Knowledge Base

PRB: Stack Violation Calling 16-Bit DLL with FoxTools

Article ID: 139070

Article Last Modified on 2/11/2000

Microsoft Visual FoxPro 3.0 Standard Edition

This article was previously published under Q139070

SYMPTOMS

Using the FOXTOOLS CallFn()function to call a 16-bit DLL and passing a "L" as an argument generates the error "Stack Violation. Please check the parameters." The same function call works well with FoxPro for Windows versions 2.x.

CAUSE

The function is registered and called as follows.

   Int_num=regfn("Fn_init","L","I", "<path to DLL>")
   Int_ret=callfn(int_num,0)

In this call, the "L" parameter, which represents a long argument, is passed to the DLL. The API function expects a short argument, and the stack allocated for the parameter is filled. The "S" argument, which represents a short integer, should be passed instead.

In the version of Foxtools shipped with FoxPro versions 2.x for Windows the "S" argument was not valid. Additionally, the version of Foxtools.fll shipped with Visual FoxPro version 3.0 checks parameters more strictly than previous versions.

RESOLUTION

When registering a function from a 16-bit DLL, pass an "S" instead of an "L" as in this example:

   Int_num=regfn("Fn_init","S","I", "<path to DLL>")
   Int_ret=callfn(int_num,0)

For more information about Foxtools.fll and the syntax you can use, please see the Foxtools.hlp file located in the Vfp\Tools directory.

STATUS

This behavior is by design.

MORE INFORMATION

Steps to Reproduce Behavior

The following example program duplicates the error. To correct it, change the "L" parameter to "S".

   SET LIBRARY TO HOME()+"FOXTOOLS.FLL"            && Locates Foxtools
   hhand=REGFN("WinHelp","ICIC","I")
   LPZFILENAME=HOME()+"FOXHELP.HLP"+CHR(0)
   WCOMMAND=258
   DWDATA="SEARCH()"+CHR(0)
   MYCALL=MAINHWND()
   HELP
   =CALLFN(hhand,MYCALL,LPZFILENAME,WCOMMAND,DWDATA)
   RELEASE HHAND
   SET LIBRARY TO      && Releases Foxtools

Keywords: KB139070