https://autohotkey.com/boards/viewtopic.php?t=791

StdoutToVar.ahk

; ----------------------------------------------------------------------------------------------------------------------
; Function .....: StdoutToVar_CreateProcess
; Description ..: Runs a command line program and returns its output.
; Parameters ...: sCmd      - Commandline to execute.
; ..............: sEncoding - Encoding used by the target process. Look at StrGet() for possible values.
; ..............: sDir      - Working directory.
; ..............: nExitCode - Process exit code, receive it as a byref parameter.
; Return .......: Command output as a string on success, empty string on error.
; AHK Version ..: AHK_L x32/64 Unicode/ANSI
; Author .......: Sean (http://goo.gl/o3VCO8), modified by nfl and by Cyruz
; License ......: WTFPL - http://www.wtfpl.net/txt/copying/
; Changelog ....: Feb. 20, 2007 - Sean version.
; ..............: Sep. 21, 2011 - nfl version.
; ..............: Nov. 27, 2013 - Cyruz version (code refactored and exit code).
; ..............: Mar. 09, 2014 - Removed input, doesn't seem reliable. Some code improvements.
; ..............: Mar. 16, 2014 - Added encoding parameter as pointed out by lexikos.
; ..............: Jun. 02, 2014 - Corrected exit code error.
; ..............: Nov. 02, 2016 - Fixed blocking behavior due to ReadFile thanks to PeekNamedPipe.
; ----------------------------------------------------------------------------------------------------------------------
StdoutToVar_CreateProcess(sCmd, sEncoding:="CP0", sDir:="", ByRef nExitCode:=0) {
    DllCall( "CreatePipe",           PtrP,hStdOutRd, PtrP,hStdOutWr, Ptr,0, UInt,0 )
    DllCall( "SetHandleInformation", Ptr,hStdOutWr, UInt,1, UInt,1                 )

            VarSetCapacity( pi, (A_PtrSize == 4) ? 16 : 24,  0 )
    siSz := VarSetCapacity( si, (A_PtrSize == 4) ? 68 : 104, 0 )
    NumPut( siSz,      si,  0,                          "UInt" )
    NumPut( 0x100,     si,  (A_PtrSize == 4) ? 44 : 60, "UInt" )
    NumPut( hStdOutWr, si,  (A_PtrSize == 4) ? 60 : 88, "Ptr"  )
    NumPut( hStdOutWr, si,  (A_PtrSize == 4) ? 64 : 96, "Ptr"  )

    If ( !DllCall( "CreateProcess", Ptr,0, Ptr,&sCmd, Ptr,0, Ptr,0, Int,True, UInt,0x08000000
                                  , Ptr,0, Ptr,sDir?&sDir:0, Ptr,&si, Ptr,&pi ) )
        Return ""
      , DllCall( "CloseHandle", Ptr,hStdOutWr )
      , DllCall( "CloseHandle", Ptr,hStdOutRd )

    DllCall( "CloseHandle", Ptr,hStdOutWr ) ; The write pipe must be closed before reading the stdout.
    While ( 1 )
    { ; Before reading, we check if the pipe has been written to, so we avoid freezings.
        If ( !DllCall( "PeekNamedPipe", Ptr,hStdOutRd, Ptr,0, UInt,0, Ptr,0, UIntP,nTot, Ptr,0 ) )
            Break
        If ( !nTot )
        { ; If the pipe buffer is empty, sleep and continue checking.
            Sleep, 100
            Continue
        } ; Pipe buffer is not empty, so we can read it.
        VarSetCapacity(sTemp, nTot+1)
        DllCall( "ReadFile", Ptr,hStdOutRd, Ptr,&sTemp, UInt,nTot, PtrP,nSize, Ptr,0 )
        sOutput .= StrGet(&sTemp, nSize, sEncoding)
    }
    
    ; * SKAN has managed the exit code through SetLastError.
    DllCall( "GetExitCodeProcess", Ptr,NumGet(pi,0), UIntP,nExitCode )
    DllCall( "CloseHandle",        Ptr,NumGet(pi,0)                  )
    DllCall( "CloseHandle",        Ptr,NumGet(pi,A_PtrSize)          )
    DllCall( "CloseHandle",        Ptr,hStdOutRd                     )
    Return sOutput
}