atexit: Clarify DLL behavior is the same as _onexit - #5952

/***
*_onexit(func), atexit(func) - add function to be executed upon exit
*
*Purpose:
*       The _onexit/atexit functions are passed a pointer to a function
*       to be called when the program terminate normally.  Successive
*       calls create a register of functions that are executed last in,
*       first out.
*
*Entry:
*       void (*func)() - pointer to function to be executed upon exit
*
*Exit:
*       onexit:
*           Success - return pointer to user's function.
*           Error - return NULL pointer.
*       atexit:
*           Success - return 0.
*           Error - return non-zero value.
*
*Notes:
*       This routine depends on the behavior of _initterm() in CRT0DAT.C.
*       Specifically, _initterm() must not skip the address pointed to by
*       its first parameter, and must also stop before the address pointed
*       to by its second parameter.  This is because _onexitbegin will point
*       to a valid address, and _onexitend will point at an invalid address.
*
*Exceptions:
*
*******************************************************************************/

_onexit_t __cdecl _onexit (
        _onexit_t func
        )
{
        _onexit_t retval;

        _lockexit();

        __try {
            retval = _onexit_nolock(func);
        }
        __finally {
            _unlockexit();
        }

        return retval;
}


static _onexit_t __cdecl _onexit_nolock (
        _onexit_t func
        )
{
        _PVFV * p;
        size_t  oldsize;
        _PVFV * onexitbegin = (_PVFV *) DecodePointer(__onexitbegin);
        _PVFV * onexitend = (_PVFV *) DecodePointer(__onexitend);

        /* overflow check */
        if (onexitend < onexitbegin ||
            ((char *)onexitend - (char *)onexitbegin) + sizeof(_PVFV) < sizeof(_PVFV))
        {
            return NULL;
        }

        /*
         * First, make sure the table has room for a new entry
         */
        if ( (oldsize = _msize_crt(onexitbegin))
                < ((size_t)((char *)onexitend -
            (char *)onexitbegin) + sizeof(_PVFV)) )
        {
            /*
             * not enough room, try to grow the table. first, try to double it.
             */
            size_t newsize = oldsize + __min(oldsize, (MAXINCR * sizeof(_PVFV)));
            if ( newsize < oldsize ||
                 (p = (_PVFV *)_realloc_crt(onexitbegin, newsize)) == NULL )
            {
                /*
                 * failed, try to grow by MININCR
                 */
                newsize = oldsize + MININCR * sizeof(_PVFV);
                if ( newsize < oldsize ||
                     (p = (_PVFV *)_realloc_crt(onexitbegin, newsize)) == NULL )
                    /*
                     * failed again. don't do anything rash, just fail
                     */
                    return NULL;
            }

            /*
             * update __onexitend and __onexitbegin
             */
#pragma warning(suppress: 22008) /* prefast is confused */
            onexitend = p + (onexitend - onexitbegin);
            onexitbegin = p;
            __onexitbegin = (_PVFV *) EncodePointer(onexitbegin);
        }

        /*
         * Put the new entry into the table and update the end-of-table
         * pointer.
         */
         *(onexitend++) = (_PVFV) EncodePointer(func);
        __onexitend = (_PVFV *) EncodePointer(onexitend);

        return func;
}

int __cdecl atexit (
        _PVFV func
        )
{
        return (_onexit((_onexit_t)func) == NULL) ? -1 : 0;
}