Allow only one installer instance

Author: Vytautas (talk, contrib)

Description

The best way is to use a kernel mutex. You can call the Win32 API using the System plug-in. See also CreateMutex plug-in.

Put this code in the .onInit function:

The Script

  System::Call 'kernel32::CreateMutexA(i 0, i 0, t "myMutex") ?e'
  Pop $R0
  StrCmp $R0 0 +3
    MessageBox MB_OK "The installer is already running."
    Abort

'myMutex' should be replaced with a unique value (e.g. ProgramSetup.exe).

The advanced way

This code will bring the already running instance to front, instead of opening a new one. Side Note (Jamyn): Strangely, if the user has first minimized the installer, and then tries to launch another copy, the following code will correctly set the first installer as the active window, but won't "pop" it back up from being minimized. Not sure how to do that simply, but that may be useful. Note (Rescator): Try something like System::Call "user32::ShowWindow(i r1,i 9) i." The 9 is the flag #SW_RESTORE see PSDK for more info on ShowWindow()

 System::Call "kernel32::CreateMutexA(i 0, i 0, t '$(^Name)') i .r0 ?e"
 Pop $0
 StrCmp $0 0 launch
  StrLen $0 "$(^Name)"
  IntOp $0 $0 + 1
 loop:
   FindWindow $1 '#32770' '' 0 $1
   IntCmp $1 0 +4
   System::Call "user32::GetWindowText(i r1, t .r2, i r0) i."
   StrCmp $2 "$(^Name)" 0 loop
   System::Call "user32::SetForegroundWindow(i r1) i."
   Abort
 launch:

Even more advanced

This uses the "advanced way" method. Unlike the function above, it will maximize the window if the installer is minimized. Note (Akshay): I tested this and it works on XPx64 but on Win7 the installer fails saying "Installer integrity check has failed. Common causes include incomplete download and damaged media. Contact the installer's author to obtain a new copy. More informations at http://nsis.sf.net/NSIS_Error"

Function .onInit
  BringToFront
; Check if already running
; If so don't open another but bring to front
  System::Call "kernel32::CreateMutexA(i 0, i 0, t '$(^Name)') i .r0 ?e"
  Pop $0
  StrCmp $0 0 launch
   StrLen $0 "$(^Name)"
   IntOp $0 $0 + 1
  loop:
    FindWindow $1 '#32770' '' 0 $1
    IntCmp $1 0 +5
    System::Call "user32::GetWindowText(i r1, t .r2, i r0) i."
    StrCmp $2 "$(^Name)" 0 loop
    System::Call "user32::ShowWindow(i r1,i 9) i."         ; If minimized then maximize
    System::Call "user32::SetForegroundWindow(i r1) i."    ; Bring to front
    Abort
  launch:
FunctionEnd