[SOLVED] How do I create a shortcut (.lnk) with a relative target?

How do I create a shortcut (.lnk) with a relative target?

I have an executable on my disk-on-key in dir\program\prog.exe I'd like to have a shortcut to the executable on the DoK's root directory, that is, prog.lnk would refer to dir\program\prog.exe.

However, it seems that prog.lnk can't have a relative target. This is a problem when the DoK will have different drive letters assigned to it, depending on which PC it's connected to.

Any suggestions, aside from the obvious one of putting prog.exe in the root dir?

(ultimately, I'd like to do this at install time using nsis)

Solution

While it is possible for shortcuts to contain a relative path to the target (.lnk files have a flag called SLDF_HAS_RELPATH) NSIS does not support creating anything other than "normal" shortcuts so you need to write the binary data directly (The .lnk format is pretty stable and has been documented by MS)

!macro FileWriteHexBytes h b
push ${h}
push ${b}
call FileWriteHexBytes
!macroend
Function FileWriteHexBytes
exch $9
exch
exch $0
push $1
push $2
loop:
    StrCpy $2 $9 2
    StrLen $1 $2
    IntCmp $1 2 0 end
    FileWriteByte $0 "0x$2"
    StrCpy $9 $9 "" 2
    goto loop
end:
pop $2
pop $1
pop $0
pop $9
FunctionEnd


Function CreateRelativeLnk
exch $9
exch
exch $0
push $1
FileOpen $0 "$0" w
StrCmp $0 "" clean
!insertmacro FileWriteHexBytes $0 "4C0000000114020000000000C000000000000046"
!insertmacro FileWriteHexBytes $0 48010400 ;flags
!insertmacro FileWriteHexBytes $0 00000000000000000000000000000000000000000000000000000000000000000000000001000000000000000000000000000000

StrLen $1 $9 ;must be < 255!
FileWriteByte $0 $1
FileWriteByte $0 0
FileWrite $0 "$9" ;relative target path

!if 0
;The icon is problematic, does not seem like it works with relative paths (but you can use system icons...)
StrCpy $9 "explorer.exe"
StrLen $1 $9
FileWriteByte $0 $1
FileWriteByte $0 0
FileWrite $0 "$9"
!else
!insertmacro FileWriteHexBytes $0 05003e2e657865 ;fake default .exe icon
!endif

clean:
FileClose $0
pop $1
pop $0
pop $9
FunctionEnd

Call it like this:

push "$temp\testlink.lnk"
push "testdir\testapp.exe" ;full path to this is $temp\testdir\testapp.exe
call CreateRelativeLnk

While the generated .lnk seems to work, I'm not sure if I would use this in production code

A much better solution is to create a little NSIS app like Oleg suggests (NSIS applications can contain embedded data at the end of the .exe that it can read from itself at runtime etc..)