DOCERR: Function Returning Zero When Redefining _nullcheck()

MS-DOS                 | OS/2       | WINDOWS

The information in this article applies to:

• Microsoft C for MS-DOS, versions 6.0, 6.0a, and 6.0ax
• Microsoft C for OS/2, versions 6.0 and 6.0a
• Microsoft C/C++ for MS-DOS, version 7.0
• Microsoft Visual C++ for Windows, versions 1.0, 1.5, 1.51, and 1.52

By default, Microsoft C checks the NULL segment before the final termination of a program in order to determine if a null pointer assignment has occurred. This check can be suppressed by defining your own function called _nullcheck(), which is the name of the library routine that is normally linked in to do the checking.

The online Help for C versions 6.0, 6.0a, 6.0ax, C/C++ versions 7.0, and 8.0 describes how to replace this function, but there is an error in those Help files. The online Help states that you just need to declare your own routine "named _nullcheck that does nothing". This is not completely true. You actually need to make your function return a value of zero; otherwise, the program exit code will be set to 255.

The following is a valid way to define the _nullcheck() function in your program:

    int _cdecl _nullcheck( void)
    {
        return (0);
    }