PRB: The SPAWN.C Sample Fails as a C++ Application

• The Microsoft C/C++ Compiler (CL.EXE) included with: - Microsoft Visual C++ for Windows, versions 1.0, 1.5, 1.51 - Microsoft Visual C++, 32-bit Edition, versions 1.0, 2.0, 2.1, 2.2,

       4.0, 4.1
  

SYMPTOMS

The SPAWN.C program fails to spawn applications correctly when it is copied to a C++ file (SPAWN.CPP).

Attempting the conversion results in these two problems:

• You receive error C2662:

     '__spawnv' : cannot convert parameter 3 from'char
     __far*[4]' to 'const char __far *const __far * '
  
  

• You receive errors stating that you have the wrong number of arguments, or the spawned program fails, which may force you to reboot the computer. These problems are caused by the fact that the definition of NULL in C++ is different from the definition of NULL in C.

RESOLUTION

These same issues are frequently encountered when converting C programs to the C++ language.

Casting the args and my_env variables with (const char *const *) will resolve the first problem. Alternatively, you can work around this problem by redefining the prototype in PROCESS.H.

To resolve the second problem, you need to understand the change in the definition of NULL. In C++, NULL is defined as 0 (zero); in C, NULL is defined as (void *). Therefore, you can solve the second problem by casting NULL with (void *). This problem is encountered in far data models only.

STATUS

This behavior is by design.

MORE INFORMATION

Sample Code

   /* Compile options needed:    none, /AL or /AC will reproduce the
   *                                            problems with NULL.
   */

   /* SPAWN.CPP: This program accepts a number in the range
    * 1-8 from the command line. Based on the number it receives,
    * it executes one of  the eight different procedures that
    * spawn the process named child. For some of these procedures,
    * the CHILD.EXE file must be in the same directory; for
    * others, it need only be in the same path.
    */
   #include <stdio.h>
   #include <process.h>
   char *my_env[] =
   {
      "THIS=environment will be",
      "PASSED=to child.exe by the",
      "_SPAWNLE=and",
      "_SPAWNLPE=and",
      "_SPAWNVE=and",
      "_SPAWNVPE=functions",
      NULL
   };
   void main( int argc, char *argv[] )
   {
      char *args[4];
      /* Set up parameters to be sent: */
      args[0] = "child";
      args[1] = "spawn??";
      args[2] = "two";
      args[3] = NULL;
      switch (argv[1][0])   /* Based on first letter of argument */
      {
      case '1':
         _spawnl( _P_WAIT, argv[2], argv[2], "_spawnl", "two",
        (void *)NULL );
         break;
      case '2':
         _spawnle( _P_WAIT, argv[2], argv[2], "_spawnle", "two",
         (void *)NULL, my_env );
         break;
      case '3':
         _spawnlp( _P_WAIT, argv[2], argv[2], "_spawnlp", "two",
         (void *)NULL );
         break;
      case '4':
         _spawnlpe( _P_WAIT, argv[2], argv[2], "_spawnlpe", "two",
         (void *)NULL, my_env );
         break;
      case '5':
         _spawnv( _P_OVERLAY, argv[2],
         (const char *const __near *)args );
         break;
      case '6':
         _spawnve( _P_OVERLAY, argv[2], (const char *const *)args,
         (const char * const *)my_env );
         break;
      case '7':
         _spawnvp( _P_OVERLAY, argv[2], (const char *const *)args );
         break;
      case '8':
         _spawnvpe( _P_OVERLAY, argv[2], (const char *const *)args,
         (const char * const *)my_env );
         break;
      default:
         printf( "SYNTAX: SPAWN <1-8> <childprogram>\n" );
         exit( 1 );
      }
      printf( "from SPAWN!\n" );
   }