How to achieve strncpy() functionality with strncpy_s() function?

Question

There're certain cases when I really need strncpy() funcitonalty - for example I have a function in a predefined interface that is passed an address of the buffer and the size of the buffer:

HRESULT someFunction( char* buffer, size_t length );

and it is documented that I can copy a null-terminated string there with length no more than length - if it is exactly of length length I don't null-terminate the string and the caller knows that the string ends at either a null character or at length length whichever happens first and it all works.

Of course I will use strncpy() for that

HRESULT someFunction( char* buffer, size_t length )
{
    const char* toCopy = ...
    size_t actualLength = strlen( toCopy );
    if( actualLength > length ) {
        return E_UNEXPECTED; // doesn't fit, can't do anything reasonable 
    }
    strncpy( buffer, toCopy, length );
    return S_OK;
}

Now I have this code and need to migrate it from Visual C++ 7 to Visual C++ 9. I compile it and see a warning that strncpy() is unsafe and I should instead use strncpy_s().

strncpy_s() is designed to always null-terminate the buffer, so I can't use it as a direct replacement in the above scenario. I'll have to return E_UNEXPECTED on strings longer than length - 1 (not length as previously) or it will just fire an invalid parameters error handler once a string is of length or longer or the program will run into undefined behavior.

The solution I applied so far is to just define a _CRT_SECURE_NO_WARNINGS and make the compiler shut up.

is there any way to use strncpy_s() as an actual replacement to strncpy()?

Answer 1

The problem you're facing here is that your function is unsafe in itself, just like strncpy() is. It is unsafe because callers of your function might forget that the returned strings are not null terminated. If this really is the desired behavior of your function I recommend not to define _CRT_SECURE_NO_WARNINGS and disable the warnings globally but use #pragmas instead:

// document here exactly why you can not use strncpy_s
#pragma warning( push )
#pragma warning( disable : 4996 )
// your code that uses strncpy instead of strncpy_s
#pragma warning( pop ) 

That way you disable those warnings only for those situations where you absolutely have to use unsafe functions.