PRB: Passing Modified Environments to Child Processes

The information in this article applies to:

• Microsoft Windows Software Development Kit (SDK) for Windows version 3.1

SYMPTOMS

Your application causes a general protection (GP) fault when starting to run under Windows.

CAUSE

To provide a new environment for Windows-based applications, the LoadModule() function must be used. LoadModule() is similar to the MS-DOS Interrupt 21h Function 4Bh call, which uses a structure to find the new environment.

The element "envseg" must be set to the selector of a memory block that contains a correctly formatted environment block. The loader makes a copy of this memory block for the child process inside LoadModule(). This copy is formatted similar to an MS-DOS 2.x environment block. It does not have the MS-DOS versions 3.x and later additional information, such as the full pathname of the task, attached to the end. If startup code (such as the Microsoft C run time earlier than C/C++ version 7.0) looks for this additional information, a general protection (GP) fault will occur. If you are certain that the task started via LoadModule() does not make this assumption, then it is safe to use it.

Another bug with LoadModule() is the ownership of the copied memory created by the loader. It's set to that of the parent, and therefore it needs to outlive the child or its environment block will be freed when the parent terminates.

RESOLUTION

If it's necessary to provide a modified environment to MS-DOS-based applications, the supported technique is to use an MS-DOS batch file. The batch file first sets the new environment settings and then starts the MS-DOS-based application.

   SET FOOS=BALL
   DOSAPP

MORE INFORMATION

Regarding using LoadModule(), below is sample code that passes the current task's environment as envseg:

Sample Code

   typedef struct tagCMDSHOW
   {
     WORD wFirst;
     WORD wSecond;
   }
   CMDSHOW;

   typedef CMDSHOW FAR * LPCMDSHOW ;

   typedef struct tagPARAMETERBLOCK
   {
     WORD      wEnvSeg;
     LPSTR     lpCmdLine;
     LPCMDSHOW lpCmdShow;
     DWORD     dwUnused;
   }
   PARAMETERBLOCK;

   typedef PARAMETERBLOCK FAR * LPPARAMETERBLOCK ;

   CMDSHOW        CmdShow;
   PARAMETERBLOCK ParameterBlock;
   char           szCmdName[] = "TASKMAN.EXE";
   char           szCmdLine[] = "";

   ParameterBlock.wEnvSeg            = HIWORD(GetDosEnvironment());
   ParameterBlock.lpCmdLine          = szCmdLine;
   ParameterBlock.lpCmdShow          = &CmdShow;
   ParameterBlock.lpCmdShow->wFirst  = 2;
   ParameterBlock.lpCmdShow->wSecond = SW_SHOW;
   ParameterBlock.dwUnused           = NULL;

   LoadModule(szCmdName, &ParameterBlock);