Microsoft KB Archive/128790

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

Article ID: 128790

Article Last Modified on 7/5/2005

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APPLIES TO


 * Microsoft Visual C++ 1.0 Professional Edition
 * Microsoft Visual C++ 1.5 Professional Edition
 * Microsoft Visual C++ 1.51
 * Microsoft Visual C++ 1.0 Professional Edition
 * Microsoft Visual C++ 2.0 Professional Edition
 * Microsoft Visual C++ 2.1
 * Microsoft Visual C++ 2.2
 * Microsoft Visual C++ 4.0 Standard Edition
 * Microsoft Visual C++ 4.1 Subscription

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This article was previously published under Q128790



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.



Sample Code
/* Compile options needed:   none, /AL or /AC will reproduce the *                                           problems with NULL. */ The following code is a correctly modified SPAWN.CPP: /* 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    #include    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> \n" ); exit( 1 ); }     printf( "from SPAWN!\n" ); }

Additional query words: 8.0 8.0c 8.00 8.00c 9.00 9.10 10.00 10.10

Keywords: kbprb kbcpponly kbcompiler KB128790

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