PRB: Passing Modified Environments to Child Processes

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
If the application being started is not a Windows-based application, the
.BAT file technique is the only supportable method.

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);
Lastnosti

ID članka: 102958 – Zadnji pregled: 14. feb. 2017 – Revizija: 1

Microsoft Windows Software Development Kit 3,1

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