automatically be changed anywhere you insert it. It also just makes your code look a lot cleaner (a single !insertmacro line vs perhaps a dozen lines) which makes it a lot easier to follow and edit.

"depending on defines and using the values of the defines"

That bit is most likely to sound confusing, but gets a little more clear once you read the definition of macros in another section of the NSIS help:

"macro definitions can have one or more parameters defined. The parameters may be accessed the same way a !define would (e.g. ${PARMNAME}) from inside the macro."

If you are familiar with scripting in other languages, this may sound familiar to you, except in the context of functions - but please do not mix this up with Functions as they exist in the NSIS language.. we'll get to those later.

What the above means is that you can have a macro take parameters, or arguments, and use those within the macro:

!macro Hello What
  DetailPrint "Hello ${What}"
!macroend
 
Section Test
  !insertmacro Hello "World"
  !insertmacro Hello "Tree"
  !insertmacro Hello "Flower"
SectionEnd

Example 2.1

Could be seen as just:

  DetailPrint "Hello World"
  DetailPrint "Hello Tree"
  DetailPrint "Hello Flower"

Example 2.2

However, you only needed a single macro definition to get these three different results. Let's take a more complex example, straight from the NSIS Help. In NSIS, a Function can only be specified as being for the Installer, or the Uninstaller (prefixed by "un."). The reason for this is that it allows the compiler to make a smaller Uninstaller if it does not need the Installer's functions, and vice-versa. However, sometimes you may have a function that both the Installer and the Uninstaller require. You could then code the Function twice:

Function SomeFunc
  ; Lots of code here
FunctionEnd
 
Function un.SomeFunc
  ; Lots of code here
FunctionEnd

Example 2.3 However, as it should be apparent, if there's lots of code involved you have two separate areas where you have to make code changes, your code looks less clean, etc.

With the use of macros, this can easily be resolved by writing a macro around the function that simply adds the "un." prefix when requested:

!macro SomeFunc un
  Function ${un}SomeFunc
    ; lots of code here
  FunctionEnd
!macroend
!insertmacro SomeFunc ""
!insertmacro SomeFunc "un."

Example 2.4

The above will then be written out as example 2.3, but now you only have a single portion of code to maintain.

For more information on this specific topic, see: Sharing_functions_between_Installer_and_Uninstaller