Visual C++ Vers 2.0 README.WRI, Part 3 Compiler

SUMMARY

The following is based on the Visual C++ version 2.0 README.WRI file:

/W4 Warning Messages from Including WINDOWS.H

Specifying warning level 4 (/W4) when you compile any program that includes WINDOWS.H may generate many warnings. Most of these are benign warnings caused by non-standard extensions used in Windows NT header files such as WINDOWS.H and files that WINDOWS.H includes. To eliminate these warnings, create a "dummy" header file that uses the #pragma warning directive to selectively turn off warnings you don't need to see. Then include this header file in any source file that also includes WINDOWS.H, or use the /FI compiler option to force its inclusion.

For example:

// FILE NOWARN.H
#pragma warning(disable:4201)
#pragma warning(disable:4214)

//FILE MYMOD.C
#include "NOWARN.H"
#include <windows.h>
void main(void) {}
				

For further examples, see AFX.H, located in the MFC\INCLUDE subdirectory of your Visual C++ version 2.0 installation. AFX.H uses #pragma warning to disable specific level 4 warnings when compiling MFC applications.

Passing Structures by Value

A pass by value of a structure greater than or equal to 4096 (4K) bytes may cause an access violation if the current stack allocation is exceeded. If the structure size is greater than or equal to 8192 (8K) bytes, an access violation is likely unless you ensure that new stack pages are not being accessed.

The __purecall function

The __purecall function is a special function (defined with C linkage) that is defined in the default run-time library, and may be called at run time if there is an error referencing the object's virtual function table. This will usually happen as a result of a program error. The following code causes this error:

struct A;
void x(A*);
struct A
{
    virtual void f() = 0;
    A() { x(this); }
};

struct B : A
{
    void f() { }
};

void x(A*p) { p->f(); }

void main()
{
    B aB;    // calls B::B calls A::A calls
}            // x calls A::f (pure virtual function)
				

Because such errors cannot be caught at compile time, the __purecall function is called, generating the following run-time error: If an application (such as a device driver) must be compiled without using the standard run-time libraries (compiler option /Zl or linker option /NODEFAULTLIB), but requires virtual function calls, you will get an "undefined symbol" linker error on __purecall. You can work around this error in one of two ways:

• Define your own __purecall function and compile with the /Gd option (specify __cdecl calling convention). The following is a minimal example:

        int __purecall( void )
        {
            /* insert your own error reporting code here */ 
            return 0;
        }
  						

  If __purecall is redefined in exactly this way, no run time error is generated if the user calls a pure virtual function. The custom __purecall function should contain appropriate error checking and recovery code, (for example, calling _exit or printing a fatal error message) as noted in the sample.
  
  -or-
• Extract the object module that contains __purecall from the LIBC.LIB library, and link the object explicitly. Using this method, all other symbols in the object module will be included. To avoid this potentially undesirable side-effect, the first method is recommended.