PRB: The SPAWN.C Sample Fails as a C++ Application (128790)
The information in this article applies to:
- Microsoft Visual C++ for Windows, 16-bit edition 1.0
- Microsoft Visual C++ for Windows, 16-bit edition 1.5
- Microsoft Visual C++ for Windows, 16-bit edition 1.51
- Microsoft Visual C++, 32-bit Editions 1.0
- Microsoft Visual C++, 32-bit Editions 2.0
- Microsoft Visual C++, 32-bit Editions 2.1
- Microsoft Visual C++, 32-bit Editions 2.2
- Microsoft Visual C++, 32-bit Editions 4.0
- Microsoft Visual C++, 32-bit Editions 4.1
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.
MORE INFORMATIONSample 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 <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" );
}
| Modification Type: | Minor | Last Reviewed: | 7/5/2005 |
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| Keywords: | kbCompiler kbCPPonly kbprb KB128790 |
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