Contact US

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

ByVal behavior in API acts like ByRef?

ByVal behavior in API acts like ByRef?

ByVal behavior in API acts like ByRef?

I am a little confused by the behavior of ByVal as it applies to declaring API functions.  I thought ByVal was supposed to prevent a function from changing the value of an input parameter.  However, it seems that this is not true, given the behavior of some API calls that I have cut-and-pasted into my programs.

I don't know why this is so.  It has been working fine, so I just believed and let it be.  Now I'm overcome with curiosity.

An example is below.  This is pasted from MS knowledge base.  The function SHGetPathFromIDList uses ByVal, but the function is still able to change the input parameter lpBuffer.

Pasted code below:
      Option Explicit

      Private Const BIF_RETURNONLYFSDIRS = 1
      Private Const BIF_DONTGOBELOWDOMAIN = 2
      Private Const MAX_PATH = 260

      Private Declare Function SHBrowseForFolder Lib "shell32" _
                                        (lpbi As BrowseInfo) As Long

      Private Declare Function SHGetPathFromIDList Lib "shell32" _
                                        (ByVal pidList As Long, _
                                        ByVal lpBuffer As String) As Long

      Private Declare Function lstrcat Lib "kernel32" Alias "lstrcatA" _
                                        (ByVal lpString1 As String, ByVal _
                                        lpString2 As String) As Long

      Private Type BrowseInfo
         hWndOwner      As Long
         pIDLRoot       As Long
         pszDisplayName As Long
         lpszTitle      As Long
         ulFlags        As Long
         lpfnCallback   As Long
         lParam         As Long
         iImage         As Long
      End Type

      Private Sub Command1_Click()
      'Opens a Treeview control that displays the directories in a computer

         Dim lpIDList As Long
         Dim sBuffer As String
         Dim szTitle As String
         Dim tBrowseInfo As BrowseInfo

         szTitle = "This is the title"
         With tBrowseInfo
            .hWndOwner = Me.hWnd
            .lpszTitle = lstrcat(szTitle, "")
         End With

         lpIDList = SHBrowseForFolder(tBrowseInfo)

         If (lpIDList) Then
            sBuffer = Space(MAX_PATH)
            SHGetPathFromIDList lpIDList, sBuffer
            sBuffer = Left(sBuffer, InStr(sBuffer, vbNullChar) - 1)

            MsgBox sBuffer
         End If
      End Sub

All this machinery making modern music can still be open-hearted.

RE: ByVal behavior in API acts like ByRef?

It's a string, so you're only passing the address of the string, not the whole string. The 'lp' in the beginning of the var name indicates that it's a pointer.

RE: ByVal behavior in API acts like ByRef?

Actually, that's not true.  I did get a solid explanation when I posed this question on tek-tips.com (I hope Mr. Murphy isn't angry about the double-posting).
see http://www.tek-tips.com/gviewthread.cfm/lev2/4/lev3/32/pid/222/qid/640085

Basically, ByVal will behave like ByRef in Windows API due to differences between how strings are defined in C vs. VB.

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members! Already a Member? Login


Low-Volume Rapid Injection Molding With 3D Printed Molds
Learn methods and guidelines for using stereolithography (SLA) 3D printed molds in the injection molding process to lower costs and lead time. Discover how this hybrid manufacturing process enables on-demand mold fabrication to quickly produce small batches of thermoplastic parts. Download Now
Design for Additive Manufacturing (DfAM)
Examine how the principles of DfAM upend many of the long-standing rules around manufacturability - allowing engineers and designers to place a part’s function at the center of their design considerations. Download Now
Taking Control of Engineering Documents
This ebook covers tips for creating and managing workflows, security best practices and protection of intellectual property, Cloud vs. on-premise software solutions, CAD file management, compliance, and more. Download Now

Close Box

Join Eng-Tips® Today!

Join your peers on the Internet's largest technical engineering professional community.
It's easy to join and it's free.

Here's Why Members Love Eng-Tips Forums:

Register now while it's still free!

Already a member? Close this window and log in.

Join Us             Close