PRB: Spawn with _P_WAIT Returns Immediately on Win32s (125213)

SYMPTOMS

If you call _spawnl() with _P_WAIT from a Win32-based application, then under Windows NT, the call returns after the spawned application has been terminated. But under Win32s, the call returns immediately, even though the spawned application is still running.

The return value from _spawnl() with _P_WAIT is zero if the spawned application is a 16-bit Windows-based application or 0x103 if the application is a Win32-based application.

CAUSE

This is due to the design of the C Run-time and a limitation of Win32s. When the CRT does a _P_WAIT spawn, it calls CreateProcess() and checks for failure. Next, the CRT calls WaitForSingleObject() on the process handle. Under Windows NT, WaitForSingleObject() pauses the application until the new process terminates, and then returns TRUE. But under Win32s, WaitForSingleObject() returns TRUE immediately. Next, the CRT calls GetExitCodeProcess() to find the exit status. It is a limitation that GetExitCodeProcess() returns zero for 16-bit Windows-based application on both Windows NT and Win32s. The return value of 0x103 for Win32-based applications is incorrect because the application has not yet terminated.

RESOLUTION

There is no way to perform the synchronous spawn with the spawn family of functions on Win32s. You can only perform an asynchronous spawn.

For an example of a synchronous spawn under Win32s using the Win32 API and the Universal Thunk, please see the following article in the Microsoft Knowledge Base:

125212 Performing a Synchronous Spawn Under Win32s

MORE INFORMATION

The following sample code can be used to demonstrate this behavior. The code spawns NOTEPAD.EXE.

NOTE: The path is hard-coded, so you may have to change it if your NOTEPAD is installed in a different directory. Under Windows NT, the message box is displayed after NOTEPAD is closed, whereas under Win32s, the message box is displayed before NOTEPAD is closed.

NOTEPAD is a 16-bit application under Win32s on Windows. To demonstrate the return value of 0x103 with a Win32-based application, substitute the path to FREECELL.EXE for the C:\\WINNT35\\SYSTEM32\\NOTEPAD.EXE in the _spawnl() call in the code below.

Sample Code

   /* Compile options needed: /MD /DWIN32 <filename>.c /link user32.lib
   */ 

   #include <windows.h>
   #include <process.h>

   void main( )
   {
      DWORD dwVersion;
      int status;
      char buf[80];

      dwVersion = GetVersion();

      if( !(dwVersion & 0x80000000) )
      {
         // Windows NT: NOTEPAD must be closed before this call completes.

         status = _spawnl( _P_WAIT, "c:\\winnt35\\system32\\notepad.exe",
            "notepad.exe", "dummy", NULL );
      }
      else if( LOBYTE(LOWORD(dwVersion)) < 4 )
      {
         // Win32s: This call completes before NOTEPAD is closed.

         status = _spawnl( _P_WAIT, "c:\\windows\\notepad.exe",
            "notepad.exe", "dummy", NULL );
      }
      else
      {
         // Windows 95: for future reference.
      }

      wsprintf( buf, "Return Value = 0x%x\n", status );
      MessageBox( NULL, buf, "Test", MB_OK );
   }